1. Patristic and Medieval Arguments for God’s Existence

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Patristic and Medieval Arguments for God’s Existence

In her The Divine Sense: The Intellect in Patristic Theology, A. N. Williams points out that for fathers of the church, humans after the fall continued to possess the light of reason. The fathers ruled out any “pessimistic apophaticism” or any effort “to impugn our capacity to know God, at least dimly,” because such views “would amount to claiming God had deliberately left us bereft of the only means to good, happy and purposeful lives, and would deny, therefore, any conceivable telos to creation.” In this chapter, I show that the fathers of the church, along with almost all medieval theologians, held that humans can rationally demonstrate the existence of God.

I focus on four fathers and three medievals. In Tertullian, we find the same proof that Xenophon attributed to Socrates: the ordered cosmos could not have made and governed itself. Tertullian insists that Christianity does not introduce a new God: despite the fall, the majority of humans already knew God, even though they also worshiped other gods. There is a human yearning for God that also testifies to God’s existence. Gregory of Nazianzus demonstrates God’s existence in three ways: through the order of the universe, through the fact that every finite thing is dependent and cannot move the whole, and from finite perfections to infinite perfection. He compares the universe to a beautiful lyre and to such things as birds’ nests and bees’ honeycombs. The universe cannot account for its order, movement, existence, and perfections, as Gregory also shows in his poetry. Augustine makes much of human desire for God, as well as the contingency of all things. His favorite demonstration of God’s existence has its starting point in the soul’s intellectual light, which discerns (as the measure of all truth) an immutable, eternal Light—eternal truth, goodness, and beauty. Like Tertullian, Augustine argues that almost all humans prior to Christ knew of the existence of a supreme Creator God, despite their idolatry. Augustine grants, however, that even when humans are aware that God is that than which there is nothing better, humans can falsely imagine that finite things are the highest, despite the fact that nothing merely living can be as good as infinite Life. John of Damascus offers various demonstrations of God’s existence, including from the fact that changeable things require a source, from the order of creation whose disparate elements could not otherwise hold together, and from the fact that things cannot spontaneously come to be and preserve themselves in being.

Turning to the medieval period, I first discuss Anselm of Canterbury. In his Monologion, Anselm demonstrates God’s existence from our desire for good, from the fact that existing things require a self-subsistent source of existence, and from degrees of perfection. But Anselm is most famous for the demonstration that he offers in his Proslogion, which begins with Augustine’s definition of God as that than which nothing better exists. Anselm argues that once we understand the terms of this definition, we can show that God must indeed exist. Gaunilo responds to Anselm, arguing that Anselm has conflated the order of knowing with the order of being; in turn, Anselm responds to certain errors that Gaunilo makes in his presentation of Anselm’s ideas. Thomas Aquinas is best known for his adaptation of the Aristotelian proofs from motion (or change), from efficient causality, and from contingency. He also sets forth demonstrations based on the degrees of perfection and the order of the universe. Although we find similar proofs in the fathers, Aquinas gives a more metaphysically detailed exposition of each proof. He adds an explanation of why Anselm’s proof in the Proslogion does not work. The last figure treated in this chapter, William of Ockham, does not think that the demonstrations from motion or efficient causality work, but he does argue that human reason can demonstrate God’s existence on the basis of the universe’s conservation in being. Ockham denies, however, that human reason can demonstrate the unity of God, thereby throwing into doubt whether human reason can demonstrate God’s existence even from the universe’s conservation in being.

It should be clear that this chapter covers a wide terrain. Most of the fundamental kinds of demonstrations of God’s existence appear in this chapter, and also, in Ockham, the first significant doubts about their efficacy. Let us now turn to the writings of these seven seminal contributors to the Christian tradition of demonstrating the existence of God.

Tertullian (ca. 150–ca. 220)

A native of Carthage and a married layman, Tertullian was a lawyer who was well educated in philosophy and rhetoric. An author of numerous seminal theological and ascetical works, he is known for developing much of the Latin terminology for the theology of the Trinity. Tertullian is sometimes thought to have separated from the church toward the end of his life. Although he adhered to the prophecy of Montanus and admired its moral rigor, Tertullian at least technically remained within the church, because during his lifetime the church in North Africa had not yet expelled the followers of Montanus’s prophecy. Later North African bishop-theologians, notably Cyprian of Carthage and Optatus of Milevis, held Tertullian in high regard. Augustine read Tertullian carefully and was deeply influenced by him, although he criticized him for teaching that second marriage was forbidden and for founding separatist congregations.

In his Apology, after describing the persecution of Christians in the Roman Empire and criticizing at length the worship of the gods, Tertullian begins his defense of Christianity by proclaiming the one God: “The object of our worship is the One God, He who by His commanding word, His arranging wisdom, His mighty power, brought forth from nothing this entire mass of our world, with all its array of elements, bodies, spirits, for the glory of His majesty.” Tertullian describes this God as invisible, incomprehensible, and infinite. Even when we know him, he remains unknown. Humans, however, can and should know him, and humans who fail to know and worship him are culpable.

In this light, Tertullian goes on to offer two ways of demonstrating the existence of God. First, we can demonstrate that God exists on the basis of the existence and order of the cosmos. Tertullian appeals to the fact that such a vast, well-ordered cosmos cannot have made and governed itself. He terms this “the proof from the works of His hands, so numerous and so great, which both contain you and sustain you, which minister at once to your enjoyment, and strike you with awe.” The second proof comes from the soul and is based on human experience. Although we are often enslaved to our bodily desires, when our soul wakes up from this enslavement we find ourselves yearning for God. As evidence of this yearning and of our knowledge of the One for whom we yearn, Tertullian points to the frequency of reference to “God” (deus) in common pagan speech. Looking toward the heavens, humans often praise God or appeal to God for help and for justice. The soul looks up to God and knows God must exist.

At this juncture, Tertullian turns to revelation. The transcendent, invisible God whom the things of this world and our own experience teach us must exist has indeed revealed himself so that we can “attain an ampler and more authoritative knowledge” of God. This ampler knowledge was first communicated to the prophets of Israel, who taught that there is one Creator God who communicated his law and his plan for eternal judgment. Of the ancient and majestic Scriptures of Israel, Tertullian states, “Whoever gives ear will find God in them; whoever takes pains to understand, will be compelled to believe.” He proceeds to set forth how the Scriptures of Israel promised the coming of the Messiah, so that “in the last days of the world, God would, out of every nation, and people, and country, choose for Himself more faithful worshippers, upon whom He would bestow His grace, and that indeed in ampler measure, in keeping with the enlarged capacities of a nobler dispensation.” Although some consider Jesus to be merely human, Tertullian emphasizes that he is divine, the Son and Word of God. Tertullian remarks that Christ, in his divinity, “is called God from unity of substance with God,” so that “Christ is Spirit of Spirit, and God of God, as light of light is kindled.” He fully shares the divine nature, although he comes forth from the Father. He took flesh in the womb of the Virgin Mary. Misunderstood by his people, he died for our sins, rose from the dead, and will come again in glory to judge the whole world.

Thus, Tertullian’s Apology starts from the one God and from two ways of proving God’s existence, and then shows that Jesus Christ reveals God (and God’s providence) in a much fuller manner. But Tertullian’s view that human reason can attain to the knowledge that God exists does not imply that he has a general admiration for the Greco-Roman philosophers. On the contrary, in his To the Nations, he argues that the philosophers managed to obscure human wisdom about God by their variety of opinions. Whereas the Platonists considered God to be within the world and to care about worldly matters, the Epicureans thought of God as nonexistent, and the Stoics thought of the world as God. The result was that “among the philosophers there was not only an ignorance, but actual doubt, about the divinity.” Tertullian goes on especially to criticize the Stoic view that the elements are divine. Elsewhere, in his The Prescription against Heretics, he warns against the way in which too strong a devotion to pagan philosophy can lead to heresies, and he observes, “What indeed has Athens to do with Jerusalem? . . . Away with all attempts to produce a mottled Christianity of Stoic, Platonic, and dialectical composition!” His affinity for the ability of human reasoning to prove the existence of God, and his placement of these proofs at the outset of his discussion of the Christian God, do not lead him to an uncritical acceptance of ancient philosophical theories. This is so even though he accepts the Stoic view that God is in some sense material, because substantial. In his Against Praxeas, he argues that “Spirit has a bodily substance, of its own kind, in its own form.”

Tertullian’s Against Marcion also provides testimony in affirmation of the demonstrations of God’s existence. Here Tertullian wishes both to defend Moses against Marcion and to hold that Moses was not the first human to know of God. After all, it would hardly make sense that God would create rational creatures and leave them incapable of knowing him. Tertullian observes that Moses himself, in writing the Torah, makes clear that the knowledge of God does not begin with God’s revelation to Moses (or to Abraham). Rather, true knowledge of God “is to be traced from Paradise and Adam, not from Egypt and Moses.” By contrast, Marcion claims to know a newly revealed God who is not the Creator, a God unknown and concealed prior to the coming of Christ. The Marcionites, says Tertullian, bring “forward a new god, as if we were ashamed of the old one!” Tertullian emphasizes that there can only be one God: either the God of Israel is the one eternal God, or the god of Marcion is, but there cannot be two. The Marcionites, however, accept that the Creator exists, even though they disparage him. Tertullian points out that if the known Creator exists, we should not believe in an unknown new God of Marcion. He underscores that the true eternal Creator God has made himself known from the beginning through the things that he created. According to Tertullian, these things manifest the existence of their transcendent Creator: “For indeed, as the Creator of all things, He was from the beginning discovered equally with them, they having been themselves manifested that He might become known as God.”

In Tertullian’s view, this knowledge of the true God never disappeared, even during the long period of idolatry. He claims that the majority of the human race, despite not knowing anything about Moses, still knew “the God of Moses” and “still spoke of Him separately by His own name as God, and the God of gods.” Insisting again that “God has for His witnesses this whole being of ours, and this universe wherein we dwell,” Tertullian refutes the Marcionite idea that the true God would ever conceal himself: “Never shall God be hidden, never shall God be wanting.” The Marcionites respond that on this view the Creator is known by things foreign to him, whereas the true God should be known by things inherent to him. Tertullian replies that “no one is proved to exist to whom nothing is proved to belong. For as the Creator is shown to be God, God without any doubt, from the fact that all things are His, and nothing is strange to Him; so the rival god is seen to be no god, from the circumstance that nothing is his, and all things are therefore strange to him.” The fact that things manifest their eternal Creator is central to Tertullian’s argument against Marcion on behalf of the God of Israel.

As a final point, I should note that Tertullian, like Paul at the Areopagus, defends Christianity against its pagan persecutors by arguing that Christianity proclaims the one God who was already known, even if not properly worshiped, by the pagans. In his The Soul’s Testimony, Tertullian emphasizes that a careful reading of Greco-Roman poets and philosophers will show that Christians “have embraced nothing new or monstrous.” He goes on to complain, “We give offence by proclaiming that there is one God, to whom the name of God alone belongs, from whom all things come, and who is the Lord of the universe.” In his view, as we have seen, the human soul itself testifies to this living God by inevitably praising and praying to God. Tertullian does not thereby abandon the proclamation of Jesus Christ or the reality that God is Father, Son, and Holy Spirit. On the contrary, he is well known for developing the Latin trinitarian vocabulary of the church in his Against Praxeas. He states there that “this is the rule of faith which I profess; by it I testify that the Father, and the Son, and the Spirit are inseparable from each other. . . . The Father is one, and the Son one, and the Spirit one, and . . . they are distinct from each other.” As he goes on to make explicit, the Father, Son, and Holy Spirit are one God, distinct from each other by order of origin but not separate from each other.

Gregory of Nazianzus (ca. 329–ca. 390)

A theologian, poet, and pastor, Gregory of Nazianzus was born on an estate near Nazianzus in the region of Cappadocia (in modern-day Turkey). Gregory’s father, a convert from a Jewish-Christian sect called the Hypsistarii, served for almost a half century as bishop of Nazianzus. As a young man, Gregory studied grammar and rhetoric both in Caesarea and in Athens. His friend Basil was among his fellow students. Gregory also traveled to Caesarea in Palestine, where Origen’s library was preserved, and he spent a year in Alexandria. In 362, his father ordained him a priest. He subsequently became bishop of Sasima before becoming bishop of Nazianzus, and then, in 379 (though without full canonical status), archbishop of Constantinople, where he presided for a brief and turbulent period over the Council of Constantinople. Gregory is best known for his articulation and defense of the divinity of the Son and Holy Spirit in the unity of the Trinity.

In his Second Theological Oration, Gregory of Nazianzus demonstrates that God exists. The context of this demonstration is provided by the First Theological Oration. There Gregory inquires into what is required to discuss theology. Theology, he argues, is properly a subject only for those who are humbly undergoing purification of body and soul, who have taken the time to study, and who are filled with faith and recollected in spirit rather than curious or contentious. The theologian cannot hope to gain more than a small knowledge of the incomprehensible God, and so the theologian’s inquiry should proceed in a modest and careful manner.

Gregory begins his Second Oration by recalling these strictures, and by “dedicating our sermon to our sermon’s subjects, the Father, the Son, and the Holy Spirit, that the Father may approve, the Son aid, and the Holy Spirit inspire it—or rather that the single Godhead’s single radiance, by mysterious paradox one in its distinctions and distinct in its connectedness, may enlighten it.” Gregory presents himself as a pastor able to communicate the fruits of his contemplation to his flock. Comparing himself to Moses and Elijah who ascended Mount Sinai, he states that he takes shelter in the “rock” who is the incarnate Word. From within faith in Christ, he sees the divine grandeur “inherent in the created things he has brought forth and governs.” God is so glorious that we can only see God through signs; even the angels cannot gaze on him directly. We can understand that God exists, but we cannot grasp the infinite in our finite minds. Thus the contemplative pastor occupies essentially the same humble position as the flock to which he preaches.

At this juncture, Gregory offers his brief demonstration that God exists. He sums up his case as follows: “That God, the creative and sustaining cause of all, exists, sight and instinctive law inform us—sight, which lights upon things seen as nobly fixed in their courses, borne along in, so to say, motionless movement; instinctive law, which infers their author through the things seen in their orderliness.” Rationality itself, in other words, requires that the universe must have received order and being from a transcendent cause. Gregory compares the universe to a “beautifully elaborated lyre.” A lyre cannot simply spring into being. It must have a craftsman who made it. Likewise, a lyre cannot make harmonious music on its own; it must have someone who plays it. A lyre’s existence and order are obvious to all, but equally obvious is the fact that of itself it cannot account for its existence and order. Just as a lyre receives existence and order from a craftsman, so too does the universe. That God exists is clear, therefore, but we should not on this basis imagine that we have formed an adequate concept of the infinite God. The Creator God infinitely surpasses every concept.

In the process of demonstrating that God fills all things without being bodily, Gregory adds another demonstration that God exists. He shows that God is not in the order of moving or moved things, since each of them depends on another, and none of them can say that they move the whole. The key question has to do with where the whole cosmos gets its being, movement, and order from. This question can only be answered by positing a transcendent mover who is not finite or dependent and who gives the whole cosmos its being and movement.

As we have seen, Gregory introduces his demonstration that God exists with a prayer to the Father, Son, and Holy Spirit. His arguments are philosophical rather than theological in the sense of requiring faith or revelation for their cogency. He contextualizes his arguments, however, fully within the life of Christian faith, and he includes in his arguments veiled references to the Psalms, 1 Corinthians, Jeremiah, and other scriptural texts. Are his arguments therefore not really rational demonstrations at all? Gregory considers that they certainly are rational demonstrations, accessible to those who think clearly about the matter. As he says after giving his comparison of the universe to a lyre that is making harmonious music, “Anyone who refuses to progress this far in following instinctive proofs must be very wanting in judgment.”

