16.2 SCG 4,2–26: Some Details
16.2.1 Scripture and the Second Person of the Trinity (SCG 4,2–9)
Is the doctrine of the Trinity as Aquinas defends it biblical? Aquinas maintains that it is, and in SCG 4,2–9 he argues for this conclusion with respect to God the Son and with specific reference to passages in both the Old and New Testament. “Sacred Scripture,” he says, “hands on to us the names of ‘paternity’ and ‘sonship’ in the divinity, insisting that Jesus Christ is the Son of God.”16 In defense of this judgment, Aquinas cites texts such as Psalm 2:7, Proverbs 30:4, Matthew 11:27, John 3:35, Romans 1:1–3, and Hebrews 1:6. He concludes, “we are taught from sacred Scripture that the Son of God, begotten of God, is God.”17
Yet, notes Aquinas, certain people “perversely presumed to measure the truth of this doctrine by their own comprehension of it,” thus embracing “opinions both vain and various.”18 In this connection, Aquinas singles out Cerinthus (fl. c. 160), Ebionites (early second century), Paul of Samosata (third century), Photinus (fourth century), Sabellius (possibly third century), and Arius (d. 336). In SCG 4,4–9, Aquinas maintains that all of these stand condemned on biblical grounds.
He lumps together Cerinthus, the Ebionites, Paul of Samosata, and Photinus as teaching that Jesus was only a human being and was God’s son only by adoption, not by nature. When attacking this position, Aquinas appeals to biblical texts such as Proverbs 8:24–30, John 1:3, John 3:13, and Philippians 2:6.
Aquinas takes Sabellius to hold that the phrase “God the Son” designates “not a subsisting person but a kind of additional property of a pre-existing person, for the Father himself, in that he assumed flesh from the virgin Mary, received the name of Son.”19 According to Aquinas, however, “Scripture makes the falsity of this position quite manifest” since it “does not call Christ merely the virgin’s son, but also the Son of God.”20
Teachings ascribed to Arius provoked a major controversy among Christians from around 325 to 381. Arius was resolute in defending the uniqueness of God as the indivisible and changeless source of creatures. Accordingly, he taught that the Son of God (Jesus), though much to be venerated, is a creature who, in some sense, had a beginning and is not eternal. Thinking of Arius as holding to this conclusion, however, in SCG 4,7 Aquinas bluntly observes: “That this opinion is manifestly repugnant to divine Scripture anyone can see who considers diligently what sacred Scripture says.”21 In SCG 4,6, Aquinas notes biblical verses that he thinks to have been cited in defense of the claim that the Son of God is a creature. In SCG 4,7 he presents a scriptural counter blast to this claim.22 Verses Aquinas cites at this point include Romans 9:5, Colossians 2:9, Hebrews 1:5, and 1 John 5:20. In SCG 4,8, therefore, we find Aquinas saying:
Since truth cannot be truth’s contrary, it is obvious that the points of scriptural truth introduced by the Arians to confirm their error cannot be helpful to their teaching. For divine scripture shows that the essence and divine nature of the Father and the Son are numerically identical, and according to this each is called true God, it must be that the Father and Son cannot be two gods but one God. For, if there were many gods, a necessary consequence would be the partition in each of the essence of divinity, just as in two people the humanity differs in number from one to the other; and the more so because the divine nature is not one thing and God another thing [see SCG 1,21]. From this it follows necessarily that, since there exists one divine nature in the Father and the Son, the Father and the Son are one God.23
You will see from this quotation, and from other parts of SCG 4, that Aquinas brings to his reading of Scripture much for which he argues philosophically in SCG 1–3.
