Chapter 2: Births
2
Births
Nature, Natural Philosophy, and the Hylomorphic Analysis of Change
Back to natura
The word natura and offshoots of it run throughout Thomas’s writings, and they play crucial roles both in his philosophy and in his theology. The word’s English cognates are familiar enough.59 Nevertheless, what Thomas takes to be its most proper meaning is now anything but current. Here is a sign of its strangeness. Using the word in what he considers its most proper sense, he says that a stone’s moving upward is “against nature.”60 Would anyone today say that? Would you even feel sure enough of its meaning to disagree with it?
Thomas can hardly mean that such an event is weird or abnormal. Boys make it happen all the time. Nor can he mean that making it happen is wrong or immoral. Picking up a piece of straw would be against nature in this same sense, and yet he judges it, in itself, morally indifferent.61 Is it that moving downward rather than upward is what stones usually do? This is closer, but it is still not quite the idea. What he means is that a stone has in itself an inclination to move downwards—that is, toward the earth’s center—and that this inclination, which he calls gravitas (heaviness), arises from within it, from its natura—whatever that is.
Now, we will probably be inclined to say that it does not matter what natura is. Such a view of gravity must be wrong in any case. The tendency of stones to move downward, we are taught, is the effect, not of anything within them, but of the force of the earth’s gravity acting to pull them down. And we would surely also say that if their yielding to this force is any sense natural, their yielding to the greater force of a throw is so too, in the same sense.
In part, I suppose, this is what is called a merely empirical question. Thomas thought that falling was a rather localized phenomenon, confined to some of the kinds of bodies that exist under the moon. Had he seen the evidence that celestial and terrestrial things differ much less than he thought they did, perhaps he would have accepted the view that gravitas affects all kinds of bodies and that it is not strictly relative to the center of the earth.62 But there is also a broader issue here, not about gravitation, but about the actions of things generally.
We may accept easily enough that a given kind of thing has certain active properties. An electron, we say, has a negative charge. But we probably do not think of the electron itself as giving rise to the charge. Its having the charge is just a brute fact about it. The very word charge suggests a load that something just happens to carry. Presumably if it did not have the charge, we would not call the thing an electron. But do we really think of the charge as essential to or inseparable from that thing? Most of us, I suspect, conceive the situation in a rather mechanistic way. We tend to picture energy, forces, and the like as free-floating entities, and bodies as their mere receptacles—quite inert receptacles, ready to be pushed or pulled anywhere, or nowhere, and to be made to do anything, or nothing. Such a body can take a charge or leave it.
A few of us might deny the need to posit such bodies at all. And in fact I think Thomas would be rather sympathetic to that view. Still, he does posit bodies of some sort, and he distinguishes between a body, or the inner makeup of a body, and the active properties that it may have. For its inner makeup is all its own, while its active properties are relative to other bodies and presuppose its makeup. But he definitely does not think of the makeup as inert. It is not itself an active property or set of properties, but it is a source of them. He does not deny that things can be also subjected to outside forces; he knows that stones can be impelled upward. But the important point, and the main one that I hope to get across in this chapter, is that, in his world, there is an order or a level of power or energy that is distinct from and prior to that of motion and change and process and interaction. We can call it the substantial level.
But let us return to the question of what Thomas means by natura. Is it just the makeup of a thing? That would not be so strange to us, would it? But it is not quite right either. For Thomas, the makeup of a thing is not a natura, in the proper sense, unless it gives rise to properties that pertain to motion and change. By contrast, we will just as easily call the makeup of a thing its nature, whether or not it has any such properties. We speak of the nature of a triangle, of a concept, of anything. Thomas does grant that natura can be used in this very broad sense. But he considers it a merely derivative sense and rather diluted—not to say denatured. Natura in the proper sense is for him something exclusively physical.
In a way, of course, this is a mere redundancy, since phusis is just Greek for nature. But clearly not everything that we call physical is something that Thomas regards as properly natural. What is more physical than a stone’s being thrown upward? And although we do sometimes speak of nature as something exclusively physical, we usually mean either the whole of corporeal reality, or else a single power or force driving and regulating the whole—something like Mother Nature. For Thomas there are many natures, as many as the kinds of physical things. Nor are the bodies themselves nature, in the strict sense. They are by nature.
I have only been trying to convey the strangeness of Thomas’s concept of nature, not to explain it. Almost inevitably, readers not alerted to its strangeness will either misconstrue much of what he says (or is reported to say) about it, or else find it all hopelessly muddled and defying construal altogether. Now I shall try to explain the concept, along with related notions. I hope that, by the end, even if still strange, it also seems serious. Thomas holds that the order of nature is instituted by divine wisdom, and that reason, virtue, and even grace, imitate it.63 If his concept of nature is not serious, many of his other concepts are not either.
A Branch of Philosophy
In Lewis and Short’s Latin-English dictionary, the first meaning listed for natura is birth. Thomas confirms this. In common usage, he tells us, natura refers to nativitas (birth) or nascentia (things born).64 English has lost the verbal connection, but as a way of saying that someone is or acts somehow by nature, we might well say that he or she was born that way. Birth or begetting or generation, the bringing forth of another individual of the same kind as the begetter or the generator, is for Thomas, as for Aristotle, the most natural event of all. By it, Aristotle says, corruptible beings share as far as they can in the immortal and the divine, achieving perpetual existence at least for their kind, even though the individuals perish.65
For Thomas, natural birth also points to the divine in a way that Aristotle could not have imagined. It reflects the birth in the deity itself, the begetting of the Son of God. However, it is not to this that Thomas refers when he speaks, as he often does, of God’s nature. It hardly could be, since the Father too has that nature, and He is not begotten. The Son is begotten, genitum, in the proper sense, but His divinity is a natura only by extension. This is because it is not physical.
Still, Thomas does find this truth about God putting ordinary physical nature, especially human nature, in a new light. Following the tradition, he holds that our being in the image of God, which sets us off from other bodily creatures, refers chiefly to the spiritual dimension of our makeup. Thomas also holds that, on the whole, angels are in God’s image more perfectly than man is. This is because their spiritual makeup, which is their entire makeup, is more perfect. But he also finds two ways in which man alone is in God’s image.66 One is that “man is from man as God from God.” Angels do not beget angels. That is a fairly easy idea.
The other way is that man’s soul is whole in his whole body and also whole in every part of it, which is how God is in the world. This is rather difficult. It rests on a concept touched upon briefly in the first chapter, that of the human soul as the human body’s substantial form. This notion, substantial form, is absolutely fundamental in Thomas’s philosophy. It is crucial even for having some positive notion of spiritual things. I mention it now because, in fact, the most proper meaning of natura will turn out to be precisely this, a body’s substantial form. Seeing how Thomas reaches this conception of nature will occupy most of this chapter.