In the same Oration, Gregory goes on to consider metaphorical and proper names of God, such as spirit, fire, light, love, wisdom, justice, mind, and so forth. He urges his listeners to think of the perfection identified by each word, and then to realize that God is infinitely greater. Those who fail to do the latter, he explains, end up worshiping creatures such as the sun, the moon, the elements, the cosmos, or notable men and women. Enslaved by their passions, many humans have even worshiped gods who share their degraded passions. Here Gregory provides one more demonstration of the existence of God. Namely, when we recognize the finite perfection in the created thing and ascend from this finite perfection to the infinite perfection of God, we discover “God through the beauty and order of things seen, using sight as a guide to what transcends sight.” Gregory explains that this is the proper work of our reason, which desires to find the maker of the sensible things we see around us. In reflecting on visible things, reason “leads us on through them to what transcends them, the very means of their continued existence.”

Gregory emphasizes that it is utterly unreasonable to suppose that the four elements, or any of the earthly or heavenly things that we can perceive, gave themselves being and ordered themselves. Speaking of a now-forgotten philosopher, Gregory states, “I commend the man, non-Christian though he was, who asked: ‘What set these elements in motion and leads their ceaseless, unimpeded flow?’” The proofs from motion, from efficient causality, from degrees of perfection, and from the teleological order of things are here in view. As Gregory asks, “Supposing chance created them, what gave them order?” What sustains them in being? Gregory answers that “it was surely their designer, who implants in all things reason whereby the universe is conducted and carried along.” He concludes that human reason, precisely because it was created by God, has the capacity to know God by reflecting on the things that we perceive by our senses. It is reason that leads “us up from things of sight to God.”

Gregory then considers the great men of Scripture—Jacob, Elijah, Manoah, Isaiah, Ezekiel, Paul, and so forth—who are depicted as in some way “seeing” God. None of them, he emphasizes, can thereby be said to have seen God’s essence despite their enhanced knowledge of God. He ponders on the nature of human rationality and bodily inclinations, and on the natures of the land animals and of fish and birds. How is it, he asks, that birds come to be able to sing so beautifully? How can birds, who lack rationality, construct such efficient nests, and bees such elaborate honeycombs? How can packs of cranes execute such complicated in-flight patterns? From whence comes the beauty of the various plants, and the scents, tastes, and medicinal qualities of their fruits, leaves, and roots? If we ponder on these things, we will be able to see how bountiful the Creator is. Similarly, he invites us to ponder how it is that the Earth is supported in the heavens, how the oceans were formed, how the weather is regulated, how the stars were made and put in order, and how the sun and moon observe their constant course. His point here is not yet another proof that God exists but rather proof of what he calls “the richness of God” in creating and sustaining the good things of life. But his reflections support his earlier proofs, since we are reminded how extraordinary the existence and order of finite things are.

He concludes the Second Oration where he began: with the holy Trinity. After a brief consideration of the nature of the angels, who sing God’s praise, he states, “If our hymn [the Second Oration] has been worthy of its theme, it is the grace of the Trinity, of the Godhead one and three; if desire remains incompletely satisfied, that way too my argument can claim success.” Reason can demonstrate that neither we ourselves nor anything that we see around us can be accounted for without appeal to the transcendent God. But in so doing, reason has to admit its own limitations as well, and thus the very success of our reasoning leads us to worship and praise the inexhaustible mystery of the Father, Son, and Holy Spirit—one God, who has revealed himself to us in Jesus Christ. Reason’s limitations do not prevent it from demonstrating that God exists, but the inexhaustibly wonderful depths of God go far beyond what reason apprehends.

Gregory’s poetry also sheds light on the paths that he finds fruitful for demonstrating God’s existence, although the genre of poetry does not allow for fully spelled-out demonstrations. For example, in his poem “Concerning the World,” he asks with respect to the combinations of things in the world (such as matter and form), “If things were commingled, how did they get to be mingled? Who mingled them if not God?” If God is the “mingler,” then surely, says Gregory, God is the Creator. He goes on to argue this more determinatively by employing the analogy of human craftsmanship. Since we can see that the universe shows clear signs of being ordered like a craft, we should accept that there must be a craftsman. Gregory states, “A potter . . . puts form to his clay when he turns the wheel, a goldsmith gives it to gold, and a sculptor to stones. Credit God with more than you do our mind, O lover of no origins.” The fact that Gregory is addressing a “lover of no origins” indicates the difficulty that some of his educated contemporaries had in accepting the idea of a Creator God.

Similarly, he makes use of the evidences of a craftsman’s work in his poem “On Providence.” As he points out, there is too much evidence of the work of a governing Mind to suppose that the universe had no Creator. Against those who appeal simply to chance to explain order in the universe, he responds forcefully, “For it’s not by chance, the nature of such and so great a world, wherewith nothing comparable can be thought—do not credit so much to chancy reasons. Who ever saw a house not built by hands? Who’s seen a self-built ship, or a fleet chariot? a shield and a helmet?” He adds that it is not only the order of nonrational things that bears witness to a Creator God. The same witness is borne by the preservation of this order over time. The vast universe could hardly have retained its order were there no provident God. As Gregory puts it, “Nor would so much have lasted through time, minus a ruler; and the choir would have stopped, I’d say, without its conductor.” He is aware that these are not formal demonstrations of the existence of God, but they show how strong he thinks the evidence is, given the existence, order, and preservation of the vast universe of nonrational things.

Again, in another poem also titled “On Providence,” Gregory responds to atheists—those who deny a creative cause of the universe—as well as to polytheists. Against atheists, Gregory states, “Away with them, those who deny what is divine, and ascribe no cause to the good, ineffable composition of the universe, whether in creation or in the upkeep of all things.” The goodness, composition, and preservation of the universe require a transcendent cause. Gregory goes on to say that “the deniers of Providence,” seemingly out of fear of being redeemed by God, “either assign the universe to an erratic ‘swoosh!’ or else credit it to the movement of the stars.” The second option particularly interests him. Could it be that a part of the universe is the source of the movement of the whole? In answer, Gregory suggests that stars cannot be unmoved movers in the strict sense, and so they too require a source of motion as well as a source of order and preservation. He inquires in this regard, “And how, and whence, and by whom are these [stars] moved? If it’s by God, how can that be first, which God turns round? . . . But, if it’s not from God, how is it to stand up out of anarchy, or again, from battling with one stronger?” Poetically, then, Gregory repeatedly calls attention to the fact that contingent things cannot be the fundamental source of their movement, order, and preservation.

Although it is not a demonstration of God’s existence, it should also be mentioned that Gregory has a keen existential sense of the contingency and obscurity of our lives. Many of the problems that lead us today to ask urgently whether God exists were well known to Gregory. For instance, in a poem titled “Conversation with the World,” he probes into the mystery of the earth’s motion, in light of which humans seem mere insects: “I’ve an issue, world, to bring up with you. Who are you, and whence, teach first, and whither do you roll? And how do you revolve me, like a wheel that carries an ant?” The same comparison of the human being to an insignificant ant comes in his poem “On the Precariousness of Human Nature,” where he speaks plaintively to the world, “Dear world, though not so very dear, why do you bear down on me, who trudge wheezing like a tiny ant distressed at his sore burden?” In this poem, he confesses that God formed the world and that the world itself proclaims its Creator, and yet he remains existentially distressed. Likewise, in his poem “On Human Nature,” he meditates closely on the fast-moving passage of time and on his ever-changing life, in which he can find no security. Describing himself as sitting in a beautiful shady grove on a sunny day, with a cool brook and the happy sound of birds and locusts, he nonetheless expresses discontent: “Who was I? Who am I? What shall I be? I don’t know clearly. Nor can I find one better stocked with wisdom. But, as through thick fog, I wander every which way, with nothing, not a dream, of the things I long for.” This existential lack and longing prepares the way, as it does for many people, for reflection on God, especially since Gregory is not expressing agnostic doubt but rather misery at his as-yet unclear and always changing existential and spiritual condition.

Augustine (354–430)

Augustine, born in Thagaste in North Africa to a pagan father and Christian mother, studied and taught rhetoric at Carthage before pursuing a career as a professional rhetorician in Rome and Milan. During his twenties, he was a Manichean, holding that earthly life was part of a cosmic drama between spirit (good) and matter (evil). His views on evil were changed by his reading of Neoplatonic philosophy, and his views on Christianity were altered by Ambrose’s allegorical interpretation of the Old Testament. After converting to Christianity at age thirty-three, Augustine soon became bishop of Hippo and spent the rest of his life preaching and writing in an effort to understand more deeply the realities of faith. In addition to his polemical works against the Manicheans, Donatists, and Pelagians, he preached on the Psalms, the Gospel of John, and other biblical texts, and he wrote an account of his own life (Confessions), of human history (City of God), and of contemplative ascent to the Trinity (De Trinitate).

In Augustine’s Confessions, he is concerned with the question that preoccupied him for the first fifteen years of his adult life and indeed, in another way, for the remainder of his life: How can we come to know the true God? He begins the book with a famous description of human desire for God as constitutive of our nature. Even if we do not yet know God, we desire him: “You stir man to take pleasure in praising you, because you have made us for yourself, and our heart is restless until it rests in you.” By seeking God, we come to find him and praise him. Indeed, God himself causes us to know him by giving us the gift of faith in Christ Jesus, through the mediation of human preachers. Augustine here undertakes a biblically rooted discussion of the divine attributes, drawing on Genesis, the Psalms, Romans, and Jeremiah.

In these opening pages of the Confessions, Augustine is primarily interested in how our wills come to seek God and cling to him. As he says, “Who will enable me to find rest in you? Who will grant me that you come to my heart and intoxicate it, so that I forget my evils and embrace my one and only good, yourself?” We have turned away from God by sin—both our own and that of our first parents. We need God to restore us by awakening us to his saving love in Christ and by enabling us to embrace Christ’s love. Augustine frequently interjects comments to God such as, “In you are the constant causes of inconstant things. All mutable things have in you their immutable origins. In you all irrational and temporal things have the everlasting causes of their life.” These comments illustrate the contingency and dependence of creatures, and creatures’ inability to be their own cause.

When Augustine tells the story of his youth, we learn how he became entranced by Cicero’s Hortensius, with its advice to pursue wisdom; how the Scriptures seemed insipid in comparison with Cicero; and how he joined the Manichees due to his inability otherwise to account for evil in the world. During this period, he believed in an ethereal but nonetheless bodily God. He also dabbled in astrology. In these chapters, Augustine intersperses very brief fragments of demonstrations of God’s existence. For example, he states, “For wherever the human soul turns itself, other than to you, it is fixed in sorrows, even if it is fixed upon beautiful things external to you and external to itself, which would nevertheless be nothing if they did not have their being from you. . . . Let these transient things be the ground on which my soul praises you (Ps. 145:2), ‘God creator of all.’”

The demonstration of God’s existence that Augustine eventually gives does not come from reflection on sensible things, although as we have seen from his fragmentary remarks, he is well aware of such demonstrations and values them. When he gives his full-fledged demonstration, he takes it from the Neoplatonists, since it was by reading their books that he finally overcame his erroneous idea that God possesses magnitude. As he says, “By the Platonic books I was admonished to return into myself.” They directed him to reflect on his soul and its acts. When he did so, he recognized that his soul’s intellectual light was guided by an immutable, eternal light. When we make judgments of truth, we are perceiving the presence of eternal and unchanging Being, Truth, and Goodness. An example of this is mathematical truth: no matter whether there were human minds to know it, the three angles of a triangle would always equal 180 degrees. Our minds here encounter eternal Truth, unchanging and without magnitude, that does not depend in any way on the existence of finite, changing things. Augustine states, “The person who knows the truth knows it [Truth], and he who knows it knows eternity.” The eternal light of Truth exists in an infinite mode, unrestricted by any limitation, and it is Good. When with awe and love we encounter the eternal light, we encounter the source of everything changing and finite. Augustine therefore proclaims, “Eternal truth and true love and beloved eternity: you are my God. . . . When I first came to know you, you raised me up to make me see that what I saw is Being, and that I who saw am not yet Being. And you gave a shock to the weakness of my sight by the strong radiance of your rays, and I trembled with love and awe.” Since we are finite and changing, we are radically different from what we here encounter, and yet by entering into our souls we can ascend upward and touch the eternal source of all that is. By contemplating the Eternal, furthermore, our souls become likened to the Eternal.

Has Augustine thereby left behind Paul’s claim that God “has been clearly perceived in the things that have been made” (Rom. 1:20)? On the contrary, the soul itself, with its ability to recognize the eternal source in which it participates, is one of “the things that have been made.” Starting with the soul and its ascent to the eternal source of its intellectual light, he arrives at the fact that God exists (and is eternal, infinite, immaterial, and so forth). This procedure then enables him, in a downward movement, to reflect on finite, contingent things. He states, “I considered the other things below you, and I saw that neither can they be said absolutely to be or absolutely not to be. They are because they come from you. But they are not because they are not what you are.” In other words, they have finite, contingent, participated existence. They are good insofar as they exist, but they are not the infinite good. This insight that insofar as something exists, it is good, enables Augustine to account for evil as a corruption, a lack or privation in the being of a finite thing (including in the being of the human will). Despite the presence of such corruption in the cosmos, nonetheless Augustine can see that corrupt parts do not destroy the goodness of the whole.

In Neoplatonic fashion, Augustine also asks himself why he considers some bodies to be beautiful and others not. He recognizes that in making judgments about beauty, he makes judgments of truth and thereby encounters the light of eternal Truth transcending his mutable mind. His mind rises from the perception of external bodies, to the internal reception of sense impressions, to the power of reasoning based on sense impressions, to the power of reasoning simply in itself, to the unchangeable light of Truth that governs the power of reasoning, to the knowledge and love of the unchangeable and Eternal. Here Augustine encounters God: “So in the flash of a trembling glance it attained to that which is. At that moment I saw your ‘invisible nature understood through the things which are made’ (Rom. 1:20).”

According to Augustine, a central problem with this intellectual demonstration is that our minds cannot focus for long on God without his help. Not only are our minds weak, but also, and more important, so are our wills. We need healing from our preference for sensible things over God. We need to learn Christ’s humility and charity in order to be cured from our absurd pride and recognize our utter dependence on the Triune God. Yet the rational demonstration of God’s existence remains an important stage in Augustine’s path to faith in God.

Elsewhere, in numerous places in his vast corpus, Augustine makes reference to ways in which we can come to know that God exists. In “Tractate 106 on the Gospel of John,” for instance, Augustine observes that God was in some way known throughout the world even before the spread of faith in Christ. This was so not only because God created humans to know him but also because God is so wondrously knowable. Augustine comments, “For such is the energy of the true Godhead, that it cannot be altogether and utterly hidden from any rational creature, so long as it makes use of its reason.” He thinks that despite the prevalence of sin, almost all humans prior to Christ recognized a supreme God who is the maker of the world, even though these same humans worshiped gods along with this God.