16.2.2 Could There Be Generation in God? (SCG 4,10–14)
In SCG 4,2–9 Aquinas is arguing as a biblical commentator, albeit one approaching the Bible with certain philosophical views. In SCG 4,10–14, however, he is clearly anxious to explain how what he takes the Bible to say about God the Son is not unreasonable. So he begins SCG 4,10 by saying:
When all things are carefully considered, it is clear and manifest that sacred Scripture proposes this for belief about the divine generation: that the Father and Son, although distinct as persons, are nevertheless one God and have one essence or nature. But one finds this far removed from the nature of creatures: that any two be distinguished as individuals while being one in essence; so, human reason, proceeding from the properties of things, experiences difficulties in a great variety of ways in this secret (secretum) of divine generation.24
What sort of ways? Continuing with SCG 4,10, Aquinas explains by summarizing these ten arguments:25
1. Generation in the world comes about as a result of change, and things that are generated perish. Therefore, there cannot be generation in God, “who is immutable, incorruptible, and eternal.”26
2. If generation involves a change, then what is generated is first potentially thus and so and then actually thus and so. But there cannot be any move from potentiality to actuality in the immutable divine nature.27
3. If God the Son is begotten by God the Father, the nature that he receives cannot be that of the Father as is the nature had by the child of a human father since this would entail polytheism. Nor can God the Son receive numerically the same nature as the Father has since this would entail that the divine nature is divisible, which it is not. We might suppose that the whole of the divine nature is passed into God the Son. Were that so, however, “it ceases to be in the Father; and so, in generation, the Father is corrupted.”28 We might think that the divine nature flows from the Father as water flows from a spring into a stream. But this would imply division in God. So, “the Son of God is neither a true son, since the Father’s species is not his; nor true God, since he does not receive the divine nature.”29
4. If God the Son is the divine essence, and if the divine essence has real or subsistent being, then the Father and Son are one subsistent thing, as Sabellius said. But if the Son is not the divine essence, then, as Arius thought, the Son is not God.30
5. What individuates a thing, what makes it to be itself and not something else, cannot be in some existing thing other than it. What different things share cannot make them to be what they are individually. Yet God’s essence individuates God, and this essence cannot be in the Son. So as Arius held, the Son is not God. Either that, or there is, as Sabellianism teaches, no real distinction between God the Father and God the Son.
6. If God the Father is distinct from God the Son, while being one when it comes to essence, “there must be in them something other than the essence by which they are distinguished, for a common essence is ascribed to each and what is common cannot be a distinguishing principle. Therefore, that which distinguishes the Father from the Son must be other than the divine essence.”31 However, this conclusion entails that neither the Father nor the Son is God, in whom there is no composition of individuality and nature.32 One might say that Father and Son “are distinguished by a relationship only, inasmuch as one is Father, and the other is the Son.”33 But “what is predicated relatively … seems not to predicate a something in that of which it is said, but rather a to something.”34 Therefore, an eternal and real distinction between Father and Son cannot be defended by saying that Father and Son, as truly God, are distinct by relationship alone. When we speak of X as being related to Y in some sense, we must suppose there to be both something of which we can say what it is in itself and something to which this thing can be related as other than it. Thus, for example, servants are individual people who have masters who govern them. In that case, however, the relationship between God the Father and God the Son has to amount to one thing being what it is in itself and another thing being related to it. So if God the Father and God the Son are related, either one or both of them have their being independently of each other, like a servant as related to a master, or they are identical in the sense in which something is identical with itself. In that case, however, “it does not seem possible that the persons of the Father and the Son are distinguished by relations only.”35
7. The relation distinguishing the Son from the Father is either real or only in thought.36 If it is the former, (1) it cannot be the divine essence since this is shared by both the Father and the Son, and (2) there will be something in the Son that is not his essence—meaning that the Son is not divine since God is God’s essence. If, however, the relation distinguishing the Son from the Father is something only in thought, it cannot “distinguish the Son from the Father personally, for things which are personally distinguished must be really distinguished.”37
8. “Every relative depends on its correlative. But what depends on another is not true God. If, then, the persons of the Father and the Son are distinguished by relations, neither of them is true God.”38