But returning to the notion of birth, what removes the begetting of God’s Son from the domain of natura in the proper sense is what I mentioned before: it involves no motion or change. The Son did not come into being. Nor did He grow up. Aristotle associates the word phusis with phuesthai, to grow.67 The Son’s birth is eternal, and He is eternally equal to the Father. Thomas does call the Son’s begetting a procession, but Thomas is no process theologian.68 In his God, “there is no change nor shadow of alteration” (Jas 1:17). Natural begetting is always a change, a coming into being, as well as the first of many other changes or processes in what is begotten. There is always a last one too, a passing away. For Thomas, subjection to processes or motions is quite generally and formally what defines the field of that large branch of inquiry which he calls natural philosophy. The subject matter of this inquiry is ens mobile, mobile being.
Thomas was not as interested in the physical world as Albert was, but he considered natural philosophy absolutely essential to a sound philosophical curriculum, and indispensable as a preparation for the study of metaphysics. Today these convictions may not be easy to appreciate, because the very notion of natural philosophy has become rather unfamiliar. It is not the same as what we call the philosophy of science. That centers on the modes of thinking involved in scientific knowledge. The closest thing to it, in Thomas’s philosophy, would be the kind of study offered in Aristotle’s Posterior Analytics, which Thomas considers part of logic. But what he calls natural philosophy is not about natural science. It is about natural reality. Indeed, in his sense of the term scientia, natural philosophy is natural science.
Obviously if by natural science we mean the type of knowledge that is gained through modern methods of experimentation, there is little or none of it in Thomas, under any name. This, however, does not mean that his natural science is mere armchair speculation, remote from observation or experience. He would not regard armchair speculation about nature as science. But experimentation is only one way of gaining experience of things, and only of some things. As for the modern tools of observation, he would surely have welcomed them.
Thomas would also have had no quarrel with our practice of dividing natural science into a multiplicity of disciplines, each concerned with a specific part or aspect of physical reality: physics, chemistry, geology, biology, and so forth. But he would call these disciplines parts of philosophy too; and it seems safe to say that he saw more unity among them, and with philosophy as a whole, than is usually ascribed to them now. The unity among them would be owing to a set of general concepts and principles that all natural sciences use, most of which are treated in Aristotle’s Physics; for instance, the very concept of motion. Like Aristotle, Thomas thought these ought to be mastered before entering into any specific science. One of his earliest works, On the Principles of Nature, offers a brief and lucid synthesis of many of them. Being very general, these principles may seem to us more metaphysical than physical. And they do overlap somewhat with matters that Thomas treats in metaphysics. But even at this general level, he thinks natural philosophy has its own perspective or focus on things—the focal point being, as we said, the mobility of things. The naturalist wants to know why things move and act as they do. This is why those general principles are for him only the beginning. Things move and act in very different ways, and his ultimate aim is an accurate understanding of the specific way of each. The rest of this chapter will lay out some of the general principles. The next will present some of Thomas’s views on one specific type of natural, mobile beings: the living ones.
Mobility and a General Definition of Nature
Now, if the field of natural philosophy is physical reality, one might wonder why the primary defining feature of its subject is said to be motion or mobility, and not corporeity. Thomas’s answer would be that there is also another kind of science that considers bodily things, but not insofar as they are natural or physical or mobile. A body is a substance with three dimensions. We can consider this in abstraction from any motions or actions, and even any tangible properties, to which it might be subject. That is what Thomas thinks mathematicians do. Their field, he says, is “substance as subject to quantity.”69 Birth and death and the processes in between are not quantities. In abstracting from them, the mathematician abstracts from what makes bodies physical or natural. Of course, natural processes are associated with quantities, and mathematics has many applications in natural science. Thomas recognizes such applications, and he thinks they play decisive roles in certain natural sciences, for example, optics and astronomy. But he would not agree with Galileo that the book of nature is written in the language of mathematics. On the whole, Thomas regards mathematics as rather secondary in the study of physical things. Such things have their own proper principles, and these have to be understood on their own terms.
On the other hand, obviously the field of mobile beings does extend beyond those that are born, in the proper sense of the term. Only living things are properly said to be born. In one place Aristotle calls motion a kind of life in the universe, but Thomas says this is a metaphor.70 Still, he does think there is something like birth among inanimate things, namely generation or coming into being. The generation of non-living things differs from birth, he says, in that what is born, generally speaking, begins as somehow joined or united to what generates it. What is born is, as we say, an “offspring” of its parent(s). For instance, a seed starts off inside, or even as part of, the tree that produces it. Inanimate things usually originate in something detached from what generates them, as a fire is started in one log by the fire in another log. But Thomas also thinks that even inanimate things can be seen to be joined with some kind of source of motion; for example, as we saw, he thinks a stone has in itself a principle of moving downward. (Even we must grant that it has in itself some sort of principle by which it can be pulled downward by the earth’s attraction. After all, not everything can. Numbers, and angels, fall only metaphorically.) As emerging from such a conjoined principle, the thing’s motion is like a birth. That, for Thomas, is how inanimate things came to be spoken of as natural. It is not that all such things are generated; indeed, he thinks some of them are not, for instance the stars (which we say are born and die). It is that they are joined to principles or sources of motions that they undergo.
This, however, raises another question. Thomas distinguishes between the natural and the artificial. But are not artifacts mobile too, and joined to principles of motion? A cellphone can fall as easily as a stone. A desk can be burned. And think of the very word automobile.
Thomas would say that, yes, in a sense such things are mobile, but not because of their being such things. A cellphone falls, not because it is a cellphone, but because it is made of heavy materials. A table can be burned, not because it is a table, but because it is wooden. And despite the name, even an automobile is mobile only in virtue of its natural components: rubber, metal, gasoline, and so forth. These things do not act as they do because they are parts of an automobile. In short, when Thomas defines the natural as the mobile, he means mobile per se, in virtue of itself. Artifacts are not mobile in virtue of what is artificial about them, but only by association, in virtue of what is natural in them. However, he does distinguish between artifacts and natural things produced under artificial conditions. For instance, he considers bread a natural substance. The baker only prepares for its production. What really produces it is the natural action of fire.71
What about the artifact-maker’s own skill? Is that not the source of a motion, namely, the very process of making the artifact? Yet, even though the maker is himself a natural being, it is not in view of this skill that we call him natural. Why not? Because the skill is not joined to what undergoes the process. The baker does not bake himself. Or at least that is true for most skills. The process, the being made, is usually not in the maker. At most, it only happens to be so in a particular case, as when a health-maker, a physician, makes himself healthy. Aristotle says that nature is like this: a physician who heals himself.72 If a thing is moved by skill, the skill may be, and usually is, in another subject. But if it is moved by nature, the nature is and must be in it.