In On Christian Doctrine, Augustine remarks that “when the one God of gods is thought of, even by those who recognize, invoke, and worship other gods either in Heaven or on earth, He is thought of in such a way that the thought seeks to attain something than which there is nothing better or more sublime.” God is known as that “than which there is nothing better.” Admittedly, this leads some people to worship the sky, the world, some luminous or infinite mass, or anthropomorphic gods. When we reflect rationally on God, however, we are able to perceive that God is elevated “above all things mutable, either visible and corporal or intelligible and spiritual,” because all people recognize that there cannot be anything greater than God. Augustine points out that “since all those who think of God think of something living, only they can think of Him without absurdity who think of Him as life itself.” Furthermore, since a foolish life is of less value than a wise life, it is necessary to conceive of God as not only infinite life but infinite wisdom. Indeed, this conception itself constitutes a demonstration of God, since the truth that we perceive is immutable and compels assent. To cling steadfastly to this truth, however, requires faith in Christ.

In City of God, Augustine has high praise for the Platonists’ knowledge of God. Augustine considers that either Plato’s travels enabled him to come into contact with the books of Genesis and Exodus, or else Plato’s superb insights simply confirm Paul’s statement in Romans 1:20 that “ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made.” Augustine describes the Platonists as “philosophers who have conceived of God, the supreme and true God, as the author of all created things, the light of knowledge, the Final Good of all activity, and who have recognized him as being for us the origin of existence, the truth of doctrine and the blessedness of life.” Yet Plato and his successors still worshiped multiple gods.

In his First Catechetical Instruction (De catechizandis rudibus), written to explain the steps of Christian catechesis, Augustine refers to the demonstration of God’s existence from the order of the universe. He states that catechumens should be led away from focus on earthly things and toward “the contemplation of the skill or design of the contriver,” so that they might “soar upward to the admiration and praise of the all-creating God.” The demonstration of God’s existence played an important role in his own coming to faith, and so it is not surprising that it has a place in his catechetical handbook.

John of Damascus (ca. 676–ca. 749)

Born into a prominent Christian family in Damascus under the Umayyad caliphate less than a half century after Muhammad died (632), John of Damascus, whose given name was Mansur ibn Sarjun, received a classical Hellenistic education. Like his father and grandfather, John served for a time in the fiscal administration of Damascus. Upon becoming a monk in Jerusalem, he took the name John. He seems to have been ordained a priest, perhaps serving the Church of the Anastasis, and to have been close to the patriarch of Jerusalem, John V, and to the metropolitan of Damascus, Peter II. In addition to liturgical poetry and homilies, John wrote against iconoclasm and monothelitism. He is best known for his three-part work The Fount of Knowledge, which helped to refine and define the content of Christian faith.

The first part of The Fount of Knowledge, titled “The Philosophical Chapters,” is devoted entirely to the study of philosophy, which he defines in various ways: “a knowledge of the nature of things which have being”; “knowledge of both divine and human things”; a knowledge of death; “the making of one’s self like God” through wisdom; and “the love of God.” This first part largely consists in a summary of Aristotelian philosophical terms. The second part examines a large number of faulty philosophical schools, Jewish sects, and Christian heresies (among which he numbers Islam). The third part bears the title “An Exact Exposition of the Orthodox Faith,” often published on its own as The Orthodox Faith. In this part, he attempts to summarize the whole of what Christians believe.

In his preface to The Fount of Knowledge, John first speaks of the purification that is necessary for the theologian who dares to speak of the highest realities. He identifies Moses as the model of the theologian. Moses received a divine theophany at the burning bush, and Moses was the first to learn the divine name “He who is.” Yet even Moses was hesitant to speak publicly and asked that Aaron be appointed in his stead. John would have done the same, but his bishop had commanded him to write The Fount of Knowledge.

John then turns to the structure of his book. He explains his lengthy first part on philosophy as follows: “First of all I shall set forth the best contributions of the philosophers of the Greeks, because whatever there is of good has been given to men from above by God, since ‘every best gift and every perfect gift is from above, coming down from the Father of lights’ [James 1:17].” Like a bee, he will gather what is good from the Greek philosophers, who as pagans are “our enemies,” but who nonetheless teach much truth, intermixed though it is with error. He notes that after treating Greek philosophy and the history of heresy, he will exposit in his third part “that truth which . . . has been embellished and adorned by the sayings of the divinely inspired prophets, the divinely taught fishermen, and the God-bearing shepherds and teachers—that truth, the glory of which flashes out from within to brighten with its radiance, when they encounter it, them that are duly purified and rid of speculations.”

John’s philosophical exposition in the first part of The Fount of Knowledge does not contain a demonstration of the existence of God. Instead, he devotes his labor to defining key philosophical terms, such as species, genus, substance, accident, predication, form, and habit, as well as to delineating the methods of dialectic and logic and to describing certain aspects of natural philosophy, such as what thunder and lightning are. In his third part, The Orthodox Faith, one might not expect to find a philosophical demonstration of God’s existence, because this third part is supposed to be devoted to the teachings of the prophets, apostles, and bishops. Yet it is here that we find demonstrations of God’s existence.

John begins this third part by noting that after the fall, not only humans but even angels need revelation in order to know God intimately. Nonetheless, God has given all humans a way to know him: “through nature the knowledge of the existence of God has been revealed by Him to all men.” Thus we do not depend solely on revelation for all our knowledge of God; human reason can know that God exists. In a brief description of the ways that God has made himself known, John first states that the harmony and order of the creation show God’s majesty. He then discusses the Mosaic law and the prophets and the revelation given through Jesus Christ. Citing Psalm 14:1, “The fool says in his heart, ‘There is no God,’” John comments that because some people deny that God exists, when the gospel was first announced the apostles did many miraculous signs and taught powerfully about God so as to “draw up those people alive in the net of their miracles from the depths of the ignorance of God to the light of his knowledge.” Now that miracles are no longer as frequent and the light of teaching no longer so pure, John invokes the aid of the Father, Son, and Holy Spirit with the goal of repeating the apostles’ teaching about God. He does so first by means of rational demonstrations of God’s existence.

The first demonstration is from movement and contingency. John begins by observing that “all things are either created or uncreated.” The created things are by definition changeable, having originated through a change. He notes that “all beings that fall within our experience, including even the angels, are subject to change and alteration and to being moved in various ways.” Material beings change by generation and corruption, or with respect to quantity, quality, or place. Immaterial beings such as angels (or demons) or our souls change “by free choice, progressing in good or receding, exerting themselves or slackening.” Things that change, John asserts, must have been created or caused by something else. Were there no uncreated Creator, then there would be an infinite explanatory regress with regard to the being of these changeable things. There must, then, be an uncreated and utterly unchangeable source of all things.

The second demonstration is from “the very harmony of creation, its preservation and governing.” If there were no God to put the universe together and to preserve it, then the universe would be unable to hold together or to continue in such an orderly fashion. The four elements are too contrary to combine together and to remain combined on their own. As the “architect,” the one who combined, arranged, set in motion, and governs the elements must be the all-powerful Creator, the one who brings all things “into being.” The only other possible supposition would be that the elements arose from spontaneous generation. Even if one were to grant that spontaneous generation brought things into existence and gave things their arrangement—which John grants only by way of supposition—one would still have to account for the preservation of this order rather than its dissolution. If only spontaneous chance were involved, then dissolution would quickly follow. Therefore, chance cannot explain the existence of the universe. What explains the existence of the universe is none other than the existence of God. John makes clear that we can know that God exists but not what God’s nature is. We cannot form an adequate concept of divine existence.

It is worth noting that in the course of his ensuing discussion of the divine attributes, John critiques the Stoic view that there are immaterial bodies. John observes that even if there were such a thing as immaterial bodies, an immaterial body would be movable, since bodies are subject to motion. If so, then such a body could not be unmoved but would require a mover or efficient cause. To avoid infinite explanatory regress, and thereby to account for the existence of immaterial bodies, we would have to arrive at an unmoved mover. An unmoved mover would not be subject to motion, and thus would not be spatially contained as bodies inevitably are. John draws the conclusion that this unmoved mover, pure spirit, is God, “and by His immovability He moves all things.”

At the end of his section on the divine existence and attributes, John states, “It has been sufficiently demonstrated that God exists and that His essence is incomprehensible. Furthermore, those who believe in sacred Scripture have no doubt that He is one and not several.” He proceeds to identify biblical texts that support God’s unity and to argue for it philosophically as well. The key point for our purposes is that God’s revelation in the Law and the Prophets and in Jesus Christ does not change what we can and should learn from God’s revelation in creation. For John, the rational demonstrations of God’s existence continue to be helpful and instructive for believers.

Anselm of Canterbury (1033–1109)

Born in Aosta in the western Alps, southernmost outpost of the kingdom of Burgundy, Anselm became a clerk at the church in Aosta before quarreling with his father and crossing the Alps into Burgundy in his early twenties. He arrived at the Benedictine Abbey of Bec in western Normandy around 1059. Under the tutelage of Lanfranc, he acquired a superb education. When Lanfranc left to become archbishop of Canterbury, Anselm became abbot of Bec. In 1093 he succeeded Lanfranc as archbishop of Canterbury. For the remainder of his life, he struggled with King William II and King Henry I over investiture. In addition to his prayers and meditations, he authored numerous works on such subjects as God, original sin, predestination and free will, the virgin birth, and why God became human.

Anselm begins his Monologion by explaining that his fellow Benedictine monks have requested him to write down his view of “how one ought to meditate on the divine essence, and about certain other things related to such a meditation.” One might imagine that such a meditation would be, for monks, a spiritual experience involving prayer, lectio divina, and ascetic mortification. But in fact Anselm observes that his fellow monks “prescribed the following form for me in writing this meditation: absolutely nothing in it would be established by the authority of Scripture; rather, whatever the conclusion of each individual investigation might assert, the necessity of reason would concisely prove, and the clarity of truth would manifestly show, that it is the case.” At first glance, given our modern idea of what medieval Benedictine monks should be, this seems quite surprising. Why did they bracket the authority of Scripture and instead seek proofs that bear “the necessity of reason”?

It should be kept in mind that the monastery at Bec was no rustic retreat center for those who wished to abandon the use of reason and enter into a purely ascetical-mystical life. By the time that Anselm arrived, the monastery at Bec had become a major center of learning, with its own school that drew students from across Europe. The monastery’s abbot, Lanfranc, was “famous as a teacher of the liberal arts” (grammar, logic, dialectic, and rhetoric) and a brilliant theologian. He was the author of a Commentary on the Epistles of St. Paul as well as a major work that challenged Berengar’s theory of the Eucharist and instead advocated an “account of the Eucharistic change” rooted in the “Aristotelian doctrine of substance.”

Even so, when Anselm speaks of a “meditation,” he does not have in mind the work of a modern professor or teacher, whose research can be and often is abstracted from the spiritual life. Richard Southern notes that for Anselm, “Cogitatio can be, and often is, concerned with worldly things and even with corrupt aims, whereas meditatio is solely concerned with pure reflection on the essences of things, whether knowable from the empirical data of the senses or from the intuitive knowledge of the mind.” In the Monologion, Anselm is engaged in meditatio, and this task belongs to the virtuous movement of the mind to God. Southern comments, “Meditation may be described as embryonic contemplation, preparatory to the perfect contemplation of Heaven.” Minds that are purified can prepare themselves for heavenly contemplation, in eternal life, by means of meditation. This was Anselm’s purpose.

Why, then, do Anselm’s fellow monks, and clearly also Anselm himself, desire that this meditation proceed “by means of a plain style, unsophisticated arguments, and straightforward disputation”? Why do they insist on demonstrative knowledge about God obtained solely on the basis of reason and without the aid of revelation in any way? The answer, I think, consists in their high view of human reason. Created by God as a finite participation in the infinite divine light (the infinite divine wisdom), human reason surely cannot have been left without a way to know God.

In the Monologion, Anselm argues that human reason receives from God the power to know him. Thus, Anselm’s first chapter begins with the claim that if anyone does not already know the one, supreme, self-sufficient, happy, omnipotent, good Creator God, then “I think that he could at least convince himself of most of these things by reason alone, if he is even moderately intelligent.” Anselm thinks that many demonstrative paths to knowledge of the one God are available, but he settles on the one that starts from the fact that we desire good things rather than bad things for ourselves. We know a number of good things that we desire. But when many things, otherwise widely diverse, share “goodness,” what is it that they are sharing? This is an especially pressing question when we recognize that things possess differing degrees of goodness. If it is possible to have more or less goodness, but if “goodness” itself is the same, then “goodness” can be distinguished from particular good things. Anselm concludes that all good things are good by sharing in goodness. On this basis, he seeks to identify what goodness is. It must be, as he says, a great good. It must also be good of itself, rather than being good by sharing in the goodness of another. In short, it must be self-subsistent goodness, having its goodness through itself rather than requiring an external source of goodness. Such a thing must be “supremely good and supremely great—in other words, supreme among all existing things.”

Anselm then points out that existence is like goodness. We know of many things that share in existence. None of these things is the source of its own existence. Yet they definitely share in existence as something common among them. Existence can therefore be distinguished from particular existents. Indeed, for particular existents to exist, they must receive or share in existence from an external source, since none of them is existence itself. As Anselm puts it, “whatever exists, exists through something”: existing things require a source of existence. Since existence is the same in all things that exist, they must share in one common source. This common source cannot itself depend on another thing for existence, since in that case it would be simply one more thing among all the things that share in existence. Instead, it must exist of itself; it must be self-subsistent existence. As such, it must exist in a supreme fashion, and therefore must be the greatest of all things (the supreme good). In this way, Anselm thinks that he has proven the existence of God, the transcendent source of the existence and goodness of all things.

Anselm also provides a short proof based on degrees of perfection. Some things are better than others. Anselm envisions a set of natures—wood, horse, human, and so forth—of which we can rightly say that there are degrees of perfection. Wood cannot move on its own; a horse can. The question is whether this chain of ascending natures stretches infinitely upward, so that there is an infinite array of ever-greater natures in existence. This strikes Anselm as an absurd claim; it is not possible that the chain of ever-greater existing natures goes on infinitely. Therefore, some nature (or natures) must be supreme, so as to be inferior to no other nature. Anselm raises the possibility that there might be more than one nature that is inferior to no other nature. This would entail that the supreme natures be equals. But, as Anselm says, being equals means possessing something that is the same. If this same thing is their nature, then they are in fact the same nature. If the same thing that they share and that makes them inferior to no other nature is something less than their nature, then they are not supreme; they would be great through something else and therefore that something else would in fact be greater than they. It follows that there is one superior nature that is inferior to none. This superior nature must exist in the greatest way and be the greatest good. This can only be the case if the superior nature is self-subsistent existence, and all other things exist by sharing in it; such self-subsistent existence must be the supreme good. Again, Anselm concludes that he has demonstrated the existence of the transcendent God.

Anselm goes on to show that this supreme nature “is not included in any common classification of substances, since every other nature is excluded from having an essence in common with him.” Further, God exists “in an unqualified sense”; he is sheer, perfect existence, transcending any limitations such as temporality or change. Anselm recognizes, therefore, that his demonstration of the existence of the supreme nature, of self-subsistent existence and goodness, does not mean that we can understand or categorize this supreme nature as though it were one nature among other natures. God is utterly ineffable.

Anselm draws out various other divine attributes on the basis of God’s self-subsistent existence. In the Monologion, he also has much to say about the Trinity via the notion of God’s Word or utterance. He also treats the human soul, which he takes to be the image of the Triune God (following Augustine’s triad of remembering, understanding, and loving). He concludes that we are made to know and love God and, indeed, that “it is he alone whom all other natures should lovingly worship and worshipfully love with all their power, from whom alone they ought to hope for good things, to whom alone they ought to flee from troubles, to whom alone they ought to pray for anything at all.”