9. If the Father is God and if the Son is God, the word “God” must signify their nature, not any accident they possess, for there are no accidents in God (see SCG 1,23). Now, there can be no plurality of subjects in one nature, even though different things might have a nature in common, as two people can be human. So, “if the Father and the Son are two persons, it seems impossible that they are one God.”39
10. Plurality is presupposed when we ascribe to things predicates that are opposed to each other. Yet we do this when we say that the Father is unbegotten and generating while the Son is begotten. So the Father and the Son cannot be one God.40
It is in response to these arguments that Aquinas develops his account of God the Father and God the Son in terms of what reflects on itself and understands: the account I summarized in chapter 16, section 1. Having noted ways in which some living things are superior to others, he says: “The ultimate perfection of life belongs to God, in whom understanding is not other than being. … Accordingly, the intention understood in God must be the divine essence itself.”41 Aquinas means that what exists in God as God eternally knows the divine essence cannot but be God. He concedes that when we come to know or understand something we inevitably acquire something accidental to us, something that arises in us but not because of what we are essentially. But he denies that this is the case when it comes to what is in God as God knows the divine nature. So, he argues, if we think of God as able to have an understanding of divinity, that understanding will not be different from the divine nature, since all that is in God is God. He adds: “From these considerations we can somehow conceive how divine generation is to be taken.”42
We cannot, says Aquinas, take it to be a matter of one thing springing from another as, say, a plant or an animal springs from another plant or animal. We can, however, intelligibly think of it as an eternal coming forth in God of what is God. He writes:
It is manifest that God understands what God is [see SCG 1,47]. Now, whatever is understood should, as understood, be in the one who understands, for the significance of the very act of understanding is this: the grasping of that which is understood by an intellect; hence, even our intellect understanding itself is within itself, not only as belonging to one thing with a particular essence, but also as grasped by itself in the act of understanding. God, therefore, must be in God as what is understood by God.43
Aquinas believes that when we arrive at an understanding of something, there comes to be in us a “word” (verbum) in our minds. He holds that our speaking in words that can be written down or heard reflects what is conceptually in us even before we speak.44 He also thinks that if I tell you what something is, I am passing on to you what is in my mind as a matter of understanding in me. So, drawing on the thought of there being a word in the mind of one who understands, he goes on to say that, reflecting on divine revelation, we might say that in God there is a word who is God.
Here Aquinas refers to the first chapter of the Gospel of John. He says that we can think of the word of God (i.e., God’s understanding of God) as being the Word of which John 1 speaks when referring to that which was always with God and which was God while becoming incarnate in time. The being (esse) of this word, Aquinas goes to say, “is identical with that of the divine intellect and, consequently, with that of God, who is God’s own intellect.”45 Aquinas accepts that when we understand ourselves, the understanding in us arises in time and is not a human being. It is a grasping by intellect of what a human being is. When it comes to God understanding God, however, matters are different.
The Word of God, precisely because he is God understood, is true God, having the divine being naturally, because the natural being of God is not one being and that of God’s understanding another. … The divine Word is not merely an intention understood, as our word is, but it is also a thing existing and subsisting in nature. For God is a true subsistent thing, since God is substantial being in the highest degree.46
Since God is entirely immaterial, Aquinas adds, it is “impossible that the divine nature be specifically one and numerically different. The Word of God, therefore, has a nature in common with God and has it with numerical identity. For this reason the Word of God and the God whose Word he is are not two gods but one God. … God is both the divine essence and the divine act of being.”47 So, Aquinas maintains, the Word of God eternally arising in God proceeds from God or is generated or conceived by God. In this sense it is God’s offspring. In this sense it is God the Son. Obviously, in human generation there are temporal stages, and what is generated is spatially and physically distinct from its generator. However, Aquinas thinks that, if there is generation in God, conception, birth, and being present to a generator are simultaneous. He says: “The Word of God is at once conceived, brought forth, and present.”48