These considerations bring us close to Aristotle’s famous definition: “nature is a principle and a cause of being moved and of being at rest in the thing to which it belongs primarily and in virtue of that thing and not incidentally.”73 This is rather a mouthful. Thomas offers some helpful remarks about it.74 The first is that the word principle here (and presumably also cause) serves to prevent us from thinking of nature as what Thomas calls an “absolute” item. The principles of different things differ.75 What nature is, in a given thing, is relative to that thing. Nature is not a single, uniform “force instilled in things” (vis insita rebus). There is no Mother Nature. Instead of speaking simply of nature, we should speak of natures, the nature of this or the nature of that.
Thomas says that the definition’s phrase, “in the thing to which it belongs,” serves to set aside artificial things, to which motion (the referent of “it”) belongs only by association with what is natural about them. Again, a table is not burned on account of any nature of table, there being no such thing (in this sense of nature), but on account of the nature of wood. As for the word “primarily,” Thomas explains that it serves to ensure that the motion in question is understood to be the effect of the nature in question. For instance, an animal is a natural thing, and falling is a natural motion; but falling does not belong primarily to animals, i.e., because of their properly animal nature. If it did, only animals would fall. Of course, animals do fall. But Thomas says this is because of the “dominant element” in them, which I suppose would be earth or water. Thomas in fact follows Aristotle in positing four elements—earth, water, air, and fire—as the simplest of the bodily natures existing below the heavens. All the others, inanimate and animate, are in some way formed out of them. Falling is primarily due to the natures of earth and water. We would perhaps say that it is due to the nature of body itself. In any case, it is not due to something proper to animals.
As for the definition’s phrase, “in virtue of itself and not incidentally,” Aristotle himself says that this serves to set aside cases like that of the doctor who heals himself. The medical art is a principle of healing that merely happens, occasionally, to be in that which is healed. It might just as well cause something else to be healed. It is not the nature of what is healed.
The definition speaks not only of motion, but also of rest. Rest is the absence of motion in something mobile.76 But rest too be may be natural to a thing. This is related to the thesis that natural things, as such, tend toward goals. A goal is something that is fit both to move toward and to rest in. When a heavy thing reaches the earth’s center, it tends to stay there.
Aristotle declares that nature, as so defined, is something whose existence is too obvious to be demonstrated; he likens those who seek proof for it to someone blind from birth arguing about colors.77 Thomas says that this is true of nature considered in a general way. The particular nature or principle of motion in a given thing, he says, might not be manifest. But things having in themselves some principle of motion or other are manifest to the senses, and so the existence of nature generally is immediately evident to us; that is, evident without any process of reasoning. It has the status of a first principle of knowledge. Natural science takes nature entirely for granted, and not even metaphysics proves it. Thomas does think, however, that we can get confused even about such obvious truths, and that metaphysics has a role in clearing up the confusion.78 Right now there does seem to be some confusion about nature.
But we have not yet finished with the question of the meaning of the word—far from it. Despite its care and rigor, the above definition of nature does not yet give us its most strict and proper sense. This is because the definition is rather too general, even ambiguous. There is more than one sense in which something can be an intrinsic principle and cause of motion and rest. It might be so in the sense of matter, or it might be so in the sense of form.
In order to understand how these—matter and form—are senses of nature, we need to stand back and take an even wider perspective. Not all instances of matter and form are natures. Following Aristotle, Thomas holds that absolutely all motions—natural, or artificial, or any other sort, if there is any other—depend on factors that function as matter and as form. What follows, then, is a sketch of Thomas’s general account of motion.79
Motion, Its Definition and Its Principles
Motions do not exist by themselves. There is always some thing—some body—that is moved. There is also always some respect in which it is moved. We may tend to think of only one respect, namely, place. We tend to identify motion with locomotion, a body’s going from here to there. But for Thomas, there is motion not only in place, but also in size—growing and shrinking—and in shape and certain other qualities.80 He calls qualitative motion alteration. Other examples of it would be getting warm or cool, soft or hard, angry or calm. We might prefer to call these processes rather than motions—although the results of the last ones cited are called emotions.
A motion or a process has termini. It has a starting-point and an endpoint. Each terminus consists in a form, or something functioning like a form: a quality, a size, a place.81 Reaching one terminus implies having left the other. This is because each implies the negation or lack of the other: being hot implies not being cold, being indoors implies not being outdoors, etc. Between the termini there is an intermediate zone. The termini are its extremes, and as such they are called contraries, either absolutely (if the zone between them is not part of a larger zone), or with respect to that zone. The motion is through the zone. If it is truly one motion and not a series of motions, it is continuous or uninterrupted across the zone.
Of course one terminus does not become the other. Heat does not become cold. Forms are not transformed. Motion needs a third factor, a subject that can have now one form, now the other. A subject of motion is a body. The motion can be one and continuous because the body is.82
As that to which one form or the other belongs and which undergoes the motion, the body is called a subject. As having in itself the capacity or the potential for either form and for the motion between them, it is called matter. Materiality, and potentiality for a form, are not to be confused with the very privation or lack of that form. There must be privation, of course, if there is to be motion; what becomes hot cannot already be hot, but must first lack heat and then obtain it. But whereas its lack of heat ceases once it is hot, its potential for heat certainly does not cease then. Otherwise it would be hot when it no longer can be so, which is absurd.
However, usually Thomas will say that something is in potency to a given form only when it does not actually have the form. When it actually has the form, he says that it is, in that respect, not in potency, but in act. Here the word act—Latin actus—does not mean action or activity. Thomas says that the word originally referred to activity, but form can also be called act, inasmuch as it is both a source and a completion of activity.83 Heat, which for him is a form, can function both as a source of the activity of heating and as its completion. In a later chapter we will see that there is also a more fundamental sense in which form is act.
The form to which a thing is moved, the endpoint, completes the motion. However, the motion itself exists only so long as the completion has not been reached. Likewise, the motion only exists once its other terminus, its starting-point, has been left behind. When the subject is at either terminus, it is not moving from one or to the other.
During the motion, then, its subject is in potency with respect to the termini. On the other hand, we do regard motion itself as a kind of being in act, an activity. To what potential in the subject does this act, the motion itself, correspond? To its potential for the endpoint. Its potential for the starting-point is in a way incidental to the motion. If, having exited the starting-point, the subject happens now to have lost its potential for the starting-point, the motion can still go on; for instance, if what is aging no longer has the potential to be as youthful as it was, it can still be aging. But if it loses its potential for the endpoint, it cannot move to that point; what cannot be decrepit cannot become so. Usually, in fact, motions are named by their endpoints, not their starting-points.84 A thing’s being heated is a process toward heat in the thing, not from it. So a motion is essentially an actualization of its matter’s potential for the motion’s endpoint.