In the prologue of his Proslogion, Anselm tells us that after publishing the Monologion, he began to wonder whether there is a simpler demonstration of God’s existence, one that would comprise not a set of arguments but a single argument. As he recalls, he was looking for “a single argument that needed nothing but itself alone for proof, that would by itself be enough to show that God really exists; that he is the supreme good, who depends on nothing else, but on whom all things depend for their being and for their well-being; and whatever we believe about the divine nature.” Eventually, he recalls, he gave up searching for such a proof. But then, all of a sudden, an idea came to him regarding how this simple proof might be constructed. For Anselm, the mind is made for knowing God, and so it should be no surprise that such a proof might be possible.

Before presenting his simple proof in the Proslogion, Anselm begins with the aspirations of prayer: “Come now, O Lord my God. Teach my heart where and how to seek you, where and how to find you.” He depicts the existential position of humans who wish to know God more directly and concretely. We cannot perceive God by our senses in the way that we perceive creatures, because God is ineffable. Here Anselm speaks as someone who, despite having been “remade” in Christ, is in many ways still a “distant exile” from God. Comparing himself to Adam before the fall, Anselm states, “He was satisfied to the full; we sigh with hunger. He had everything he needed; we go begging.” From this perspective, Anselm pleads with God to provide a path of intellectual knowledge of God. As he says, “Lord, if you are not here, where shall I seek you, since you are absent? But if you are everywhere, why do I not see you, since you are present?” He bewails his fallen condition: “From our homeland into exile; from the vision of God into our blindness; from the joy of immortality into the bitterness and terror of death.”

In describing his neediness, Anselm cites eleven psalms, as well as Jeremiah and Job. His position is one of lamentation; to amplify this stance, he never explicitly refers to Christ or salvation but instead speaks of himself as “wretched,” “woeful,” lacking joy, “bitter” in heart, and “bent double” so as to be unable to raise his eyes to God. In praising God, he begs to be renewed: “I acknowledge, Lord, and I thank you, that you have created in me this image of you so that I may remember you, think of you, and love you. Yet this image is so eroded by my vices, so clouded by the smoke of my sins, that it cannot do what it was created to do unless you renew and refashion it.” Since he goes on to describe himself as a believer, his complaints about his fallen condition may simply be depicting the renewal and refashioning that we all still need, even when Christ has revealed God to us. But it is clear that he leaves Christ out of this section on purpose so as to maximize the sense of neediness and of the real difficulty we have in knowing God. He explains that he is a believer in God’s existence and that he is seeking understanding of what he believes about God.

He then proceeds to his proof. First, he states that he believes not only that God exists but also that God is “something than which nothing greater can be thought.” The question, then, is whether it is possible for “something than which nothing greater can be thought” not to exist. Certainly the fool can say, “There is no God” (Pss. 14:1; 53:1). But can even a fool say that “something than which nothing greater can be thought” does not exist? In answering this question, Anselm first notes the distinction between existing in one’s understanding and existing in reality. Certainly, even the fool would grant that “something than which nothing greater can be thought” exists in the fool’s understanding, insofar as the fool can understand what is meant by this phrase. But many things exist in one’s understanding that do not exist in reality. Anselm gives the example of a painter’s idea for a new painting. Before the painter has completed his painting, his painting exists in his mind but not in reality. When the painter completes his painting, then and only then does the painting also exist in reality.

Does this situation hold also for “something than which nothing greater can be thought”? The content of this phrase (like the idea for a painting) can certainly exist in our mind, since we can understand what it means. But when we do understand what the phrase means, we realize a strange paradox: if the content of this phrase only exists in our mind, then it is in fact not yet “something than which nothing greater can exist.” This is so because it is greater to exist both in the mind and in reality than to exist solely in the mind. In a nutshell, “If that than which a greater cannot be thought exists only in the understanding, then that than which a greater cannot be thought is that than which a greater can be thought”—an obvious contradiction in terms. Once we apprehend “something than which nothing greater can exist,” therefore, we have necessarily apprehended something that exists both in the mind and in reality. When we have truly understood it and it exists in our mind, we realize that it must exist in reality too. God literally “cannot be thought not to exist,” as the fool should be able to see once he understands the term “God” (and thereby ceases to be a fool).

Having completed his proof, Anselm gives thanks to God for the illumination that God has made possible for him through the light of reason. Reason’s ability to know God’s existence ensures, Anselm says, “that even if I did not want to believe that you exist, I could not fail to understand that you exist.” God made our minds to know him; inscribed in our very rationality is the reality that even to think that God does not exist is a contradiction in terms, a sheer absurdity, once one realizes what “God” means. On this basis, Anselm in the Proslogion proceeds to spell out the attributes of God, which follow from the fact that God is “that than which nothing greater can exist.” These attributes include intelligence, omnipotence, mercy, justice, and impassibility. Anselm shows that God is “life itself, light, wisdom, goodness, eternal happiness and happy eternity” and that God “exists always and everywhere.” At the same time, Anselm expresses his longing to see more, to know more of God. Beyond what he has seen, he sees “darkness”—not because God is dark but because Anselm’s mind “is both darkened in itself and dazzled by you [God]. Indeed it is both obscured by its own littleness and overwhelmed by your vastness.” Anselm proclaims, “How great is that light, for from it flashes every truth that enlightens the rational mind! . . . What purity, what simplicity, what certainty and splendor are there. Truly it is more than any creature can understand.” It is because God’s “unapproachable light” (1 Tim. 6:16) is so dazzling that we cannot see God, even though God is perfectly present everywhere.

Given God’s dazzling light, Anselm returns to his original complaints about his inability to see God and finds that his proof only serves to make him more desirous of seeing God. His proof does not fill or satisfy his need for knowledge of God even though it is true knowledge of God. As Anselm puts it, “My soul hoped for satisfaction, and once again it is overwhelmed by need. I tried to eat my fill, but I hunger all the more.” He begs God for further strength so that he might strive again to know God more. Continuing his inquiry into God’s attributes, he discerns God’s simplicity, eternity, self-subsistent and unqualified existence, and goodness—the ineffable, glorious goodness of which we are promised a share as “heirs of God and fellow heirs with Christ” (Rom. 8:17). Anselm shows that God is so unimaginably good, and the communion of saints will be so joy-filled in rejoicing at God’s sharing of his goodness, that indeed the saints’ “whole heart, mind, and soul will be too small for the fullness of their joy.” After citing various biblical passages about the joy that is promised us (John 16:24; Matt. 25:21; 1 Cor. 2:9), Anselm concludes with a final prayer: “O Lord, I pray that I will know and love you that I might rejoice in you. And if I cannot do so fully in this life, I pray that I might grow day by day until my joy comes to fullness. Let the knowledge of you grow in me here, and there let it be full. Let your love grow in me here, and there let it be full.”

Anselm quickly received a reply “on behalf of the fool” from Gaunilo, a Benedictine monk of Marmoutiers. Gaunilo notes that he can think of numerous things that have no existence in reality. How can we be sure that what Anselm is describing exists in reality? How is it, asks Gaunilo, that “this being merely needs to be thought in order for the understanding to perceive with complete certainty that it undoubtedly exists?” Especially given that God is beyond our understanding, as opposed to something that we can perceive directly, it seems possible to conceive of God and yet to suppose that God does not exist. Gaunilo sums up Anselm’s argument as being about “that which is greater than everything else that can be thought” or about “something greater than everything else.” He grants that this idea “exists in my understanding,” but he does not grant that it necessarily follows that “this thing also exists in reality.” We must first establish that God actually exists, before we can know that our concept of God describes a truly existing reality. Here Gaunilo provides an example of what he means: he pictures himself learning of the greatest possible island and then, since such an island now exists in his mind, being compelled to suppose that the island thereby must also exist in reality, since otherwise it would not be the greatest possible island. His point is that the fool would still need to be shown that “the island truly exists.” Thus, according to Gaunilo, the fool’s position is a perfectly tenable and consistent one.

Anselm replies by pointing out that “that than which a greater cannot be thought cannot be thought of as beginning to exist.” If one is thinking of something for which there is even a possibility that it does not exist in reality, then one is quite simply not thinking of that than which nothing greater can be thought. The real question, then, is whether “that than which a greater cannot be thought can be thought at all.” Anything that can be conceived of as even possibly not existing in reality could be conceived of as existing in a greater way. Anselm repeats his point that “if that than which a greater cannot be thought exists only in the understanding, it is that than which a greater can be thought.” Therefore, something “than which a greater cannot be thought” is something that literally must exist, since its existence is necessary to its definition.

Anselm easily deals with Gaunilo’s example of the greatest possible island. This example does not capture the paradox that applies only to that than which a greater cannot be thought, namely, that it either exists in reality or else it does not meet the definition. We can conceive of an island (even the greatest possible one), and have it possibly not exist. But if we are attempting to conceive of that than which nothing greater can exist, even the mere possibility of it not existing in reality would already mean that it is not what we are looking for, that it does not fit the definition of absolute existence. If we think that it is possible that “that than which nothing greater can be thought” does not exist in reality, then we have not yet in fact been able to conceive what is requisite for “that than which nothing greater can be thought.” Existing in reality is implied necessarily in the very idea, in the very phrase. God is such that to conceive of him as possibly not existing is to fail to conceive of him properly. By contrast, every creature can be thought of as possibly not existing.

Anselm also points out that Gaunilo errs by describing Anselm’s argument as involving “that which is greater than everything else,” since Anselm in fact does not use this phrase. It is solely with regard to “that than which a greater cannot be thought” that Anselm denies that it can be thought possibly not to exist. Since its existence is absolute, it is not simply greater than everything else, but rather it has existence in such a way that even the possibility of actual nonexistence would mean that one would be thinking of something lesser. Anselm observes, “For whatever does not exist is capable of not existing, and whatever is capable of not existing can be thought not to exist. Now whatever can be thought not to exist, if it does exist, is not that than which a greater cannot be thought.” By contrast, something that is “greater than everything else” need not (theoretically speaking) have necessary existence in its very definition. Again, Anselm believes himself to have phrased the matter in such a way that the thing understood cannot even have a possibility of not existing in reality. We can conceive of “that than which a greater cannot be thought” via inference from “the things that have been made” (Rom. 1:20). When we conceive of this, we are conceiving of something that cannot even have a potency for nonexistence—that cannot even be thought as not existing in reality.

Thomas Aquinas (ca. 1225–74)

Born into a noble family at Roccasecca, near Aquino in southern Italy, Thomas Aquinas was sent at age five to be educated at the Benedictine Abbey of Monte Cassino. While studying at the University of Naples, Aquinas joined the relatively new Order of the Friars Preachers, despite the opposition of his family. He studied philosophy and theology under Albert the Great at Cologne, and he later taught at the University of Paris and helped to found a Dominican studium in Naples. Like Albert, Aquinas is well known for integrating Aristotle’s philosophy into the exposition of Christian theology, although both Albert and Aquinas were deeply influenced as well by the late-Platonic reception of Aristotle and also by Boethius and Avicenna, among others. Aquinas wrote or dictated numerous commentaries on books of Scripture and on the works of Aristotle, and he also commented on Peter Lombard’s Sentences, the Apostles’ Creed, the Divine Names of Pseudo-Dionysius, the Liber de causis (drawn from Proclus’s Elements of Theology), and Boethius’s De Trinitate. He compiled the Catena Aurea, which brings together the commentary of numerous Latin and Greek fathers on the four Gospels. In addition to his liturgical poetry and hymns, which are still in use, he is best known for his Summa theologiae and his Summa contra gentiles.

In On Being and Essence, one of his earliest writings, Aquinas distinguishes essence (beings) from esse (being). There are many kinds of beings, such as “man” or “a phoenix.” In the definition of these beings or essences, however, an actual act of being is not included. Aquinas observes in this regard that “every essence or quiddity [nature] can be understood without knowing anything about its being [esse, act of being]. I can know, for instance, what a man or a phoenix is and still be ignorant whether it has being in reality.” The definition of what a phoenix is (or, to take a contemporary example, what a passenger pigeon is) can be known even if there is no phoenix (or passenger pigeon) in existence. The nature of a thing thus differs from its act of being. Furthermore, a thing’s nature cannot cause its act of being. If it could, then “that thing would then be its own cause and it would bring itself into being, which is impossible.” The fact that something has the nature of a “man” does not mean that a man could give himself his act of being. We can think of a man without thinking of him actually existing, and so the act of being does not come from the nature. Aquinas states, “It follows that everything whose being is distinct from its nature must have being from another.”

On this basis, Aquinas shows that there must be a reality whose nature is its act of being (Pure Act), and there can only be one such reality. If there were more than one Pure Act, more than one reality whose nature is subsistent act-of-being, then we could differentiate one instance of this nature from another. But such differentiation is not possible for Pure Act, because the differentiation could only consist in the addition of a specific difference (as, for example, the genus “animal” is found in various species) or in the addition of matter (as, for example, the species “man” is found in different individual humans). In both of these cases, Pure Act would no longer be Pure Act: in the first case, “it would not be being alone but being with the addition of a form [the specific difference],” and in the second case, “it would not be subsistent, but material, being.”

Given that there could only be one Pure Act, does Pure Act really exist? Aquinas argues that it does, because everything whose nature is not “to be” must receive its act of being from another. An essentially ordered series of beings is one in which the beings exist at the same time, each causing the act of being in the next in the series. Although Aquinas does not explain it here, he states that going “on to infinity” in essentially ordered dependent causes is impossible—since all the causes would be intermediate, dependent ones, and so the causal series could never get started. As Aquinas says, “Because everything that exists through another is reduced to that which exists through itself as to its first cause, there must be a reality that is the cause of being for all other things, because it is pure being.” Fundamentally, Aquinas is arguing that no natural reality we experience is the cause of its own existence. Nothing that we see around us exists necessarily by reason of the kind of thing it is or the essence or nature it has. This invites us to probe further to find the cause of why things exist until we come to something that exists by its own nature or in which there is no real distinction of nature and existence.

Book 3 of the Summa contra gentiles contains an important brief reflection on human knowledge of God. In chapter 38, Aquinas denies that humans possess self-evident knowledge of God, but he affirms that “there is a common and confused knowledge of God which is found in practically all men.” What explains this “common and confused knowledge of God”? Aquinas reasons that “when men see that things in nature run according to a definite order, and that ordering does not occur without an orderer, they perceive in most cases that there is some orderer of the things that we see.” Aquinas grants that there is also a lot of disorder in the things of nature. His point is simply that the ordering toward a goal that we see in particular nonrational things is the kind of ordering that requires reason. (A modern thinker might appeal to the process of evolution, but this would not negate Aquinas’s point, since even evolution presupposes underlying order.) Yet such knowledge of God, of the one who is the ultimate source of order in nonrational things, is “confused” because, as Aquinas says, “who or what kind of being, or whether there is but one orderer of nature, is not yet grasped immediately in this general condition.” For this reason, people who have a confused knowledge of God drawn from the order that they find in nonrational things can end up worshiping a finite thing as God or worshiping multiple gods.

In book 1 of the Summa contra gentiles, Aquinas speaks of a “twofold truth of divine things,” namely, truth about God that can be known by the human mind’s own powers and truth about God that requires divine revelation in order to be known. With regard to truth about God that can be known by the mind’s powers, Aquinas seeks “demonstrative arguments.” He notes that as far as human reason is concerned, knowledge of God first requires the demonstration that God exists “as the necessary foundation of the whole work. For, if we do not demonstrate that God exists, all consideration of divine things is necessarily suppressed.”