Given what he has been saying about the coming forth in God of God the Son, should Aquinas not conclude that God the Son can form a concept of himself, thereby generating another son? Indeed, should he not conclude that generation in God goes on without end? In keeping with what he takes to be orthodox teaching on the Trinity, Aquinas holds that in God there is but one divine Son. In SCG 4,13, though, he does allude to the questions I have just raised. He writes:
It seems to follow from the foregoing both that the divine Word has another word and the divine Son another son. For it was shown that the Word of God is true God. Whatever, therefore, belongs to God must belong also to the Word of God. But God necessarily understands God. Therefore, the Word of God also understands the Word of God. If, then, one says that because God understands God there is in God a Word begotten by God, it seems to follow that in the Word so far as he understands himself one must allow another word. And thus there will be a word of the Word and a son of the Son. And that word, if divine, will again understand himself and will have another word. In this way, the divine generation will proceed to infinity.49
How does Aquinas deal with this difficulty to there being only one Word or Son in God? He does so by reminding his reader that God the Son is not another God distinct from God whose Word he is. God the Son, says Aquinas, is not an intellect distinct from the intellect of God the Father. God the Son is nothing but God’s understanding of God, not something forming a concept of itself and, therefore, not something generating one or more divine sons. So, Aquinas argues: “Just as the Word is not another god, so neither is he another intellect; consequently, not another act of understanding; hence, not another word.”50
What, though, of the ten arguments in SCG 4,10? In SCG 4,14 Aquinas says that they have been refuted by what he argues in SCG 4,11–13. Generation in God, he notes, does not involve material change or a move from potentiality to actuality. It is not the same as human generation of children, so it does not raise questions as to whether God the Father gives the divine nature to God the Son wholly or partially. Again, says Aquinas, distinction in God does not imply multiplication of deities. The Word of God is God and is the divine essence. There is distinction in God only because relations exist in God that subsist in the divine nature and are not accidents ascribable to God. These relations, Aquinas insists, are real (secundum rem) and “not in understanding alone” (non solo intellectu). God the Son is what God the Father generates, and God the Father is what generates God the Son. Here, uniquely, we have relation in what is but one essence. Or, as Aquinas puts this thought:
From the fact that in God there is unity of essence and distinction of relations it becomes manifest that nothing stops one’s finding opposites in the one God, at least those opposites which follow the distinction of relation: begetting and begotten, for instance, which are opposed relatively, and begotten and unbegotten which are opposed as affirmation and negation. For wherever there is distinction one must find the opposition of negation and affirmation. Things which differ in no affirmation or negation are entirely undifferentiated, for the first would have to be in every respect one with the second, and thus they would be thoroughly identified, and in no way distinct.51
Here Aquinas is saying that if everything said of one thing can be said of another, we lack reason for distinguishing between them just on the basis of what is said of them. But if we can say of X that it is distinguishable from Y, there is a real distinction between X and Y. And, thinks Aquinas, there is a real distinction between God the Father and God the Son since the Father and the Son are distinguishable even though they are one in essence. Distinguishable how? With respect to their relation to each other.
16.2.3 The Holy Spirit as Divine (SCG 4,15–26)
In SCG 4,15 Aquinas observes that “the authority of the divine Scriptures not only tells us about the Father and the Son in divinity, but together with these also numbers the Holy Spirit.”52 Yet should Christians think of the Holy Spirit as being divine as are the Father and God the Son? In SCG 4,15–26, Aquinas argues that they should.
As with his discussion of God the Son, Aquinas begins his discussion of God the Holy Spirit by appealing to biblical texts (SCG 4,15–18). But, and again reflecting the progression of thought in SCG 4,2–14, he continues in SCG 4,19–26 to consider how we might take the claim that the Holy Spirit is God to be thinkable without just quoting from the Bible. He writes:
Taught by holy Scripture, we maintain this firmly about the Holy Spirit: that he is true God, subsistent, personally distinct from the Father and the Son. But one ought to consider how a truth of this kind must be grasped somehow, in order to defend it from the attacks of unbelievers.53
When it comes to the Holy Spirit being God, in SCG 4,15 Aquinas cites Matthew 28:19, John 15:26, and 1 John 5:7. In SCG 4,16 he notes biblical texts that might be thought to suggest that the Holy Spirit is a creature; but in SCG 4,17 he denies that they do this while quoting yet more biblical texts that he thinks of as teaching that the Holy Spirit is truly divine. He takes it to be clear that Scripture tells us that the Holy Spirit is God and is something in God distinct from the Father and the Son: a point he reiterates in SCG 4,18.