But it is an incomplete actualization. As long as the motion is going on, the endpoint has not been reached. When it is reached, the motion stops. In fact, so long as the motion is going on, there must be more of it still to come. Otherwise it has already stopped. A motion is both an incomplete act and act of something incomplete, something with a potential that is not yet fully actualized. And so Aristotle’s definition of motion, which Thomas fully endorses, is this: “the act of that which is in potency, insofar as it is in potency.”85 The phrase “insofar as it is in potency” means with respect to the very act toward which the thing is moving and to which it is in potency as long as it is moving. This sets the motion off from any act, complete or not, that is merely incidental to it. For instance, if what is being heated is white, then it has both the act of being heated and the act of being white. But it has the being heated insofar as it is in potency to more heat. Its being white does not, as such, entail its being in potency to anything.
That definition of motion is not easy to understand. Some of the anti-Scholastic thinkers of early modernity found it simply unintelligible. But even if we can understand it, we might wonder why we need it. Can we not distinguish what is moved from what is at rest quite well without it? Probably so. But Aristotle and the Scholastics used it to distinguish motion not only from rest, but also from two other things: activity that is not motion, which will be treated in the next chapter, and immediate change, about which something must be said here.
By immediate change, I mean change with no zone between its termini. Such change is therefore instantaneous. The change from being at rest to being moved is of this sort. It is not an act distinct from its termini. It is nothing but the sequence or succession of them. In modern thought, the distinction between motion and immediate change has come to look rather insignificant, because of the practice of treating the continuous as a synthesis of indivisible units (e.g., of treating lines as sets of points). On this view, a motion can be treated as a series of immediate changes. Aristotle, however, had judged that such a view cannot be strictly true.86 Dividing a continuous thing, he saw, does yield indivisible boundaries of the divided parts, as dividing a line yields endpoints. But, he insisted, if the continuous thing is not actually divided, the boundaries are only potential and do not actually exist. We do not start with indivisible units and build up a continuous thing out of them. Points, for example, have no size. How could the mere multiplication of points, by itself, yield something with any size?
This is not to say that immediate changes are not important. One reason why they are important for Thomas is that motions only exist in bodies, but immediate changes can occur in incorporeal beings too. Earlier it was mentioned that not only God but also angels stand outside natural philosophy, because they are not mobile. But only God, for Thomas, is absolutely changeless. Angels can undergo immediate changes, of a spiritual sort; e.g., from not understanding or willing something to understanding or willing it. Of course, we too can undergo such changes. And immediate changes are important in the philosophy of nature itself, because they include substantial change. We shall begin to address that notion shortly.
But returning to the general account of motion, we saw that what is moved, as such, is always on the way toward something. It is ordered to some form or act that it does not yet have. As Thomas sees it, this is unintelligible unless there is also something else in the picture: an origin of the motion, and of the form or act reached through it. A motion needs an agent, a mover.87 Nothing is brought from being in potency to being in act except by something already in act, as water is heated by something already hot. We first conceive of action or agency, Thomas says, in encountering motion. And this conditions all our action-concepts, even those that exclude motion and potentiality, such as the concepts of the Trinitarian processions.88
Thomas is not saying that the mover must literally possess the very same kind of act as that toward which the motion’s subject is moved. Heating your coffee with a cool microwave does not refute him. He is saying that the mover must have an act sufficient to produce the motion. We would probably rather say that it must have sufficient energy. But then, energy comes from Aristotle’s energeia, which passed into Thomas’s Latin precisely as actus.89
Also, Thomas does not think that the mover must be acting throughout the motion’s duration. It may only deposit a kind of determination or impulse, as it were a kinetic germ, out of which the motion later unfolds. This is how Thomas conceives the heaviness of a heavy body. It was received along with and through the body’s nature, from the body’s producer or generator. The motion that follows on it may only occur later on. If there is an agent acting right then when the body falls, it is only an indirect one, removing some obstacle to the fall, as when you let go of a stone that you are holding. And even that sort of agent operates only at the start of the fall.
Finally, not even the presence of an agent is enough to make a motion fully intelligible. We could at least imagine the agent’s being able to move the thing, having the sufficient energy, but not actually doing so. The agent must also have some inclination or tendency toward ordering the thing to an endpoint and producing the motion to it. If it had no such tendency, it would not act in this way rather than that. It would do nothing.90 The endpoint must engage its energy, so that it move the thing in that direction. The endpoint, although it does not yet exist, is already making a difference to what is going on. The agent acts because of it, for its sake. The endpoint is an end—finis—not only as a stop, but also as a goal.
Thus, in very broad terms, the general account of motion invokes the famous doctrine of the four kinds of cause: material, formal, agent or efficient, and final.91 Later we will see how the kinds of cause can be traced back to matter and form themselves, at least in natural things. But first we must identify the matter and the form that constitute senses of natura.