Before turning to the arguments for the existence of God that he considers to be demonstrative, Aquinas reflects on Anselm’s Proslogion. Anselm’s argument begins, as Aquinas says, with the fact that “by the name God we understand something than which a greater cannot be thought.” Since it is greater to exist in reality than not to do so, “God” cannot exist solely in the intellect. Indeed, for Alselm, God is such that his nonexistence literally cannot be conceived, once one recognizes that God must be absolute existence, without any potency (by contrast to all creatures). To Anselm’s position, Aquinas adds two arguments that proceed in a similarly a priori fashion. The first is that we naturally desire God as our ultimate end, and so the existence of God is self-evident to us. The second is that we know truth through knowing God. God is at the root of all our knowledge because “the divine light is the principle of all intelligible knowledge; since the divine light is that in which intelligible illumination is found first and in its highest degree.” Here Aquinas compares the divine light with the light of the sun; our knowledge requires God just as much as our perception of things requires the sun. God grounds all truth, and so God’s existence must be self-evident to the reflective mind.

In response to these arguments for the self-evidence of the existence of God, Aquinas states that they exhibit “a failure to distinguish between that which is self-evident in an absolute sense and that which is self-evident in relation to us.” Absolutely speaking, God’s existence is self-evident because he is supremely intelligible. But in relation to our weak minds, God’s existence is not self-evident. Even among those who profess to worship “God,” there exists confusion about what this name entails. Furthermore, pace Anselm, even “granted that everyone should understand by the name God something than which a greater cannot be thought, it will still not be necessary that there exist in reality something than which a greater cannot be thought.” A conception in the mind exists in the mind, and one cannot thereby conclude that it exists in reality, no matter how compelling the logic is. Simply on the basis of a definition of God that shows that it would be a contradiction in terms for God not to exist, our weak minds cannot rule out the possibility that in reality God does not exist. Furthermore, although we have a natural desire for God because we desire happiness, in order for this natural desire to proceed, we need only know finite likenesses of God’s goodness. Likewise, we need not know God before we can know truth about things, even though it is certainly the case that “all our knowledge is caused in us through His influence.”

After dismantling these arguments for God’s existence, in chapter 13 of book 1 of the Summa contra gentiles Aquinas seeks to demonstrate God’s existence along broadly Aristotelian lines. Before proceeding, he raises two problems: first, Aristotle considered that in order to demonstrate whether a thing exists, we must begin with a definition of what the thing is, which is impossible in the case of God; second, Aristotle also held that we rely on our senses for our knowledge of the principles of demonstration, whereas God utterly transcends sensible things. Appealing to Romans 1:20, Aquinas explains that the demonstration of God’s existence works by moving from effects to cause. Admittedly, we do not know what God is, and we cannot define his “essence” in a concept. But since we are not seeking to demonstrate what God is, we do not need to employ a definition of God in our demonstration. Instead, we can use God’s sensible effects (finite things) as the middle term of a demonstration that God exists.

Aquinas then follows this path of demonstration. The first argument for God’s existence that he describes is from “motion” (or change), which is a movement from potency to actuality. The proof goes as follows: Since everything that is moved is moved by another, and we see that the sun (for example) is in motion, we must inquire into its mover(s). There are two options: either it has an infinite number of moved movers, or there is an unmoved mover. An infinite number of moved movers is impossible, and so an unmoved mover exists.

This proof, Aquinas observes, depends on two theses: “everything that is moved is moved by another,” and “in movers and things moved one cannot proceed to infinity.” With respect to the first thesis, however, why could not things simply move themselves? Aquinas explains that Aristotle answers this question in three ways, which we find in books 7–8 of Aristotle’s Physics. The first way begins with the fact that a self-mover must “have within itself the principle of its own motion” and must be moved by itself rather than by a part of itself. Yet a self-moving being must also have parts, or else it would not be movable. Aristotle then argues that such a being must be “primarily moved—that is, moved as a whole rather than having one part move while another part is at rest. But if it is “primarily moved,” then it depends on the motion of another and is not in fact a self-mover. Aquinas explains that “the force of Aristotle’s argument lies in this: if something moves itself primarily and through itself, rather than through its parts, that it is moved cannot depend on another. But the moving of the divisible itself, like its being, depends on its parts; it cannot therefore move itself primarily and through itself.” Thus, things that we might suppose to be self-movers, which are movable because they have parts, are actually moved by another.

The second way that Aristotle demonstrates the thesis “everything that is moved is moved by another” is by noting that all things that are moved either by accident, by external force, or by parts of themselves (such as the soul), are in fact not moved by themselves. Since “whatever is moved is moved through itself or by accident,” everything that is moved is moved by another.

The third and final way that Aristotle demonstrates this thesis is by arguing that “the same thing cannot be at once in act and in potency with respect to the same thing,” and “everything that is moved is, as such, in potency.” Since the thing moved is in potency for this motion, the thing moved cannot itself be the mover, because the mover must be in act with respect to the motion. It follows that “with respect to the same motion, nothing is both mover and moved. Thus, nothing moves itself.”

Given these three ways of showing the validity of the first thesis on which the demonstration depends, Aquinas turns to the second thesis, namely, “there is no procession to infinity among movers and things moved.” Aristotle offers three ways of proving this second thesis. First, since whatever is moved must be divisible or composite, and since mover and the thing moved must exist at the same time, each mover would itself be being moved at the same time as it is moving something else. If this chain of movers went on infinitely, then an infinite chain of movers, all moving together, would move in a finite time—which is impossible.

The second and third ways of proving that “there is no procession to infinity among movers and things moved” address the requirements of “an ordered series of movers and things moved.” Aristotle explains that if one removes the first mover from an ordered series, no other mover in the series will either move or be moved. If the ordered series extends in an infinite chain, then by definition there can be no first mover. In such a scenario, the motion could never have gotten started. An infinite series of intermediate or instrumental causes does not suffice for actual motion.

Having demonstrated both that “everything that is moved is moved by another” and that “there is no procession to infinity among movers and things moved,” Aristotle has demonstrated the validity of his first argument for the existence of a first unmoved mover: the argument from motion. Aquinas next summarizes Aristotle’s second argument for the existence of a first unmoved mover. In this second argument, Aristotle asks whether, if every mover is moved, this is necessary or accidental. If it is not necessary, then it is possible that no mover is moved. If so, at some time there would be no motion. Since there cannot have been a time in which there was no motion, it must be necessary that every mover is moved by another. Further, if the mover and the thing moved were only accidentally related, then one could find a mover without a thing moved. But if every mover is moved then “the mover must be moved either by the same kind of motion as that by which he moves, or another.” In fact, the same kind of motion is not possible, since it would require, for example, that a healer must be being healed. The mover could be moved by a different kind of motion, but this could not proceed to infinity, since the kinds of motion are finite in number. It follows that there has to be a first mover. But might this first mover be self-moved rather than unmoved? In a self-mover, the whole is not moved by the whole; rather, “one part of the self-moved mover is solely moving, and the other part solely moved.” This shows that the first mover must be an unmoved mover.

Further, Aristotle holds that “some self-moved being must be everlasting,” because motion is everlasting and the generation of self-moving beings (animals) has no temporal beginning or end. But no corruptible self-moving being could be the cause of endless motion; neither could endless motion be caused by all the corruptible self-moving beings together, since they do not exist simultaneously. Therefore, there must “be some endlessly self-moving being.” This incorruptible, endlessly self-moving being, however, must itself have a mover that “is not moved, either through itself or by accident,” because “no self-moved being is moved everlastingly whose mover is moved either by itself or by accident.”

This second argument of Aristotle’s for the existence of the unmoved mover is found in book 8 of his Physics. For Aquinas, the question is whether Aristotle’s demonstration has arrived simply at an unmoved mover who is merely the greatest part of the universe and so could not be God. In Aquinas’s view, therefore, we find the completion of Aristotle’s proof in book 12 of Metaphysics. Here Aristotle seeks an unmoved mover who is not merely “part of the self-moved mover” but rather “is absolutely separate.” Everything that is part of the universe moves by appetitive desire and thus is a moved mover. The object that is desired, by contrast, does not itself desire anything and is thus utterly unmoved. Since without such an object there would be no motion, Aristotle concludes that there must “be an absolutely unmoved, separate first mover.”

Since Aristotle’s two arguments for God’s existence presuppose the eternity of the world (endless motion), Aquinas asks whether such arguments can be employed by a Christian who knows that the world had a beginning. He answers that the arguments would be even more potent “if the world and motion had a first beginning,” since “that which comes to be anew must take its origin from some innovating cause; since nothing brings itself from potency to act, or from non-being to being.” Aristotle’s arguments, however, also presuppose that the first-moved being is self-moved, that is to say that the highest celestial sphere is conscious and animate. Aquinas points out that if one rejected this claim about the highest celestial sphere, one would simply arrive more directly at the “unmoved separate first mover.”

From books 1 and 4 of Aristotle’s Metaphysics Aquinas briefly draws two additional arguments for the existence of God. The first has its basis in efficient causality. In an ordered series of efficient causes, if we suppress the first efficient cause, there will be no effect. In such a case, the intermediate efficient causes could not be causes. An infinite chain of intermediate causes, absent a first cause, would mean that there could be no effect and thus no intermediate causes, which is contrary to what we see around us. The second additional argument for God’s existence hinges on the consideration that “what is most true is also most a being.” Further, of two false things, one can be falser than the other. This means that there is a measure of absolute truth, and this absolute truth is absolute being: God.

Last, in chapter 13 of the Summa contra gentiles, Aquinas offers a demonstration of God’s existence drawn from John of Damascus. This argument comes from the government of the world, namely, the fact that the world comprises “contrary and discordant things” that are nonetheless part of one intelligible order. If there were no providential orderer of the world, then the contrary and discordant things would at best rarely, or only by chance, come together in an intelligible order. As it stands, there is one stable order that comprises a tremendous number of widely diverse things. There must, then, be an orderer, who is God.

In his Summa theologiae, Aquinas very briefly offers five ways of demonstrating God’s existence. These are found in question 2, article 3 of the Prima Pars. The first way is the argument from motion or change. Aquinas states that “motion is nothing else than the reduction of something from potentiality to actuality,” and he adds that “nothing can be reduced from potentiality to actuality, except by something in a state of actuality.” The same thing cannot be in potency and actuality in the same respect, and so a thing cannot be both mover and moved in the same respect. This shows that “whatever is in motion must be put in motion by another,” but this cannot proceed to infinity, since if there is no first mover, there can be no intermediate movers either. The first unmoved mover, who is pure actuality and the source of all act and potency composites, is God.

The second way is the argument from efficient causality, understood again in terms of act and potency. Nothing is the efficient cause of its own finite actuality (or act of existing), and it is not possible to proceed to infinity in essentially ordered efficient causes, since without a first cause—which itself needs no efficient cause of its act of existing and is therefore pure actuality—there can be no intermediate causes and no ultimate effect. Since there obviously are intermediate causes and an ultimate effect, there must be a first cause, which is God.

The third way is from possibility (or contingency) and necessity: if everything were “possible to be and not to be” rather than existing absolutely and necessarily, then at some time (given infinite time on an endless continuum) everything would have not existed, since “that which is possible to be at some time is not.” If so, then there would now be nothing in existence, since nothing can come from nothing. The fact that something now exists, therefore, means that there must be some thing or things whose existence is necessary. As shown by the argument from efficient causality (the second way), it is impossible to proceed to infinity in necessary things that are caused by another. There must be one uncaused necessary being that causes all others, and this is God.

The fourth way is from the degrees of perfection found in finite things: “Among beings there are some more and some less good, true, noble, and the like.” The predication of “more” or “less” good or true requires that there be a measure of the degree to which something “resembles” goodness or truth. This measure must be maximal goodness or truth, for otherwise it would itself be measured rather than being the measure. For a maximum in perfection to exist, it must be maximal actuality, “for those things that are greatest in truth are greatest in being.” This maximum, as perfect actuality, “is to all beings the cause of their being, goodness, and every other perfection.”

The fifth and final way comes from the governance of the world. Nonrational things cannot direct themselves to an end, and yet nonrational things in the universe generally repeat the same actions to achieve the same ends. This could not be the result of chance. Thus nonrational things are ordered to their ends by an intelligent orderer who, as the one who orders this-worldly things to their end, transcends and governs this world.

It follows that the things we see all around us—movement, causality, contingency, degrees of perfection, and teleological order—are paths to God. Nonetheless, Aquinas recognizes the difficulty of knowing God. He holds that we all naturally desire happiness and therefore we all naturally desire God, but we do not necessarily know that it is God for whom we are yearning. Indeed, Aquinas considers that “human reason is very deficient in things concerning God.” Even the best philosophers have disagreed with each other and have fallen into numerous errors about God. In his Commentary on the Gospel of John, Aquinas says of the pagan philosophers that “if they did have some speculative knowledge of God, this was mixed with many errors: some denied his providence over all things; others said he was the soul of the world; still others worshipped other gods along with him.” Such errors are grave, since in a certain sense those who err “only slightly in their knowledge of God” are “entirely ignorant of him.” In addition, since metaphysics is the last of the sciences, it takes a long time to learn and bears fruit relatively late in life. Aquinas comments in the Summa theologiae that “many are unable to make progress in the study of scientia, either through dullness of mind, or through having a number of occupations and temporal needs, or even through laziness in learning, all of whom would be altogether deprived of the knowledge of God, unless divine things were brought to their knowledge under the guise of faith.”

The difficulty of metaphysical reasoning helps to explain why Aquinas, in the sed contra immediately prior to his elucidation of the five ways, quotes God’s words in Exodus 3:14, “I am who am.” (or “I am who I am”). God names himself in this way to Moses in the course of commissioning Moses to lead the people of Israel out of slavery in Egypt. The God who can name himself in this way—and only God could name himself “I am,” because only God is infinite existence rather than a finite mode of existence—is the same God who redeems and restores his idolatrous people. It is this redemption, and not philosophical knowledge of God, that grounds Christian faith.

William of Ockham (ca. 1288–ca. 1349)

Born in Ockham, a village in Surrey not far from London, William of Ockham was either given to the Franciscan order to be educated or joined the order on his own volition at a young age. In 1306, he was ordained a subdeacon, having been a Franciscan since at least 1302. Ordained a priest, he studied philosophy and theology in London and Oxford, and he lectured on Peter Lombard’s Sentences from 1317–19, largely in dialogue with the views of Henry of Ghent, Giles of Rome, John Duns Scotus, and Peter Aureoli. Between 1321 and 1324, he wrote commentaries on Aristotle’s works on logic, a Summa logicae, a commentary on Aristotle’s Physics, treatises on certain problems regarding predestination and the Eucharist, and quodlibetal disputations. Accused of heretical teaching, he spent four years at the papal court of John XXII in Avignon. In 1328 he fled Avignon along with the superior general of the Franciscan order, Michael of Cesena, first to Italy and then to Munich under the protection of the German emperor, Louis the Bavarian. Excommunicated by Pope John XXII, Ockham devoted the rest of his life to writing political treatises against Popes John XXII and Benedict XII on the grounds that the papacy had fallen into heresy with respect to apostolic poverty and the beatific vision.

For Ockham’s views on whether God’s existence can be demonstrated, I begin with his Quaestiones in librum I physicorum. Regarding the demonstration of a first (uncaused) efficient cause, Ockham responds to John Duns Scotus’s position that “essentially ordered effects [i.e., effects that are simultaneous in time] have a cause; therefore there is some cause which does not belong to this totality, otherwise one and the same thing would be the cause of itself.” For Scotus, the totality of essentially ordered caused things cannot be dependent on something within this totality. Scotus argues that “where there is no first principle, nothing is essentially prior”; and so if we could not demonstrate a first (uncaused) efficient cause, then we would be left with an infinite number of essentially ordered causes existing at the same time, which is impossible. Ockham, however, judges that none of these reasons is sufficient to demonstrate a first efficient cause “in regard to production as specifically distinct from conservation.”