Yet how can we understand the Holy Spirit if this teaching is true? In one sense, of course, Aquinas does not think that we can understand the Holy Spirit since he takes the Holy Spirit to be God and since he denies that we can understand what God is (see chapter 5 here). It is often said that the doctrine of the Trinity presents us with something more mysterious than the claim that God exists, but such is not Aquinas’s view. He thinks that our powers of understanding have seriously broken down as soon as we entertain the assertion “God exists.” Yet, just as he tries to beat against the barriers to our understanding when it comes to “God exists,” he aims to indicate that people are not talking nonsense when they affirm the doctrine of the Trinity. Hence what we find in SCG 4,11–14, and hence what we find in SCG 4,19 and 23.
Here Aquinas begins by noting that something with understanding must also will or desire. He argues:
In every intellectual nature a will must be discovered. For an intellect is made to be in act by an intelligible form so far as it is understanding, as a natural thing is made to be in act in its natural being by its proper form. But a natural thing, through the form by which it is perfected in its species, has an inclination to its proper operations and to its proper end, which it achieves by operations … and it tends to what is fitting for itself. Hence, also, from an intelligible form there must follow in people who understand an inclination to their proper operations and their proper end. Of course, this inclination in an intellectual nature is the will, which is the principle of operations in us, those by which we who understand operate for an end. For end and the good are the will’s object. One must, therefore, discover a will in everyone who understands.54
Here Aquinas is saying that, as understanding, we are drawn to what we think of as good for us. And he thinks that this being drawn is love in us. He holds that we love what we desire or are attracted to.55 We are drawn to what we delight in. “To be affected toward something,” he says, “is to love that thing.”56 He adds:
From the fact that we love something we desire that thing if it is absent; we rejoice, of course, if it is present; and we are sad when we are kept from it; and we hate those things which keep us from what we love, and grow angry against them.57
Aquinas then goes on to say that what is loved is in the intellect of the lover as well as in the lover’s will. He takes it to be in the lover’s will as an end to which the lover is drawn, and he thinks that if the lover attains such an end, then the will of the lover comes to be at rest as delighting in what it has gained.
Now, Aquinas is clear that there is intellect in God (see chapter 7 here). So, he has no problem in thinking of there being will in God (see chapter 8 here). Given his thoughts on divine simplicity, however, he cannot suppose that knowledge or will in God amount to accidents that come to exist in God as God moves from one way of being to another. Consequently, he argues that both knowledge and will in God are God and that love in God is God.
What, though, can God will just by being God and not by creating creatures to love? Obviously, thinks Aquinas, the answer to this question has to be that God as loving by essence has to love God. So, he argues, we can think of the Holy Spirit as the love that eternally comes forth as God the Father and God the Son know and love each other. Aquinas has already maintained that God the Son proceeds from God the Father by generation. He has said that God the Son is nothing but God as generated by God the Father and that God the Father is nothing but God as generating God the Son. In SCG 4,19 Aquinas suggests that we can think of God the Holy Spirit as eternally coming forth as God the Father and God the Son love each other. We can think of there being in God not just generating and being generated. We can think of there being love as the Father loves the Son and as the Son loves the Father. This love, says Aquinas, is the Holy Spirit, which is not an accident in God but is what God eternally and unchangeably is. He writes:
A thing’s proceeding in order to be in the will as the beloved is in the lover is not a proceeding by way of generation, just as a thing’s proceeding in order to be in the intellect does have the essentials of generation. … Therefore, God proceeding by way of love does not proceed as begotten. … But, because the beloved in the will exists as inclining, and somehow inwardly impelling the lover toward the very thing beloved, and an impulse of a living thing from within belongs to a spirit, this is suitable: that God proceeding by way of love should be called God’s “spirit,” as it were a kind of existing spiration.58