Substance and Accidents
Motions, we said, are with respect to a body’s place, size, or quality. These are categories of what Aristotle calls accidents. Bodies are in the category of substance. For us, neither term is very felicitous. Accident can sound like a mistake or a coincidence, something unintended. Aristotle too uses it in that sense sometimes.92 But when he applies it to certain categories, he simply means an accompaniment or an addition, a further modification of something (viz., of a substance). It may very well be intended, and it may be very important. Thus, as Thomas explains, no substance is complete or perfect merely by being the substance that it is. Its full perfection, its unqualified goodness, needs additional being, which is through accidents: size and shape, qualities, relations.93 Virtue, for example, is a quality.94 Friendship is a relation.95
Still, no matter how important accidents may be for the perfection of a thing, they are not the thing. The thing is a substance. Accident and substance differ radically, so radically that in a sense only substances exist. Calling a thing a substance means that it is a genuine subject of being. A substance is something that truly is. We do speak of accidents as though they were subjects of being, for instance when we say that courage is a virtue. But really it only confers some being on a substance.96 What is courageous is not the soldier’s courage; it is the soldier. Accidents do not even exist in the sense in which parts of a substance do. A part, for instance a nose, is a true subject of being, albeit only partial. It shares in the being of the whole. So parts too are substances.97 Thomas says that accidents “inhere” in substances, but it is not as parts do. For accidents inhere in parts too, as snubness inheres in noses. But whereas parts can have parts, accidents cannot have accidents.98
As for the term substance, it sounds like what stands under the rest. And since motion is with respect to accidents, it also sounds like something static or inert. One imagines a stage or a platform, with everything happening above it. Aristotle, however, says the most proper mark of a substance is that, “while remaining numerically one and the same, it is capable of admitting contrary qualities through a change in itself.”99 Notice that he does not say simply “while remaining one and the same,” without qualification, as though it stayed unvaried in all respects; that would indeed make it inert, and also changeless. He says “numerically” one and the same—that is, the same individual. Socrates changes from fat to thin, vicious to virtuous, bearded to clean-shaven, etc., only if he remain Socrates. Fatness, vice, beardedness—these do not change while they remain; they simply appear or disappear. Notice too “a change in itself.” Again, heat does not change into cold. A hot substance changes into a cold substance. Also statements and opinions, which can change from being true to being false, do so only through changes in what they are about, not in themselves.100 Aristotle is saying that if there were no substance, there would be no motion. Of course he does posit an unmoved substance. But it makes all the others move. It is pure activity. In general, the substances are the things that act or are acted upon, or both. A nose can grow, bleed, be punched. Its snubness cannot. Not even its growing or bleeding or being punched can. Motions do not undergo motion.101
Substantial Change and Protomatter
Aristotle’s unmoved substance is everlasting. Unlike many of his predecessors, however, he holds that a large number of substances come into being and pass away. Plants and animals, for instance, are born and die. Obviously his predecessors knew that. But on their views, these things were not substances. They thought the substances, the real beings, were the underlying components or materials. Those were the primary subjects of change, and they did not come into being or pass away. According to Aristotle, Thales attributed this function to water; Heraclitus, to fire; Empedocles, to the four elements of earth, water, air, and fire.102 Similarly, Democritus posited a huge multitude of tiny, highly mobile bodies that were indivisible—atomic, in the original sense—as the first and permanent subjects of change. All else was made out of them.103
For Aristotle, even the simplest earthly bodies, the four elements, come into being and pass away. Fires are lit and extinguished. Water condenses and evaporates. Minerals dissolve. Also in support of this view, he judged, is that simpler bodies can be blended into new kinds, such as living kinds—plants or animals. The blends have their own natures. They do not act in the way that mere combinations would act. They may show some of the properties of the elements, and when they corrupt, the elements may be regenerated out of them; but while they exist, the elements are in them only potentially, not actually. We will return to this idea.104
Our present concern is the very idea of substances coming into being and passing away. It is not an easy idea. A major difficulty arises from Aristotle’s own hylomorphic analysis of motion and change. Substantial generation and corruption, if they exist, consist in the conversion of one substance into another. They are supposed to be quite commonplace occurrences. They are not a substance’s sheer popping into or out of existence, out of or into nothing. They allegedly happen when you eat and digest an apple. They happen when an animal dies. Thomas, I suppose, would say that they happen when hydrogen and oxygen are made into water. The problem then is, what is it that underlies the conversion of one substance into another? What functions as the matter?
We must not forget that matter, in this philosophy, is just like nature: it is a relative term, not absolute. It does not at all mean stuff. Relative to a statue or to the process of making it, marble or wood is matter. Relative to a syllable, letters are matter. What is matter for one thing may have another as its matter. Bricks are matter for a house, and (say) earth is matter for bricks. The Presocratics were saying, in effect, that there is some kind of thing, or set of kinds, having nothing else as matter, and functioning as the first matter for everything else.105
We may find their physics crude, but with respect to the doctrine of matter, similar views have also appeared much more recently. Descartes, for instance, held that what underlies all bodily changes was what he called res extensa, extended reality. (It sounds just like substance as subject to quantity—Thomas’s description of the field of mathematics.) The res extensa is ingenerable and incorruptible, never turned into anything else nor what anything else ever turns into. It is the first matter, and substance, of all physical things. It receives all motion and all kinetic properties. It differs from the Presocratics’ substance only in being utterly inert and passive.
But returning to Aristotle and Thomas: if there is substantial change, there has to be a subject or a matter underlying it. Substances function as matter for accidental changes. But this matter will be prior to them and will have no matter of its own. It will be the first matter. Thomists writing in English commonly call it prime matter, though William Wallace calls it protomatter.106 Here too, however, matter is only a relative term. That which is called prime matter or protomatter is called matter relative to those substantial changes. But what is it just in itself?
Nothing. That is, nothing actual, nothing definite. If it were something, it would already be a substance. What is something just in itself, with its own being and own intelligibility, existing separately and not just relative to something else, is precisely a substance. Of course, matter is not an accident either, added on to a substance. But accidents are essentially only modifications of substances, and prime matter is essentially only potency for substances. If we want to distinguish prime matter from accidents, we can call it substance in a qualified sense or substance in potency. But if we remove all reference to substance in the unqualified sense, which is actual substance, both accidents and prime matter simply disappear. And especially matter does. Accidents are at least forms and bring some actual being with them, even if what has the being is a substance. But prime matter is only potency, and it has no form of its own. This is why Thomas holds that not even God can make it exist by itself, apart from any kind of substance. In that case it would actually be, but there would be nothing that it actually is, which is absurd. “It is the same to say that matter is in act and that matter has form. So, to say that matter is in act without form is to say that contradictories hold together. Hence, it cannot be made by God.”107
If there is nothing that prime matter is in itself, is it only a being of reason, not real at all? That might be the verdict if we reached it by mere logical analysis, stripping away whatever is predicated of a subject and leaving something that seems to be totally indeterminate and unidentifiable. This would perhaps not show that there is any such thing in reality. But Thomas insists that prime matter is only fully brought to light through the analysis of change.108 The reality of prime matter stands or falls with the reality of substantial change. If substantial change is real, then so is prime matter; and, unlike what is nowadays called dark matter, it is no mere conjecture.109 However, in speaking of it as we do, we do give it more being than it really has. We give it a name, an identity, and things are named and identified according to their forms.110 But matter does not have a form of its own. It only has an order toward form, habitudo ad formam.111 We treat this as a sort of form in its own right. That is how we can make matter a subject of propositions. In reality it is not a full-fledged subject, but only potency for a subject.
We should not think of prime matter as immediately ready to be just anything you please. A sow’s ear cannot become a silk purse at once. This is so even though generation and corruption, in themselves, are immediate changes, not motions.112 An example is an animal’s death. Between a cow and its carcass there is no middle zone. Nevertheless, a substance has accidents that are characteristic of it, and its corruption occurs as the terminus of a process (or processes) of accidental change; e.g., aging or cooling or being cut up. And its generation must be preceded by processes disposing for its characteristic accidents. So there are orders regularly followed in the sequences of generations and corruptions. As Aristotle observes, by a series of processes water can become wine, and wine can immediately become vinegar, but vinegar cannot immediately become wine; it must go back to water first.113
Substantial Form and the Meanings of natura
Now, just as substantial change implies an underlying matter that is substance in potency, so too it implies acts by which the substances serving as its termini are successively in act. Aristotle calls such acts substance as form. Thomas calls them substantial forms. It would be hard to overstate the importance of substantial form in Thomas’s philosophy.