Ockham reaches this conclusion because he thinks that if one were to judge from natural reason alone, an infinite multitude of caused things could indeed be caused from within the multitude, just as Scotus admits would be the case with an accidentally ordered (nonsimultaneous) set of effects or causes. One member of the multitude could be caused by another member—for instance, one human by another human, stretching to an infinite number of humans, caused by different members of the multitude—without there needing to be a first efficient cause. Ockham observes that “not every thing which mediately or immediately produces a thing coexists with the thing produced,” so that even in an essentially ordered series one cannot appeal to the danger of proceeding to infinity. Thus Ockham thinks it is possible for there to be an infinite series of efficient causes regardless of how those causes are related (essentially or accidentally). In sum, “as regards mere production it cannot be proved that one efficient cause is not produced by another, and so ad infinitum.”

By contrast, Ockham holds that the conservation of all things in being can be the basis of a successful proof of God’s existence, since with respect to conservation, one cannot proceed to infinity in an essentially ordered series of causes. From the conservation of all things in being, one can show that there must be a first efficient cause. Ockham explains that “every preservative cause is a productive cause.” Unlike in the case of efficient causes, “everything that conserves something else, be it mediately or immediately, exists at the same time with that which is conserved.” Since this is so, one cannot proceed to infinity in conserving causes without producing the absurdity of an actual infinity.

But given that every conserving cause is also a productive cause, why has Ockham not thereby also vindicated the proof from efficient causality? In answer, he returns to the thesis (shared by Scotus and Aquinas) that “in essentially ordered causes all causes are required at the same time for the production of the effect.” It would seem that there must be a first cause in order for there to be an essentially ordered chain of caused things; otherwise, either there would be an infinity of things actually existing, which is impossible, or else one of the caused things would be the cause of the whole multitude of caused things, which would fall into the absurdity of supposing that something is the cause of itself. To this line of argument, Ockham repeats his fundamental objection: it is possible that “one part is caused by one thing which is part of this multitude, and another by another thing, and so on ad infinitum.” But this objection, he observes again, does not undermine the proof of God’s existence from conservation in being.

In question 1 of Quodlibet 1 Ockham asks whether natural reason can prove that there is only one God. He contrasts two possible understandings of “God”: “some thing more noble and more perfect than anything else besides Him” and “that than which nothing is more noble and more perfect.” In Ockham’s view, the second understanding of “God” can be proved to be true but not the first. There must indeed be some thing “than which nothing is more noble and more perfect.” The only alternative would be to proceed to infinity, arriving at the absurd conclusion that there are an infinite number of beings. But to prove that there is some thing “than which nothing is more noble and more perfect” differs from showing that there is only one such thing. There may be more than one thing that fits this description. To know that there is only one thing “than which nothing is more noble and more perfect” requires faith.

In the same place, Ockham addresses various arguments put forward by Scotus in favor of the view that natural reason can demonstrate that there is only one God. Scotus proposes that if there were two Gods, each would love itself more than it would love the other, despite the other being infinitely lovable. Ockham replies that “it cannot be demonstrated that each of them is intensively infinite, since this is merely believed.” Similarly, against Scotus’s view that it would be impossible for an effect to have “two primary and total ends” (i.e., two Gods), Ockham argues that it would in fact be possible for one effect to have two “sufficient” final causes, even though neither of the final causes would be “total.” He holds that the existence of only one “total” final cause cannot be demonstrated. In response to Scotus’s argument for the impossibility of two necessary beings, Ockham considers that the divine nature could determine itself to be in two Gods rather than one. In such a case, “the concept necessary being would be univocally predicated of them—and they would be distinguished by their very selves.” The key move in Ockham’s responses consists in his denial that God can be proven to be infinite, unbounded by any limitation. For an unbeliever, says Ockham, it would make sense to “claim that infinity is incompatible with being and that the opposite cannot be proved.”

If one could show that God is in no way composite and is unable to enter into composition with another, would this prove that God is infinite? For Ockham, even this would not be demonstrative. He holds that both an angel and a spiritual soul are in no way composite without thereby being infinite. For Aquinas, an angel is a composite of being and essence, but Ockham rejects this notion of ontological composition. He argues that God is no more or less simple or devoid of composition than any other spiritual substance.

Question 1 of Quodlibet 2 takes up whether God can be shown to be the first efficient cause of all things. Ockham holds that God can neither be shown to be the immediate efficient cause of all things nor the efficient cause of any particular thing. Ockham suggests that angels could be sufficient causes for many things, and thus God need not be the immediate cause of everything. The angels, too, might well be uncaused, so far as rational demonstration can tell. As he shows in the first question of Quodlibet 1, Ockham thinks it possible that there is more than one necessary being, and he also thinks that not every effect need be reducible to the same first cause. In his view, it is entirely plausible that “the totality of things that are caused is not caused by some one thing, but is instead caused by many things. For one of the things caused is caused primarily by one efficient cause, and another is caused by another, and so on.” He emphasizes that he is not saying that things do not have a first efficient cause. A first efficient cause is necessary so as to avoid an infinite regress, but it need not be the same first cause in each case.

In his Ordinatio and elsewhere, Ockham accepts Scotus’s position that although “we cannot know God in this life in a concept that is simple and proper to Him,” nonetheless “we can know God in a common concept that is predicable of Him and others.” This concept, as such, applies univocally to God and creatures in the sense that the concept has the same meaning in both cases. Like Scotus, Ockham is well aware that “nothing real is univocal to God and creatures, if we take ‘univocal’ in its strict meaning; for nothing that exists in a creature, whether it be essential or accidental, has perfect similitude with something which really exists in God.” When “univocal” is taken in this strict meaning, there can be nothing univocal between God and creatures.

In his Reportatio, Ockham comments that “univocal” has various senses. The sense of “univocal” that pertains to God and creatures involves “a concept common to many things which have no likeness, either substantial or accidental.” Strictly speaking, God and creatures have absolutely no likeness. Ockham states that “in God and in creatures there is nothing at all, intrinsic or extrinsic, which is of the same kind.” God’s goodness is not like our goodness. Yet the concept “good,” as such, can be affirmed of both God and creatures. Ockham thinks that the particular sense that he gives to univocity accords with what others try to affirm through the doctrine of analogy. When we say that creatures are good and that God is good, Ockham thinks it indisputable that we are using one concept (rather than two), and he makes clear that this concept is neither purely univocal nor purely equivocal.

Since we know God through his created effects, God “is known either in a concept common to Him and others, or in something real which is different from Him.” The latter cannot be the case, because if it were true, then we would know a creature, not God. Ockham reasons that “when I know God in this life, I know Him in a concept which is common to Him and to other things, so that the term of the act of knowing is some one thing and not a plurality, and is common to many things.” He goes on to remark that it is possible “to abstract from an imperfect creature a concept of wisdom which does not refer more to creatures than to what is not a creature.” The concept itself, as such, refers neither to God nor to creatures; it is simply a concept that is predicable not only of material things but also of spiritual things. Ockham recognizes that “the distinction between the wisdom of a creature and the wisdom of God is as great as the distinction between God and a stone.” But the concept “wisdom,” abstracted from any particular essence, is predicable of both a creature and God.

Conclusion

Let us recall and briefly evaluate the ways in which the fathers and medievals argued for (and in Ockham’s case also against) the demonstrability of the existence of God. Tertullian seeks to show that we can know God from the existence and order of the universe, but he does not do this claim any favors by holding that God, as a substance, must be in some sense material. Gregory of Nazianzus pushes the discussion forward significantly by means of Aristotelian reasoning, as interpreted through a late-Platonic lens. For Gregory, as for Tertullian, the universe’s motion and order demonstrate the existence of God. Gregory adds the points that finite things are dependent, that no finite thing can move the whole, and that God is not in the order of moved or moving things but rather is a transcendent mover. Gregory also argues, from the existence of finite perfections, that there must be an infinitely perfect source. By bringing to bear the Greek tradition of philosophical proofs of God’s existence, Gregory goes beyond the limitations of Tertullian’s Stoic-influenced reasoning, valuable though the latter is.

Augustine, in various comments strewn throughout the Confessions, provides a profound sense of our creaturely contingency. Augustine’s major contributions to the discussion are Platonic in origin and are found in the Neoplatonists such as Plotinus: namely, our restless desire for being, goodness, and beauty, which can only be quenched by union with transcendent being, goodness, and beauty; the soul’s intellectual light as leading us to eternal and unchanging Truth, as we ascend from physical things to intelligible realities; and the fact that we participate in a finite way in existence, so that there must be infinite Being in which we participate.

If Gregory of Nazianzus adds broadly Aristotelian dimensions to Tertullian’s Stoic proofs, and Augustine adds Platonic (or Neoplatonic) dimensions, does John of Damascus contribute anything? John repeats the demonstrations of God’s existence from the order of the universe and from changeable things. His perspective is fundamentally Aristotelian, like Gregory’s. Thus he criticizes the view that God is in some sense material by noting that even a spiritual body would be moveable and would itself require an unmoved cause. He adds the point that the four elements, as contraries, could not hold together in a unity for very long unless they had a transcendent governor, who would also bring the elements into being and preserve them in being. John deepens the Aristotelian basis (filtered through late Platonism) of the patristic demonstrations of God’s existence.

Anselm’s approach is generally more Augustinian or Neoplatonic. He emphasizes that our desire for good things should lead us to recognize the existence of self-subsistent goodness in which all good things participate. Like Augustine, Anselm also attends to the contingency of things: other than God, “whatever exists, exists through something.” Anselm’s argument that there are degrees of perfection in being and that the chain of ever-greater existing natures cannot go on infinitely accords with Augustine’s argument that a living thing is better than a nonliving thing, and a wise living thing better than one that has no intelligence, and so forth to the highest being. Anselm also draws from Augustine his definition of God as “that than which nothing greater can be thought.”

The broadly Aristotelian reasoning, inflected with late-Platonic insights, about the existence of God that we find in the Greek-speaking East in Gregory and John of Damascus is continued in the Latin-speaking West by Thomas Aquinas. Aquinas’s approach to the existence of God is Aristotelian in the sense that he relies on such principles as ontological composition (essence/being, form/matter), causal interdependency, and above all the act/potency distinction. Like Gregory and John, Aquinas is not solely “Aristotelian” but is influenced by a number of thinkers, such as (in his case) Augustine, Boethius, Pseudo-Dionysius, and John of Damascus himself. He rejects arguments that claim God’s self-evidence—not only Anselm’s but also those from our desire for the good and from our knowledge of truth. Significantly, Aquinas’s demonstrations of God’s existence often appeal to the same points that we saw already in Tertullian, namely, that the universe could not have brought itself into being and ordered itself. In the Summa contra gentiles, Aquinas devotes a great deal of effort to summarizing and refining demonstrations that originate in Aristotle. These demonstrations hinge on Aristotle’s understanding of motion or change, specifically the claim that “everything that is moved is moved by another” and that “with respect to the same motion, nothing is both mover and moved.” In an essentially ordered series of causes, it is impossible to proceed to infinity in movers and things moved; there must be a first unmoved mover or else the motion of the intermediate causes could not be accounted for. To the demonstrations of God’s existence along these lines, Aquinas adds John of Damascus’s demonstration from the fact that “contrary and discordant things” are part of one universal order.

In the Summa theologiae, Aquinas offers five demonstrations that sum up the main lines of the patristic witness (especially if we also include his attention to the proofs from desire for the good and knowledge of the truth, which he thinks to be evocative even if they do not strictly work). From the more “Aristotelian” side, as represented by Gregory and John of Damascus, he sets forth the demonstrations from motion and from efficient causality as well as a demonstration from possibility and necessity, which is a broadly Aristotelian version of Augustine’s emphasis on creaturely contingency. These demonstrations presume the principle of noncontradiction—something cannot both be and not be in the same way at the same time—and they also presume that being is intelligible as being and can be analyzed in terms of act and potency, so that diverse modes of being are analogously distinguished. His fourth way is from degrees of perfection, and this approach echoes a proof found in Gregory of Nazianzus and is also deeply Augustinian (and Neoplatonic). His fifth way is from the order of the universe, since no nonrational thing could order itself consistently in a teleological fashion. The fifth way takes us back to the basic demonstration that we find in Tertullian and in almost all the fathers, and indeed already in Xenophon.

What then of Ockham? He supposes that the conservation of all things in being shows that a first efficient cause exists, but he does not consider that the demonstration from efficient causality works. For Ockham, it is possible that in a multitude of things, there are enough sources of actualization that one transcendent “first mover” is not needed. In my view, this conclusion rests on failing to appreciate adequately the contingency of all finite things. After all, any accidentally ordered series of finite causes depends on an essentially ordered series in order to exist. Even if the finite causes in an accidentally ordered series are infinite in number (as Aquinas grants), they cannot account for the existence of the series, since the existence of the infinite series would itself be dependent and in need of a cause. With regard to whether natural reason can prove God’s unity, the question here is again Aristotelian: Can there be more than one Pure Act? Ockham argues that we can answer in the negative only on the basis of faith, not on the basis of reason. His reasoning, however, relies on his view that all spiritual substances—God, the angels, and spiritual souls—are equally simple. In denying that angels and spiritual souls, whose being is not their essence, are composite in a way that God is not, Ockham overlooks the unique simplicity of Pure Act. When we demonstrate the existence of pure actuality (a first cause, unmoved mover, source of all things), we demonstrate the existence of something whose being and power are absolutely simple, infinite, unlimited, and unrestricted. In this respect, Ockham would have been aided by a deeper attention to the patristic tradition.

Even Ockham, however, holds that we can demonstrate the existence of God on the basis of the conservation of all things in being. The common thread of this first chapter is thus not completely absent even in Ockham. But since Ockham’s demonstration of God’s existence from conservation does not include God’s unity, the argument from conservation has not yet demonstrated the existence of the true God (who is one). Indeed, with respect to causality, act/potency, and simplicity, Ockham’s Aristotelianism lacks some crucial elements that we find in John of Damascus and Aquinas, no doubt in part because Ockham largely excludes Neoplatonic insights. His work therefore foreshadows the broad Reformation and Enlightenment movement away from Aristotelian and Neoplatonic demonstrations of God’s existence.

. A. N. Williams, The Divine Sense: The Intellect in Patristic Theology (Cambridge: Cambridge University Press, 2007), 4. Williams focuses in particular on five authors: Clement of Alexandria, Origen, Gregory of Nazianzus, Gregory of Nyssa, and Augustine. The whole purpose of the mind, for these fathers, is to contemplate divine things. The fall did not take away the mind’s ability to achieve this purpose, although the fall did focus the will (and the passions) on created things, making it easier for the mind to ignore God. See also Jaroslav Pelikan, Christianity and Classical Culture: The Metamorphosis of Natural Theology in the Christian Encounter with Hellenism (New Haven: Yale University Press, 1993), 57–73, 185–99. Pelikan attends especially to Gregory of Nyssa, whose assurance regarding the easy knowability of the first cause seems similar to John Calvin’s. Athanasius shares the patristic confidence about the demonstrability of God’s existence: see Athanasius, On the Incarnation 12, trans. and ed. a Religious of CSMV (Crestwood, NY: St. Vladimir’s Seminary Press, 1993), 39.