The idea here is that the Holy Spirit can be thought of as distinct in relation to its origin, as God the Son is distinct from God the Father when it comes to the relationship between generator and generated. Distinct how? Not as what is known can be thought of as distinct from a knower, but distinct as proceeding as love between things. Aquinas thinks that God the Father finds in God the Son a likeness of what God the Father is. He also thinks that it is not stupid to say that God the Father and God the Son can love what they see in each other, this love being the Holy Spirit. Aquinas takes this thought to commit him to saying that the Holy Spirit “proceeds” from the Father and the Son and not from the Father alone. As Herbert McCabe notes:
It is essential to Aquinas’s doctrine that the Holy Spirit proceeds from both the Father and the Son, and not merely from the Father. The reason for this is that the only distinction admissible in the Trinity is that of being at opposite ends of a relation based on a procession of origination. If the Holy Spirit does not proceed from the Son, there is no such relation between them and therefore no distinction between the Son and the Holy Spirit. Thus in Aquinas’s account there are two processions in God, one of the intellect, God’s knowing himself, which is generation, and one of the will, God’s enjoying himself. … Each of these gives rise to a relationship with two (opposite) ends, the origin and the originated. There are thus four of these relations. This does not, however, result in four distinct persons, for in order to be distinct, a person must be at the opposite end from both other persons. The Father is opposed to the Son by generating and to the Spirit by spiration. The Son is opposed to the Father by being generated and to the Spirit by spiration. The Spirit is opposed to both the Father and the Son by being spirated.59
In adopting this position Aquinas, of course, is taking sides on a matter that has divided Eastern Christian churches and the Western (Latin) church for centuries.60 The belief that the Spirit proceeds from both the Father and the Son was defended by the Council of Lyons (1274) and the Council of Florence (1439) and was widely held in the West long before those councils. It features, for example, in the so-called Athanasian Creed (c. 381–428). However, this belief is not explicitly asserted in the Nicene Creed or in the original text of the Nicene-Constantinopolitan Creed. Nevertheless, Aquinas upholds it and turns to its defense in SCG 4,24–25.
He begins by arguing that it coheres with the New Testament and the teaching of eminent Christian theologians since New Testament times, such as St. Augustine. Then he says that the truth that the Spirit proceeds from the Son as well as the Father is “also clarified by straight reasoning” (etiam evidentibus rationibus apparet). He writes:
Among things, with the material distinction gone (and in the divine persons such can have no place), one discovers no differentiation except by some opposition. For things which have no opposition to one another can be simultaneously in something identical; thus, no distinction can be caused by them. Take white and triangular. Although they are diverse, they can, because they are not opposed, be in an identical thing. But one must set down, according to the documents of the Catholic faith, that the Holy Spirit is distinct from the Son; otherwise there would not be a Trinity, but a duality of persons. Therefore, a distinction of this kind must take place through some opposition.61
Aquinas goes on to say that when it comes to the persons of the Trinity, opposition cannot be, for example, that of affirmation and negation, which distinguishes what is from what is not. Nor can it be opposition of contrariety (i.e., difference springing from what Aquinas refers to as “form” or “nature”) since each divine person is divine and is but one form, that of God, whose nature is ipsum esse subsistens (“subsistent Being Itself”; see chapter 4 here). So, Aquinas concludes:
One divine person is not distinguished from another except by the opposition of relation: thus, the Son is distinguished from the Father consequently to the relative opposition of father and son. For in the divine persons there can be no relative opposition except according to origin. … Therefore, if the Holy Spirit is distinguished from the Son, he is necessarily from the Son. … The Son is from the Father and so is the Holy Spirit. Therefore, the Father must be related both to the Son and the Holy Spirit as a principle or origin to that which is from the principle or origin. He is related to the Son by reason of paternity, but not to the Holy Spirit; for then the Holy Spirit would be the Son, because paternity is a relation to nothing but a son. There must, then, be another relation to the Father by which he is related to the Holy Spirit; and we call this “spiration.” In the same way, since there is in the Son a relation by which he is related to the Father, the name of which is sonship, there must also be in the