Earlier, in discussing motion, we remarked that an accident or accidental form confers a certain being on the substance to which it belongs. It makes the subject be in a certain way, as heat makes water be hot. However, accidents do not give what Thomas calls esse simpliciter, unqualified being, the being according to which a substance simply is or exists. Accidents presuppose the substance and its unqualified being, and they add further, qualified being: being hot, being so large, etc. What makes a substance simply be, however, is its substantial form. Exactly how a thing’s substantial form relates to its being is a topic for metaphysics, and we will take it up in detail later. But we must at least say something about substantial form as a principle of being here, because it bears directly on the notion of nature.
Aristotle says that what has a nature is always a substance, and that what exists by nature includes substances and their accidents and their motions.114 The nature of a thing, then, is not solely a principle of the thing’s motion. That may be why it is called a nature, but in fact it is also a principle by which the thing exists. Now, the analysis of the generation and corruption of natural substances shows that their existence does indeed have principles: their matter and their form. However, the matter is only potential for a substance. A substance exists without qualification, not when it is merely in potency, but when it is in act. It is in act in virtue of its form. So the form is more truly and properly a principle of its being than the matter is.
And the form is also more truly its nature. With it, in fact, we have finally reached what Thomas considers the primary sense of the word natura.115 This fits with the fact that the nature of a thing is intrinsic to it. We do not say that something has the nature of a tree, for example, until it has in it the form of a tree—until it actually is a tree. Moreover, nature is an intrinsic principle of motion and rest, and things of different kinds have very different ways of moving and resting. So they have very different natures. But what decides a thing’s kind is its form.
Here, however, lies an important distinction, and also still another sense of nature. The distinction is between a thing’s substantial form and its kind. They are close, but Thomas will not quite identify them. A sign of their closeness is the fact that, to the chagrin of his readers, Aristotle uses the same word for both: eidos. Thomas uses forma for form; for kind, he uses species. Our aspect is related to this, and both eidos and species refer to a look or an aspect. In this case, though, it is not visible but intelligible. In fact, one way to explain the notion of kind is with reference to intellect. We understand a thing insofar as we know what it is. But knowing what it is is tantamount to knowing its kind. What is Lassie? A dog; her kind is dog.
Her substantial form, however, is not dog. It is only a principle of her being a dog. And at least on Thomas’s view, it is not the only principle. It is only part of what makes Lassie a dog, though indeed, it is the really decisive part. To show how it is the decisive part, Aristotle offers a helpful comparison, using the syllable ba.116 He likens the letters a and b to the elements out of which a natural thing is generated. Then he observes that what makes the letters be the syllable ba is not at all another element. It is rather the order of the letters. This is a sort of form. (Notice that, unlike shape or color or other typical examples of form, this one cannot be pictured in itself. Neither can a substantial form.) And this form cannot even be thought of apart from the letters or from ba. But it can be distinguished from them, and whereas b and a are indeterminate as between ba and ab (and as between them and no syllable at all), it is quite determinate, and determining. Its presence or absence is what immediately decides the existence or the non-existence of ba.
Nevertheless the letters are clearly principles of ba too. If we were to define the syllable ba, we would certainly have to mention a and b. And certain bodily parts are essential to being a dog. This is precisely because a dog is essentially something natural and mobile. The definition of a natural thing’s kind expresses not only its form but also its matter; not the matter by itself, of course, since (unlike the letters) prime matter is nothing determinate by itself, but the matter as subject to the quantitative and qualitative conditions necessary for moving and acting as that kind of thing does: having flesh or nerves or DNA or protons. So both form and matter are in a thing’s essence. This is the whole set of factors constituting its kind.117
And this, for Thomas, is yet another meaning of natura: the essence of a natural thing, as expressed by the definition of its kind. He will often even treat it as a sort of form in its own right. (Again, not everything that the mind treats as a form really is one, or is even thought to be one.) A definition, after all, is a formula—a “little form.” When we put the kind’s name abstractly, as in dogness, we treat the essence as a simple unit. And we use this as the formal principle, the distinguishing factor, for judging between what is and what is not of that kind.
Finally, since the essence of a natural thing is called nature, any and every essence can be called a nature, “by a certain metaphor and extension of the name.”118 This is that very broad sense of nature that applies to just about everything, whether or not it is physical or mobile. But the chief and most proper sense of nature is the substantial form of a mobile substance.
Taking a bird’s-eye view of the meanings of natura, Thomas says that generation or birth is what we first call nature, and form is last, because we know the forms of things by their acts. But really nothing has nature except insofar as it has form; and so, “chiefly and properly what is called nature is . . . the form of things having in themselves a principle of motion, as such. For matter is said to be nature because it is receptive of form. And generations have the name of nature because they are motions proceeding from form and also toward forms.”119
Substantial Form as Unitive Energy
Now let us return to the idea mentioned at the start of this chapter, the idea of a thing’s makeup being a source of its active properties. Perhaps we can make sense of this idea, if we focus on the role of substantial form. It is analogous to the role played by the form of ba. The form of ba is what unites the letters and makes one syllable of them. What we need to see is that a body needs a form, simply in order to be a body; in order, that is, to be one body.
Quite generally, Aristotle insists, a continuous thing—a line, a motion, an extension, a body—must have something indivisible in it that makes it one.120 This seems undeniable. What is continuous, as such, is divisible into continuous parts; why then are it and they not divided, in any of the indefinitely many ways in which they might be, but rather are one? This question is not asking for an agent or an origin of the unity, an outside cause putting the thing together. It is asking about the togetherness itself, asking what the thing’s unity consists in. A body is something continuous. Continuity entails infinite divisibility, and hence potency. So a body’s unity entails some act, one that is distinct from the body itself.121
Now, when we call a body one, what do we mean? We do not mean only that its surface looks or feels continuous. We also mean that it moves and acts as a unit. The whole rises or rolls or runs together.122 Of course, what moves as a unit may turn out to be many bodies somehow joined—by nails or glue or hooks or whatever. Many artifacts are like that. But not all bodies can be like that. A nail is not nailed together. And even if particle-forces hold larger bodies together, what holds a particle together? There have to be bodies that are not smaller bodies held together by something else. They may be generable and corruptible, but while they exist, they hold together by . . . nature. Running through each is a unifying factor that is not a body and not divisible, but simple—all or none. This is a substantial form.123
These bodies are what qualify as the truly natural ones. Aristotle thinks that they come in many kinds. Some of them are quite large; e.g., you. And the same form making a body be of the kind that it is also makes it be what he calls a this: a determinate and separate entity, an undivided whole, united in itself and distinct from others. A thing’s being of a given kind is tied to its having a specific way of moving and acting, and so a specific way of being one whole subject of motion and action. There is no such thing as being one and whole but in no specific way. A substance is a this something: this stone, this dog, this human being.