. See David Rankin, Tertullian and the Church (Cambridge: Cambridge University Press, 1995); Alistair Stewart-Sykes, Introduction to Tertullian, Origen, and Cyprian, On the Lord’s Prayer, trans. Alistair Stewart-Sykes (Crestwood, NY: St. Vladimir’s Seminary Press, 2004), 13–39, at 13–17.

. Tertullian, Apology 17, trans. S. Thelwall, in Latin Christianity: Its Founder, Tertullian, in Ante-Nicene Fathers 3, ed. Alexander Roberts and James Donaldson, rev. A. Cleveland Coxe (Peabody, MA: Hendrickson, 1994), 31. Subsequent references to Tertullian’s works are from this edition.

. For discussion, see Eric Osborn, Tertullian: First Theologian of the West (Cambridge: Cambridge University Press, 1997), 77–79.

. Tertullian, Apology 17, p. 32.

. Tertullian, Apology 17, p. 33.

. Tertullian, Apology 17, p. 33.

. Tertullian, Apology 21, p. 34.

. Tertullian, Apology 21, p. 34.

. Tertullian, Ad Nationes 2, p. 130.

. Tertullian, The Prescription against Heretics 7, p. 246.

. Tertullian, Against Praxeas 7, p. 602.

. Tertullian, Against Marcion 1.10, p. 278.

. Tertullian, Against Marcion 8, p. 276.

. Tertullian, Against Marcion 10, p. 278.

. Tertullian, Against Marcion 10, p. 278.

. Tertullian, Against Marcion 10, p. 278.

. Tertullian, Against Marcion 11, pp. 278–79.

. Tertullian, The Soul’s Testimony 1, p. 175.

. Tertullian, The Soul’s Testimony 2, p. 176.

. Tertullian, Against Praxeas 9, p. 603.

. See Christopher A. Beeley, Gregory of Nazianzus on the Trinity and the Knowledge of God: In Your Light We Shall See Light (Oxford: Oxford University Press, 2008), 3–62; John A. McGuckin, Saint Gregory of Nazianzus: An Intellectual Biography (Crestwood, NY: St. Vladimir’s Seminary Press, 2001); Frederick Norris, Faith Gives Fullness to Reasoning: The Five Theological Orations of Gregory Nazianzen (Leiden: Brill, 1997); Pelikan, Christianity and Classical Culture.

. St. Gregory of Nazianzus, On God and Christ: The Five Theological Orations and Two Letters to Cledonius, trans. Frederick Williams and Lionel Wickham (Crestwood, NY: St. Vladimir’s Seminary Press, 2002), Second Oration 28.1, p. 37.

. Gregory of Nazianzus, Second Oration 28.3, p. 39.

. Gregory of Nazianzus, Second Oration 28.6, pp. 40–41.

. Gregory of Nazianzus, Second Oration 28.6, p. 41.

. Gregory of Nazianzus, Second Oration 28.6, p. 41.

. Gregory of Nazianzus, Second Oration 28.13, p. 47.

. Gregory of Nazianzus, Second Oration 28.16, p. 49.

. Gregory of Nazianzus, Second Oration 28.16, p. 49.

. Gregory of Nazianzus, Second Oration 28.16, p. 49.

. Gregory of Nazianzus, Second Oration 28.16, p. 49.

. Gregory of Nazianzus, Second Oration 28.16, p. 49.

. Gregory of Nazianzus, Second Oration 28.27, p. 59.

. Gregory of Nazianzus, Second Oration 28.31, p. 63.

. Gregory of Nazianzus, On God and Man: The Theological Poetry of St. Gregory Nazianzus, trans. Peter Gilbert (Crestwood, NY: St. Vladimir’s Seminary Press, 2001), 1.1.4, p. 48.

. Gregory of Nazianzus, On God and Man 1.1.4, p. 49.

. Gregory of Nazianzus, On God and Man 1.1.5, p. 53.

. Gregory of Nazianzus, On God and Man 1.1.5, p. 53.

. Gregory of Nazianzus, On God and Man 1.1.6, p. 76.

. Gregory of Nazianzus, On God and Man 1.1.6, p. 76.

. Gregory of Nazianzus, On God and Man 1.1.6, p. 76.

. Gregory of Nazianzus, On God and Man 1.2.11, p. 129.

. Gregory of Nazianzus, On God and Man 1.2.12, p. 130.

. Gregory of Nazianzus, On God and Man 1.2.14, p. 132.

. For discussion of Gregory’s poetry, see Brian E. Daley, SJ, “Systematic Theology in Homeric Dress: Poemata Arcana,” in Re-Reading Gregory of Nazianzus: Essays on History, Theology, and Culture, ed. Christopher A. Beeley (Washington, DC: Catholic University of America Press, 2012), 3–12.

. See Serge Lancel, Saint Augustine, trans. Antonia Nevill (London: SCM Press, 2002); Peter Brown, Augustine of Hippo: A Biography, 2nd ed. (Berkeley: University of California Press, 2000); John M. Rist, Augustine: Ancient Thought Baptized (Cambridge: Cambridge University Press, 1994); Matthew Levering, The Theology of Augustine: An Introductory Guide to His Most Important Works (Grand Rapids: Baker Academic, 2013).

. Augustine, Confessions 1.1.1, trans. Henry Chadwick (Oxford: Oxford University Press, 1991), 3. For development of this theme in contemporary philosophy, see John Haldane, Reasonable Faith (London: Routledge, 2010), chap. 5: “The Restless Heart: Philosophy and the Meaning of Theism.” See also C. S. Lewis, Mere Christianity (New York: Macmillan, 1952), 119–21; Lewis, “The Weight of Glory,” in Lewis, The Weight of Glory and Other Addresses, ed. Walter Hooper (New York: Macmillan, 1980), 3–19; S. L. Frank, The Meaning of Life, trans. Boris Jakim (Grand Rapids: Eerdmans, 2010), chap. 5.

. Augustine, Confessions 1.5.5, p. 5.

. Augustine, Confessions 1.6.9, p. 7.

. Augustine, Confessions 4.10.15, p. 62. The interior quotation is of Ambrose’s hymn “Deus creator omnium.”

. Augustine, Confessions 7.10.16, p. 123.

. Augustine, Confessions 7.10.16, p. 123.

. Augustine, Confessions 7.10.16, p. 123.

. Augustine, Confessions 7.10.16, p. 124.

. Augustine, Confessions 7.17.23, p. 127. For discussion of book 7, see Phillip Cary’s “Book Seven: Inner Vision as the Goal of Augustine’s Life,” in A Reader’s Companion to Augustine’s Confessions, ed. Kim Paffenroth and Robert P. Kennedy (Louisville: Westminster John Knox, 2003), 107–26. Cary exaggerates, however, when he concludes, “Augustine clearly prays and worships and sings to the god of the Platonists, believing that this is no different from the Truth signified by Holy Scripture, to which Jesus Christ is the way. Our problem with Book Seven of the Confessions, I suggest, is what to make of this. Augustine wants us to worship and pray to the god of philosophy in the name of Jesus Christ. Shall we follow him in this?” Rather than praying, worshiping, and singing to the god of philosophy, Augustine is arguing that the Platonists were able to know truths about the living God—though it would not be appropriate to pray, worship, and sing to the “god of the Platonists” because they also had a number of false ideas.

. Augustine, Lectures or Tractates on the Gospel according to St. John, trans. John Gibb and James Innes, in Augustine: Homilies on the Gospel of John, Homilies on the First Epistle of John, Soliloquies, ed. Philip Schaff (Peabody, MA: Hendrickson, 1995), 400.

. Augustine, On Christian Doctrine 1.7, trans. D. W. Robertson Jr. (New York: Macmillan, 1958), 11.

. Augustine, On Christian Doctrine 1.7, p. 11.

. Augustine, On Christian Doctrine 1.8, p. 12.

. See Augustine, City of God 8.12, trans. Henry Bettenson (New York: Penguin, 1984), 315.

. Augustine, City of God 3.9, p. 311.

. Augustine, City of God 1.8, p. 311.

. Augustine, The First Catechetical Instruction 12, trans. Joseph P. Christopher (Westminster, MD: Newman, 1946), 41–42.

. John Rist highlights Augustine’s argument for the existence of God in book 2 of On Human Responsibility, which I have not treated here. See Rist, Augustine, 67–71. See also L. P. Gerson, “Saint Augustine’s Neoplatonic Argument for the Existence of God,” The Thomist 45 (1981): 571–84.

. See Andrew Louth, St. John Damascene: Tradition and Originality in Byzantine Theology (Oxford: Oxford University Press, 2002), 3–14.

. John of Damascus, The Philosophical Chapters, in Writings, trans. Frederick H. Chase (Washington, DC: Catholic University of America Press, 1958), chap. 3.

. John of Damascus, The Fount of Knowledge, in Writings, trans. Frederick H. Chase (Washington, DC: Catholic University of America Press, 1958), pref., p. 5.

. John of Damascus, The Fount of Knowledge pref., p. 5.

. John of Damascus, The Fount of Knowledge pref., pp. 5–6.

. John of Damascus, The Orthodox Faith 1.1, in Writings, trans. Frederick H. Chase (Washington, DC: Catholic University of America Press, 1958), 166.

. John of Damascus, The Orthodox Faith 1.3, p. 168.

. John of Damascus, The Orthodox Faith 1.3, p. 169.

. John of Damascus, The Orthodox Faith 1.3, p. 169.

. John of Damascus, The Orthodox Faith 1.3, p. 169.

. In The Consolation of Philosophy, similarly, Boethius argues that if there were no transcendent God, three kinds of order that we see in the universe could not be sustained. First, the universe’s diverse parts would never have come together into a unified whole. Second, even if they had come together, they would break apart rather than remaining a unified whole. Third, the orderly movements of the cosmos would not remain orderly. Boethius concludes that “whatever this is by which created things continue in being and move, I call by the name used by all, God.” See Boethius, The Consolation of Philosophy 3.12, trans. S. J. Tester, in Boethius, The Theological Tractates and The Consolation of Philosophy (Cambridge, MA: Harvard University Press, 1973), 299.

. John of Damascus, The Orthodox Faith 1.3, p. 170.

. John of Damascus, The Orthodox Faith 1.4, p. 171.

. John of Damascus, The Orthodox Faith 1.5, p. 172.

. See R. W. Southern, Saint Anselm: A Portrait in a Landscape (Cambridge: Cambridge University Press, 1990); G. R. Evans, “Anselm’s Life, Works, and Immediate Influence,” in The Cambridge Companion to Anselm, ed. Brian Davies and Brian Leftow (Cambridge: Cambridge University Press, 2004), 5–31.

. Anselm, Monologion prol., in Anselm, Monologion and Proslogion, with the Replies of Gaunilo and Anselm, trans. Thomas Williams (Indianapolis: Hackett, 1995), 3. Subsequent references to Anselm’s writings are found in this edition.

. Anselm, Monologion prol.

. Southern, Saint Anselm, 39, 49.

. Ibid., 79.

. Ibid., 78.

. Anselm, Monologion prol., p. 3.

. Anselm, Monologion 1, p. 10.

. Anselm, Monologion 1, p. 12.

. Anselm, Monologion 3, p. 12.

. Anselm, Monologion 27, p. 46.

. Anselm, Monologion 28, p. 47.

. Anselm, Monologion 29, pp. 48–49.

. Anselm, Monologion 80, p. 90.

. Anselm, Proslogion prol., p. 93.

. Anselm, Proslogion 1, p. 97.

. Anselm, Proslogion 1, p. 97.

. Anselm, Proslogion 1, p. 98.

. Anselm, Proslogion 1, p. 97.

. Anselm, Proslogion 1, p. 98.

. Anselm, Proslogion 1, pp. 98–99.

. Anselm, Proslogion 1, p. 90.

. Anselm, Proslogion 2, p. 99. For this description of God, see Augustine, On Christian Doctrine 1.7. See also Boethius, The Consolation of Philosophy 3.10: “Since nothing better than God can be conceived of, who can doubt that that, than which nothing is better, is good?” (ibid., 277). For discussion of the Proslogion’s demonstration of God, see Brian Davies, OP, “Anselm and the Ontological Argument,” in Cambridge Companion to Anselm, 157–78. See also, from a critical perspective, Graham Oppy’s Ontological Arguments and Belief in God (Cambridge: Cambridge University Press, 1995).

. Anselm, Proslogion 2, p. 100.

. Anselm, Proslogion 3, p. 101.

. Anselm, Proslogion 4, p. 101.

. Anselm, Proslogion 14, p. 108.

. Anselm, Proslogion 14, p. 109.

. Anselm, Proslogion 14, p. 109.

. See Anselm, Proslogion 16, p. 109.

. Anselm, Proslogion 18, p. 111.

. Anselm, Proslogion 25, p. 116.

. Anselm, Proslogion 26, p. 117.

. “Gaunilo’s Reply on Behalf of the Fool,” 122.

. “Gaunilo’s Reply on Behalf of the Fool,” 123.

. “Gaunilo’s Reply on Behalf of the Fool,” 124.

. “Gaunilo’s Reply on Behalf of the Fool,” 125.

. “Anselm’s Reply to Gaunilo,” 129.

. “Anselm’s Reply to Gaunilo,” 130.

. “Anselm’s Reply to Gaunilo,” 132.

. “Anselm’s Reply to Gaunilo,” 134.

. “Anselm’s Reply to Gaunilo,” 134.

. “Anselm’s Reply to Gaunilo,” 134–35.

. See Jean-Pierre Torrell, OP, Saint Thomas Aquinas, vol. 1, The Person and His Work, trans. Robert Royal (Washington, DC: Catholic University of America Press, 1996); M.-D. Chenu, OP, Toward Understanding St. Thomas, trans. A.-M. Landry, OP, and D. Hughes, OP (Chicago: Henry Regnery, 1964); Denys Turner, Thomas Aquinas: A Portrait (New Haven: Yale University Press, 2013). For detailed discussions of Aquinas’s demonstrations of God’s existence, see especially Edward Feser, Aquinas (Oxford: Oneworld, 2009); Thomas Joseph White, OP, Wisdom in the Face of Modernity: A Study in Thomistic Natural Theology (Ave Maria, FL: Sapientia, 2009); John F. Wippel, The Metaphysical Thought of Thomas Aquinas: From Finite Being to Uncreated Being (Washington, DC: Catholic University of America Press, 2000); Gregory Rocca, Speaking the Incomprehensible God: Thomas Aquinas on the Interplay of Positive and Negative Theology (Washington, DC: Catholic University of America Press, 2004); Rudi A. te Velde, Aquinas on God: The “Divine Science” of the Summa Theologiae (London: Ashgate, 2006); Pierre-Marie Emonet, OP, God Seen in the Mirror of the World: An Introduction to the Philosophy of God, trans. Robert R. Barr (New York: Crossroad, 2000).

. Thomas Aquinas, On Being and Essence 4, trans. Armand Maurer, 2nd rev. ed. (Toronto: Pontifical Institute of Mediaeval Studies, 1968), 55.

. Aquinas, On Being and Essence 4, p. 55.

. Aquinas, On Being and Essence 4, p. 56.

. Aquinas, On Being and Essence 4, p. 56.

. Aquinas, On Being and Essence 4, p. 56.

. Aquinas, On Being and Essence 4, p. 57.

. Aquinas, On Being and Essence 4, pp. 56–57.

. Thomas Aquinas, Summa contra gentiles 3.38, in Thomas Aquinas, Summa contra gentiles, Book Three: Providence, Part One, trans. Vernon J. Bourke (Notre Dame: University of Notre Dame Press, 1975), 125.

. Aquinas, Summa contra gentiles 3.38, p. 125.