Holy Spirit another relation by which he is related to the Father, and this is called procession. And thus, in accord with the origin of the Son from the Father, there are two relations, one in the originator, the other in the originated: that is, spiration and procession. Therefore, paternity and spiration do not constitute two persons, but pertain to the one person of the Father, for they have no opposition to one another. Therefore, neither would sonship and procession constitute two persons, but would pertain to one, unless they had an opposition to one another. But there is no opposition to assign save that by way of origin. Hence, there must be an opposition of origin between the Son and the Holy Spirit so that the one is from the other.62
Aquinas is not here denying primacy in the Trinity to God the Father. Nor is he suggesting that the Holy Spirit is inferior to the Father and the Son. He is arguing that if there is distinction in God as taught by the doctrine of the Trinity, the distinction has to arise (albeit eternally) from there being what is at opposite ends of a relation based on a coming forth. When it comes to the Holy Spirit, he thinks, this is a coming forth based on the generation of the Son by the Father, something that amounts to the love between them. For Aquinas, the Holy Spirit is not a likeness of God, as he takes God the Son to be as conceived by God the Father as knowing himself. Rather, the Holy Spirit is an enjoyment or delight that eternally exists as the Father and the Son love each other, and it is in this sense that Aquinas thinks of God the Holy Spirit as a person.
What, though, can it mean to say that there are divine persons? Aquinas knows that what he took to be orthodox theology speaks of there being three persons in God. When it comes to the Trinity, however, he is not remotely inclined to think that “person” has to mean “human being” or “center of consciousness” or something along those lines. Three thinking individuals (whether bodily or not) cannot be one thinking individual. Yet, says Aquinas, we can use the word “person” to note what Father, Son, and Spirit each are. He thinks that the Father is a person and that the Son and Holy Spirit are persons. Why so? Because he thinks that “person” can be used to mean “individual” in some sense.
There is a famous definition of “person” deriving from Boethius (c. 480–524), and Aquinas often quotes it with approval. According to this definition, a person is “an individual substance of rational nature” (Persona est rationalis naturae individua substantia).63 It is, however, clear that, for Aquinas, the persons of the Trinity are (1) not individuals (considered as members of a kind), (2) not substances (considered as individually existing things that exist independently of other things), (3) not rational (as going through processes of reasoning), and (4) not things having a nature to be distinguished from what they essentially are.
When it comes to his discussion of the Trinity in SCG 4, it is not the Boethian understanding of “person” that is at the forefront of Aquinas’s thinking. What is at the forefront is the notion of relation. In his discussion of the Trinity in the SCG, as well as in his discussion of it in the Summa Theologiae, Aquinas seems intent to show that there is a unique sense in which Father, Son, and Holy Spirit can be thought of as distinct even though they are not members of a kind, not individually existing things that exist independently of other things, not things going through processes of reasoning, and not things having a nature to be distinguished from what they are. As McCabe observes, “For Aquinas the key to the Trinity is not the notion of person but of relation.”64 As McCabe also notes, we should not read Aquinas as being sexually biased as he cheerfully talks about God the Father and God the Son. Aquinas inherits such gender language from the Bible; but when he talks about the Trinity, gender is not an issue. As I have shown, in SCG 4,24 he writes as though conception always results in a son. He says: “Paternity is a relation to nothing but a son,” which is manifestly false. Yet one can hardly suppose that Aquinas seriously thought otherwise. He knew that there can be fathers of daughters. As he writes about Father and Son when it comes to the Trinity, he appears to be somewhat mesmerized by biblical texts in which God is said to have a son. He is also, I presume, influenced by the thought, common in his day, that women did not contribute physically to the human generative process. But his discussion of the Trinity in SCG 4 could, without losing anything of importance in its reasoning, be rephrased so as to leave Aquinas saying, for example, that God the Son is generated from God the parents. As McCabe notes when commenting on Aquinas’s Trinitarian thinking, “the plural ‘parents’ would be no more misleading than is the sexual connotation of ‘Father.’”65