Once all this is grasped, there should be less mystery in Thomas’s claim that the human soul—a substantial form—is in every part of the body, and whole in each. It unifies every part, and it is whole in each because in itself it is all or none, simple. Otherwise it would need a unifier. Of course he knows that a human body’s parts also differ greatly. He thinks the soul is responsible for the diversity too. But this is a peculiarity of soul.124 Common to all substantial forms is to be whole in every part of what they inform. We must not think of a substance’s parts as having distinct unifying principles of their own, like those of an automobile. An automobile does have a form. Its form is like that of ba, the order among its parts. But this is an accidental form. Substantial form runs through all the parts as well, giving them unity too. In whatever it is found, it is the very first form. This is why Thomas insisted that a given substance can only have one substantial form. If a water-molecule is a substance, then the form of water is also the form of its parts. Even if it was produced by combining hydrogen and oxygen, it cannot actually contain such substances or have parts that behave exactly as they do. This does seem to be the case. For instance, hydrogen is immediately combustible, but no part of water is.
So a good way to think of substantial form is as a body’s chief inner cause of unity: unity of its extended parts, of its attributes, and also of its motions and actions over time. The term inner is important; again, the form is not a unifying agent, which would be extrinsic. There is only one this. And most substantial forms cannot exist separately from what they inform at all.125
Matter and form are often said to be “components” of a bodily substance, and Thomas too speaks this way, but it can be misleading. It would at least be better to say that the body’s matter is composed—that is, unified—according to or through its form. Matter and form are distinct, and they are real, but neither is an unqualified this. They are not two kinds of thing needing to be unified by yet a third kind. They are the potency and the act of one entity. Their unity is immediate.126 For the same reason, however, they are not at all on an equal footing. Form is by far the dominant partner. Aristotle even says it is more of a being than matter is.127
This then is how a thing’s makeup can be a source of its active properties. What is a subject of motion or activity must already be an actual unit.128 It must already have a kind of strength or energy, not yet for motion or action, but first of all for its unity, and hence for its being. Thomas in fact calls substantial form virtus essendi, strength to be.129 He also thinks of it as inclination toward being: form “holds a thing in being.”130 Whether a thing is in motion or at rest, it is enduring in its own identity, which is its substantial unity.131 As Thomas sees it, this is itself a sort of event. Even at this level, a thing’s nature is at work.132 And even kinds that do not reproduce or beget—rocks, water, stars—display such energy and tendency. Is it not easy to see how form, so conceived, can also be a source of motion and action?
Perhaps we even catch a glimpse here of why he says that the order of nature is instituted by divine wisdom, and that other principles such as reason, virtue, and even grace, imitate it. Wisdom orders all things to the good, and Thomas understand the good of each thing to be nothing other than the fullness of its being according to its nature. A substantial form is nothing if not an ordering in that direction. And since it is a thing’s very first form, all other good principles must presuppose it and in some way conform to it.
And of course the motions and actions that flow directly from a thing’s substantial form must conform to it. They must be in the direction of the fullness of its being according to its kind. Thomas holds this even in the case of such apparently indifferent motions as the falling of heavy things. The places to which things naturally tend, he argues, must have some power to conserve those things; that must be why they tend there. It is their motion’s final cause.133
Interestingly, in the same passage, he insists that this does not mean that the place aimed at has some “attractive power” (virtutem attractivam), “unless this be understood in the way that an end attracts.” In other words, the place is not acting upon the thing. Heavy things tend to the earth’s center, but the tendency is not produced by the earth’s center. To repeat, its only producer is their generator. Would Thomas have to retract this judgment in view of the mere universality of gravitation? I doubt it. He would only need to modify (and not much) the account of the good that gravitational motion favors. Just as each thing naturally tends toward its own being according to its kind, each also naturally tends toward others of the kind. It is almost the same tendency: toward the fullness of being of the kind. Thomas finds the old proverb, that birds of a feather flock together, expressing a very general truth. He formulates it thus: “Speaking per se, the similar is naturally desirable and lovable.”134
This is not to say that there is no such thing, for Thomas, as acting upon something so as to attract it. Even in his day there was, for example, such a thing as advertising. That sort of action, however, cannot be exercised upon all bodies, or even all natural bodies, but only upon some of those whose forms are what Thomas calls souls.
59. On the history of nature and related terms, very helpful is Lewis, Studies in Words, chapter 2, 24–74.
60. STh, I–II, q. 71, a. 2, obj. 2.
61. See STh, I–II, q. 18, a. 10.
62. This is not to say that he would accept the idea of a force of gravitational attraction. See below, 50.
63. On virtue and grace as imitating the order of nature, see STh, II–II, q. 31, a. 3. On reason as doing so, see chapter 6.
64. In Meta., V, lect. 5, §808; In Phys., II, lect. 1, §145[5]. I draw on these two lectiones throughout this chapter.
65. De anima, II.4, 415a28–b7. See In De an., II, lect. 7, §311–17.
66. STh, I, q. 93, a. 3.
67. Metaph., V.4, 1014b16–18.
68. See Michael J. Dodds, The Unchanging God of Love.
69. STh, I, q. 85, a. 1, ad 2.
70. STh, I, q. 18, a. 1, obj. 1 & ad 1; see Physics, VIII.1, 250b14–15.
71 STh, III, q. 75, a. 6, ad 1.
72. Physics, II.8, 199b31–32.
73. Physics, II.1, 192b21–23.
74. In Phys., II, lect. 1, §145.
75. Cf. Metaph., VII.16, 1040b19–21; XII.4–5, 1070a31–1071b2.
76. Physics, III.2, 202a3–5; IV.12, 221b12–14; VI.2, 226b12–16; Metaph., XI.12, 1068b22–25.
77. Physics, II.1, 193a3–9.
78. See below, 101–3, 166–71.
79. See De princ. nat., cap. 1–2; In Phys., I, lect. 11–13. On Thomas’s reading of Aristotle’s account of motion in terms of matter and form—in Greek, hulē and morphē, whence the name hylomorphism—see McInerny and O’Callaghan, “Saint Thomas Aquinas,” sections 6 and 8. For a presentation of Thomas’s hylomorphism from a contemporary perspective, see Brower, Aquinas’s Ontology of the Material World.