. Aquinas, Summa contra gentiles 3.38, p. 125.

. Aquinas, Summa contra gentiles 1.9, in Thomas Aquinas, Summa contra gentiles, Book One: God, trans. Anton C. Pegis, FRSC (Notre Dame: University of Notre Dame Press, 1975), 77.

. Aquinas, Summa contra gentiles 1.9, p. 77.

. Aquinas, Summa contra gentiles 1.9, p. 78.

. Aquinas, Summa contra gentiles 1.10, p. 79.

. Aquinas, Summa contra gentiles 1.10, p. 80.

. Aquinas, Summa contra gentiles 1.11, p. 81.

. Aquinas, Summa contra gentiles 1.11, p. 82.

. Aquinas, Summa contra gentiles 1.11, p. 83.

. For discussion of Aquinas’s reinterpretation of Aristotle’s proofs, see White, Wisdom in the Face of Modernity, 76–96; Rocca, Speaking the Unknowable God, 200–220.

. Aquinas, Summa contra gentiles 1.13, p. 86.

. Aquinas, Summa contra gentiles 1.13, p. 86.

. Aquinas, Summa contra gentiles 1.13, p. 86.

. Aquinas, Summa contra gentiles 1.13, p. 87.

. Aquinas, Summa contra gentiles 1.13, p. 88.

. Aquinas, Summa contra gentiles 1.13, p. 88.

. Aquinas, Summa contra gentiles 1.13, p. 88.

. Aquinas, Summa contra gentiles 1.13, p. 89.

. Aquinas, Summa contra gentiles 1.13, p. 89.

. Aquinas, Summa contra gentiles 1.13, p. 89.

. Aquinas, Summa contra gentiles 1.13, p. 91 (italics added).

. Aquinas, Summa contra gentiles 1.13, p. 92.

. Aquinas, Summa contra gentiles 1.13, p. 93.

. Aquinas, Summa contra gentiles 1.13, p. 93.

. Aquinas, Summa contra gentiles 1.13, pp. 93–94.

. Aquinas, Summa contra gentiles 1.13, p. 94.

. Aquinas, Summa contra gentiles 1.13, p. 94. Commenting on “Thomas’s long and laborious presentation in SCG I, c. 13 of two arguments from motion for God’s existence,” Wippel points out that

to some extent each runs the risk of ending at best with a besouled self-mover of the outermost heavenly sphere rather than with God, at least when each is placed within its Aristotelian and medieval physical world-view. . . . If, as Thomas has indicated, Aristotle’s argumentation in Physics VIII leads only to a besouled self-mover, by appealing to Metaphysics XII Thomas believes he can enable it to conclude to the existence of a perfectly immobile first mover which is separate and which is God. (Wippel, Metaphysical Thought of Thomas Aquinas, 431)

As Wippel says, “Thomas introduces a much simpler and more direct argument based on motion in ST I, q. 2, a. 3 as his first way” (ibid.). See also Norman Kretzmann, The Metaphysics of Theism: Aquinas’s Natural Theology in Summa contra gentiles I (Oxford: Clarendon, 1997).

. Aquinas, Summa contra gentiles 1.13, p. 95–96.

. Aquinas, Summa contra gentiles 1.13, p. 95.

. Aquinas, Summa contra gentiles 1.13, p. 95.

. Thomas Aquinas, Summa theologiae Ia.2.3, in Thomas Aquinas, Summa Theologica, trans. Fathers of the English Dominican Province (Westminster, MD: Christian Classics, 1981).

. Aquinas, Summa theologiae Ia.2.3.

. In an essentially ordered series of causes, all of the causes must act simultaneously in causing the effect. By contrast, an accidentally ordered series of causes does not depend on all the causes being simultaneously in act. An example of an accidentally ordered series of causes is grandfather-father-son, in which the grandfather may no longer be alive when the son is begotten. Aquinas grants that it is philosophically possible that one could have an infinite accidentally ordered series of causes.

. Edward Feser clarifies that “Aquinas is not in fact trying to argue in the Fourth Way that everything that we observe to exist in degrees (including heat, smelliness, sweetness, etc.) must be traceable to some single maximum standard of perfection. Here (as elsewhere in the Five Ways) his archaic scientific examples have led modern readers to misread him” (Feser, Aquinas, 104). Aquinas instead has transcendental perfections in view, such as true, good, and noble.

. Boethius presents a version of this proof, as does Augustine. Boethius argues that perfect goodness must exist because imperfect finite goods exist: “For everything which is called imperfect is held to be imperfect because of some diminution of what is perfect” (Consolation of Philosophy 3.10, p. 275). If the perfect does not exist, then to call something imperfect would not make sense. There could not be an imperfect good if there were no perfect good, because without a standard of perfection there cannot be degrees of perfection. He states, “Hence it happens that if in any class something seems to be imperfect, there must also be something perfect of that class; for if we take away perfection altogether, it cannot even be imagined how that which is held to be imperfect can exist” (Consolation of Philosophy 3.10, p. 275). Boethius explains that only perfect being could be the source of imperfect beings. This is so because any imperfect being is dependent on something prior for its being; an imperfect being is a mere mode of being rather than the source of being. If God were not the perfect good, then God “could not be the principle of all things; for there would be something possessing perfect good more excellent than he, which in this would seem to be prior and more excellent” (Consolation of Philosophy 3.10, p. 277). The cause of all that exists must also be the highest good, and so God’s being is sheer goodness.

. Feser again makes a helpful clarification: Aquinas

is not arguing (as advocates of the design argument might) that it is improbable that complex structures could arise by chance, which would invite the response that natural selection shows how such structures might nevertheless arise by non-fortuitous but impersonal processes. For . . . he is not interested here in complexity per se in the first place; . . . even a simple physical phenomenon like the attraction between two particles would suffice for his purposes. What he is saying is rather that it is impossible that every apparent causal regularity can be attributed to chance, for chance itself presupposes causal regularity. (Feser, Aquinas, 113)

Feser goes on to point out that the countless nonintelligent causes, ordered to particular ends, require not merely a cosmic designer in the past but an intelligent governer who sustains the universe here and now.

. See Aquinas, Summa theologiae Ia.2.1, ad 1.

. Aquinas, Summa theologiae II-II.2.4.

. Thomas Aquinas, Commentary on the Gospel of John: Chapters 13–21, trans. Fabian Larcher, OP, and James A. Weisheipl, OP, ed. Daniel A. Keating and Matthew Levering (Washington, DC: Catholic University of America Press, 2010), 195–96. Aquinas is commenting on John 17:25, where Jesus says, “O righteous Father, the world has not known thee.”

. Aquinas, Commentary on the Gospel of John, 196. For discussion see Thomas Joseph White, OP, “‘Through Him All Things Were Made’ (John 1:3): The Analogy of the Word Incarnate according to St. Thomas Aquinas and Its Ontological Presuppositions,” in The Analogy of Being: Invention of the Antichrist or the Wisdom of God?, ed. Thomas Joseph White, OP (Grand Rapids: Eerdmans, 2011), 246–79, esp. 247n3, 249–51, and 276–78.

. Aquinas, Summa theologiae II-II.2.4.

. For further discussion of Aquinas’s proofs see Lawrence Dewan, OP, “The Existence of God: Can It Be Demonstrated?,” Nova et Vetera 10 (2012): 731–56; Antoine Guggenheim, “The ‘Five Ways’ and Aquinas’s De Deo Uno,” trans. Robert McKeon, Analecta Hermeneutica 2 (2010); Guggenheim, Les preuves de l’existence de Dieu (Paris: Parole et Silence, 2008); John F. X. Knasas, “The ‘Suppositio’ of Motion’s Eternity and the Interpretation of Aquinas’ Motion Proofs for God,” in God: Reason and Reality, ed. Anselm Ramelow (Munich: Philosophia Verlag, 2014), 147–78.

. See Philotheus Boehner, OFM, introduction to William of Ockham, Philosophical Writings: A Selection, trans. Philotheus Boehner, OFM, revised by Stephen F. Brown (Indianapolis: Hackett, 1990), ix–li; William J. Courtenay, “The Academic and Intellectual Worlds of Ockham,” in The Cambridge Companion to Ockham, ed. Paul Vincent Spade (Cambridge: Cambridge University Press, 1999), 17–30; Léon Baudry, Guillaume d’Occam. Sa vie, ses oeuvres, ses idées sociales et politiques, vol. 1, L’homme et les oeuvres (Paris: Vrin, 1950); Marilyn McCord Adams, William Ockham, 2 vols. (Notre Dame: University of Notre Dame Press, 1987); Gedeon Gál, OFM, “William of Ockham Died Impenitent in April 1347,” Franciscan Studies 42 (1971): 90–95.

. Ockham, Philosophical Writings, 119. For discussion of Ockham’s response, see Armand A. Maurer, CSB, The Philosophy of William of Ockham in the Light of Its Principles (Toronto: Pontifical Institute of Mediaeval Studies, 1999), 173–79.

. Ockham, Philosophical Writings, 119. For Scotus’s viewpoint, see the texts and commentary in William A. Frank and Allan B. Wolter, Duns Scotus, Metaphysician (West Lafayette, IN: Purdue University Press, 1995); John Duns Scotus, A Treatise on God as First Principle, trans. and ed. Allan B. Wolter, OFM (Chicago: Franciscan Herald Press, 1966). See also Timothy O’Connor, “Scotus on the Existence of a First Efficient Cause,” Philosophy of Religion 33 (1993): 17–32; James F. Ross and Todd Bates, “Duns Scotus on Natural Theology,” in The Cambridge Companion to Duns Scotus, ed. Thomas Williams (Cambridge: Cambridge University Press, 2003), 193–237; Rega Wood, “Scotus’s Argument for the Existence of God,” Franciscan Studies 47 (1987): 257–77; and Alexander W. Hall, Thomas Aquinas and John Duns Scotus: Natural Theology in the High Middle Ages (London: Continuum, 2007).

. Ockham, Philosophical Writings, 120.

. Ockham, Philosophical Writings, 121.

. Ockham, Philosophical Writings, 121.

. Ockham, Philosophical Writings, 123.

. Ockham, Philosophical Writings, 123.

. For the limitations of this proof even in Ockham, see Adams, William Ockham, 2:970.

. Ockham, Philosophical Writings, 123.

. Ockham, Philosophical Writings, 124.

. Ockham, Philosophical Writings, 125. See also the translation of this text in William of Ockham, Quodlibetal Questions, Volumes 1–2, Quodlibets 1–7, trans. Alfred J. Freddoso and Francis E. Kelley (New Haven: Yale University Press, 1991), 5–12.

. Ockham, Quodlibetal Questions, 9.

. Ockham, Quodlibetal Questions, 11.

. Ockham, Quodlibetal Questions, 12.

. See also question 1 of Quodlibet 3, where Ockham argues that since “all things are finite,” it follows that “even if one presupposes an efficient causation with respect to all things and an understanding of all things, God’s infinity cannot be proved on this basis” (Ockham, Quodlibetal Questions, 169). He explains that “the infinity of the cause cannot be proved from a finite effect or from finite effects producible at one and the same time” (Ockham, Quodlibetal Questions, 169). He considers, too, that “it is possible for something to be the most eminent in the sense that nothing is better than it, and yet for it to be finite” (Ockham, Quodlibetal Questions, 170).

. Ockham, Quodlibetal Questions, 176 (Quodlibet 3.1). See also Quodlibet 7.11–17, where Ockham examines whether God’s infinity can be demonstrated from God’s efficient causation, cognition, simplicity, final causality, eminence, or other arguments given by Aristotle.

. See Quodlibet 7.13.

. Ockham, Quodlibetal Questions, 96 (Quodlibet 2.1).

. For further discussion of Ockham on the demonstrations of God’s existence, see Adams, William Ockham, 2:967–79. Graham Oppy similarly opposes Aquinas’s second way on the grounds that “there is nothing in the premises that justifies drawing the conclusion that there is exactly one first cause” and that “there is nothing in the premises to rule out circles of causes, for example, a situation in which a causes b, b causes c, and c causes a” (Oppy, Arguing about Gods [Cambridge: Cambridge University Press, 1999], 99–100).

. Ockham, Philosophical Writings, 102; see also Quodlibet 5.14, where Ockham states that “the concept being is univocal to God and all other things. This is evident from the fact that everyone concedes that we have some sort of noncomplex cognition of God” (Quodlibetal Questions, 449). For discussion see Stephen Dumont, “Henry of Ghent and Duns Scotus,” in Medieval Philosophy, ed. John Marenbon (London: Routledge, 1998), 291–328.

. Ockham, Philosophical Writings, 106.

. Ockham, Philosophical Writings, 107.

. Ockham, Philosophical Writings, 107.

. Ockham, Philosophical Writings, 110.

. Ockham, Philosophical Writings, 111.

. Ockham, Philosophical Writings, 112.

. Ockham, Philosophical Writings, 112.

. For further discussion see Alfred J. Freddoso, “Ockham on Faith and Reason,” in Cambridge Companion to Ockham, 326–49, esp. 341–43. John Lee Longeway describes Ockham as “the founder of European empiricism. Like Locke and Hume, he relied on the logical analysis of language to ground a rejection of Platonic metaphysics” (Longeway, introduction to Demonstration and Scientific Knowledge in William of Ockham: A Translation of Summa Logicae III–II: De Syllogismo Demonstrativo, and Selections from the Prologue to the Ordinatio, trans. John Lee Longeway [Notre Dame: University of Notre Dame Press, 2007], 1–140, at 1).

. For an introduction to the distinction between act and potency, see Feser, Aquinas, 9–12. For detailed discussion of the relation of Aquinas’s thought to Aristotle’s on this point, see John F. Wippel, “Thomas Aquinas and the Axiom That Unreceived Act Is Unlimited,” in Wippel, Metaphysical Themes in Thomas Aquinas II (Washington, DC: Catholic University of America Press, 2007), 123–51; W. Norris Clarke, SJ, “The Limitation of Act by Potency in St. Thomas: Aristotelianism or Neoplatonism?,” in Clarke, Explorations in Metaphysics: Being—God—Person (Notre Dame: University of Notre Dame Press, 1994), 65–88. See also Clarke, “Action as the Self-Revelation of Being: A Central Theme in the Thought of St. Thomas,” in Clarke, Explorations in Metaphysics, 45–64; Steven A. Long, “On the Natural Knowledge of the Real Distinction of Essence and Existence,” Nova et Vetera 1 (2003): 75–108.

. For a contemporary Thomistic engagement with those who argue that the universe, once in existence, does not need to be conserved in existence by God (and indeed does not need to be caused by God), see Edward Feser, “Existential Inertia and the Five Ways,” American Catholic Philosophical Quarterly 85 (2011): 237–67. For the position that Feser contests, see especially Bede Rundle, Why There Is Something Rather Than Nothing (Oxford: Oxford University Press, 2004). Reading the classical proofs through a Humean lens, Rundle argues that “the notion of beginning to be has no application to the universe, and hence . . . there is in this instance no coming into existence for which we have to find a cause” (ibid., viii). The same view is found in John Stuart Mill, Three Essays on Religion: Nature, The Utility of Religion, Theism (Amherst, NY: Prometheus, 1998), 142–43. Mill argues that since the elements of the universe “are not known to us as beginning to exist,” it follows that “causation cannot legitimately be extended to the material universe itself, but only to its changeable phenomena” (ibid., 143). For Mill, then, the only possible “first cause” would be “a certain quantum of Force, combined with certain collocations” (ibid., 145). But Aquinas’s demonstrations of God’s existence are not predicated on the universe having a beginning of existence.

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