80. See In Phys., V, lect. 3–4. In Meta., XI, lect. 12. Shape is a quality pertaining to quantity: In Phys., VII, lect. 5, §917[5].
81. A place is not properly a form, because it does not inhere in that which has it. But, like a form, it does make that which has it to be in a certain way. Just as having a shape makes a thing be so shaped, e.g., be round, so having a place makes a thing be so placed, e.g., be indoors.
82. See Physics, IV.11, 219a10–13.
83. De pot., q. 1, a. 1.
84. Physics, V.1, 224b7–8. See STh, I, q. 23, a. 1, ad 3.
85. Physics, III.1, 201a10–11.
86. See Physics, VI.1, 231a21–232a22.
87. See In Phys., III, lect. 5, §324[17]; STh, I, q. 2, a. 3, Prima via.
88. STh, I, q. 41, a. 1, ad 2.
89. See the very interesting paper by Thomas McLaughlin, “Act, Potency, and Energy.”
90. See STh, I–II, q. 1, a. 2, and In Meta., VI, lect. 2, §1183.
91. A full treatment of the causes would involve several distinctions under each kind. See, e.g., Aristotle, Physics, II.3–9; Thomas, De princ. nat., cap. 3–6 (which also reflects Metaph., XII.1–4, 1069a18–1070b35). Of particular importance is the distinction between direct and indirect movers, as in the example of falling. Thomas also uses this distinction in moral matters; see, e.g., STh, I–II, q. 6, a. 3, and I–II, q. 76, a. 1.
92. For instance, in Metaph., VI.2, 1026a33–1027a28.
93. STh, I, q. 5, a. 1, ad 1; I–II, q. 18, a. 1.
94. STh, I–II, q. 55, a. 4.
95. STh, I–II, q. 49, a. 1.
96. See, e.g., STh, I, q. 9, a. 2, ad 3; q. 45, a. 4; q. 90, a. 2; I–II, q. 110, a. 2, ad 3.
97. Categories, 2, 1a24–25, and 5, 3a29–32; Metaph., VII.2, 1028b8–13. Ultimately this requires some qualifying; see below, 112n9.
98. See Metaph., IV.4, 1007b2–16.
99. Categories, 5, 4b17–19.
100. Ibid., 4a21–b13.
101. Physics, V.2, 225b13–226a18; Metaph., XI.12, 1068a13–b15.
102. Metaph., I.3, 983b6–984a12; also Physics, II.1, 193a9–28.
103. Metaph., VII.13, 1039a7–11.
104. See below, 48, 56–57.
105. Aristotle, Metaph., IX.7, 1049a24–26.
106. In The Modeling of Nature, the word first appears on p. xi, then on pp. 8 ff. This book is an interesting attempt at a synthesis of modern science and the Aristotelian conception of natural things.
107. Quodl., III, q. 1, a. 1. Cf. De princ. nat., cap. 2; STh, I, q. 66, a. 1.
108. See In Meta., VII, lect. 2, §1285–87; also VIII, lect. 1, §1689.
109. On prime matter and modern science, in addition to Wallace’s book see also Gilson, “In Quest of Matter.”
110. STh, I, q. 13, a. 11.
111. In De Trin., q. 5, a. 3; De ver., q. 10, a. 5.
112. See In Phys., V, lect. 3, §662[3]–665[6]; In Meta., XI, lect. 12, §2376–84.
113. Metaph., VIII.5, 1044b29–1045a6.
114. Physics, II.1, 192b32–193a1.
115. In Meta., V, lect. 5, §826; In Phys., II, lect. 153[5]–155[7].
116. Metaph., VII.17, 1041b11–33.
117. On matter in the definitions of natural kinds, see Physics, II.2, 193b31–194a15; Metaph., VI.1, 1025b28–1026a7. On the essences of natural things as including matter, see In Meta., VII, lect. 9, §1467–69; here Thomas takes issue with Averroes for equating substantial form and essence. He insists on the point in STh, I, q. 75, a. 4.
118. In Meta., V, lect. 5, §823. Yet another extended sense of nature, close to essence, is differentia; see below, 79–80. Thomas rehearses the senses of nature in his discussion of the definition of person: STh, I, q. 29, a. 1, ad 4.
119. In Meta., V, lect. 5, §826.
120. De an., III.6, 430b17–20.
121. STh, I, q. 3, a. 1.
122. See Metaph., V.6, 1015b35–1016a15.
123. As mentioned above, 43, there is no motion, but only instantaneous change, with respect to substance. Substantial kinds are not on a continuum; see Categories, 5, 2b15–28, 3b33–4a9. This may help to explain why substance and substantial form seem to have little place in modern science: they cannot be directly quantified. (Cf. De an., III.6, 430b14-17; STh, I, q. 76, a. 8.) Modern science generally treats causality in terms of correlations among quantities. Thomas does say that “cause and effect are proportioned to each other” (In Meta., 6, lect. 2, §1185), but what he means by “proportion” is not exclusively quantitative. Is modern science then incompatible with his thought? Perhaps not, if it can be seen as taking only a partial or abstract view. As Aristotle says about mathematics, abstracting does not yield falsehood: Physics, II.2, 193b35.
124. See below, 57, 114.
125. Cf. De anima, III.6, 430b18. The exception, for Thomas, would be the human soul (see below, 114–16).
126. Metaph., VIII.6, 1045a7–1045b23.
127. Metaph., VII.2, 1029a5–6; see In Meta., VII, lect. 2, §1278. This does mean that there just might be forms that are able to exist separately from matter. That is a topic for metaphysics; see below, 109–14.
128. See STh, I, q. 76, a. 4. obj. 2.
129. See Scg, I, cap. 20, §174, and II, cap. 30, §1073; In De caelo, I, lect. 6, §62[5].
130. STh, I, q. 59, a. 2.
131. See Metaph., V.15, 1021a11.
132. See STh, I, q. 10, a. 1, ad 2: just as time regards motion (and rest: STh, I, q. 10, a. 4, ad 3), so the “stretching forth” (protensio) of duration in being seems to regard activity (operatio, “being at work”). Also STh, I, q. 66, a. 1: “if unformed matter preceded [its formation] in duration, then it was already in act, since duration implies this; . . . but this very thing, act, is form. So, to speak of matter preceding form is to speak of a being in act without act.”
133. In Phys., IV, lect. 1, §412[7].
134. In Eth., VIII, lect. 1, §1547; the proverb is mentioned in §1545. He grants that, per accidens, what is similar may be undesirable, and what is dissimilar, desirable. Of course things that are of the same general kind, such as body, may be of different specific kinds. Even though birds, as heavy things, naturally tend downward, Thomas would not at all deny that flying is natural for them, as birds. The point goes back to the general definition of nature; see above, 32–33.