2 Metaphysics

 

Even among contemporary philosophers who are otherwise unfamiliar with his work, it is fairly well known that Aquinas held that the existence of God, the immortality of the soul, and the content and binding force of the natural moral law could be established through purely philosophical arguments (as opposed to an appeal to divine revelation). But those arguments themselves are in general very badly misunderstood by those who are not experts on Aquinas. The reason is that most contemporary philosophers have little or no awareness of just how radically different the fundamental metaphysical assumptions of ancient and medieval philosophers are, in general, from the assumptions typically made by the early modern philosophers and their successors. A distinctive conception of causation, essence, form, matter, substance, attribute, and other basic metaphysical notions underlies all of Aquinas’s arguments in philosophy of religion, philosophy of mind, and ethics; and it is a conception very much at odds with the sorts of views one finds in Descartes, Locke, Hume, Kant, and the other founders of modern philosophy. While most contemporary philosophers would probably not identify themselves as Cartesians, Lockeans, Humeans, Kantians, or the like, their thinking about the metaphysical concepts just noted nevertheless tends, however unconsciously, to be confined within the narrow boundaries set by these early modern thinkers. Hence when they come across a philosopher like Aquinas, they unthinkingly read into his arguments modern philosophical presuppositions he would have rejected. The result is that the arguments are not only misinterpreted, but come across as far less interesting, plausible, and defensible than they really are. In rejecting them, as contemporary philosophers tend to do, they do not realize that what they are rejecting is a mere distortion or caricature of Aquinas’s position rather than the real McCoy.

 

An overview of Aquinas’s general metaphysics is therefore a necessary preamble to a consideration of his views in these other areas of philosophy. Such an overview would be of value in any case, for Aquinas’s metaphysical ideas are important and interesting in their own right. We shall also see that they are as defensible today as they ever were, and (ironically enough) that some work by contemporary philosophers, quite outside the camp of Thomists and otherwise unsympathetic to Aquinas’s overall project, tends to support this judgment.

 

Act and potency

 

The Greek philosopher Parmenides (c. 515–450 B.C.) notoriously held that change is impossible. For a being could change only if caused to do so by something other than it. But the only thing other than being is non-being, and non-being, since it is just nothing, cannot cause anything. Hence, though the senses and common sense tell us that change occurs all the time, the intellect, in Parmenides’ view, reveals to us that they are flatly mistaken.

 

The tendency of philosophers like Parmenides to pit the intellect against the senses and common sense is one that was firmly resisted by Aristotle. At the same time, Aristotle was loath simply to dismiss a theory like Parmenides’ on the grounds that it was odd or counter intuitive; it was important to understand exactly why such a theory was mistaken. Aquinas, who (as we have seen) esteemed Aristotle above all other philosophers, followed him in these attitudes, and also in his specific reply to Parmenides, which appealed to the distinction between act and potency.

 

Parmenides assumed that the only possible candidate for a source of change in a being is non-being or nothing, which (of course) is no source at all. Aristotle’s reply was that this assumption is simply false. Take any object of our experience: a red rubber ball, for example. Among its features are the ways it actually is: solid, round, red, and bouncy. These are different aspects of its “being.” There are also the ways it is not; for example, it is not a dog, or a car, or a computer. The ball’s “dogginess” and so on, since they don’t exist, are different kinds of “non-being.” But in addition to these features, we can distinguish the various ways the ball potentially is: blue (if you paint it), soft and gooey (if you melt it), and so forth. So, being and non-being are not the only relevant factors here; there are also a thing’s potentialities. Or, to use the traditional Scholastic jargon, in addition to the different ways in which a thing may be “in act” or actual, there are the various ways in which it may be “in potency” or potential. Here lies the key to understanding how change is possible. If the ball is to become soft and gooey, it can’t be the actual gooeyness itself that causes this, since it doesn’t yet exist. But that the gooeyness is non-existent is not (as Parmenides assumed) the end of the story, for a potential or potency for gooeyness does exist in the ball, and this, together with some external influence (such as heat) that actualizes that potential – or, as the Scholastics would put it, which reduces the potency to act – suffices to show how the change can occur. Change just is the realization of some potentiality; or as Aquinas puts it, “motion is the actuality of a being in potency” (In Meta IX.1.1770), where “motion” is to be understood here in the broad Aristotelian sense as including change in general and not just movement from one place to another.

 

So far this may sound fairly straightforward, but there is more to the distinction between act and potency than meets the eye. First of all, some contemporary analytic philosophers might object that a thing is “potentially” almost anything, so that Aristotle’s distinction is uninteresting. For example, it might be said by such philosophers that we can “conceive” of a “possible world” where rubber balls can bounce from here to the moon, or where they move by themselves and follow people around menacingly. But the potentialities Aristotle and Aquinas have in mind are ones rooted in a thing’s nature as it actually exists, and do not include just anything it might “possibly” do in some expanded sense involving our powers of conception. Hence, while a rubber ball has the potential to be melted, it does not, in the Aristotelian sense, have the potential to bounce to the moon or to follow someone around all by itself.

 

Second, and as indicated already, though a thing’s potencies are the key to understanding how it is possible for it to change, they are merely a necessary and not a sufficient condition for the actual occurrence of change. An additional, external factor is also required. Potential gooeyness (for example), precisely because it is merely potential, cannot actualize itself; only something else that is already actual (like heat) could do the job. Consider also that if a mere potency could make itself actual, there would be no way to explain why it does so at one time rather than another. The ball melts and becomes gooey when you heat it. Why did this potential gooeyness become actual at precisely that point? The obvious answer is that the heat was needed to actualize it. If the potency for gooeyness could have actualized itself, it would have happened already, since the potential was there already. So, as Aquinas says, “potency does not raise itself to act; it must be raised to act by something that is in act” (SCG I.16.3). This is the foundation of the famous Aristotelian–Thomistic principle that “whatever is moved is moved by another” (In Phys VII.2.891). (The principle is true, incidentally, even of animals, which seem at first glance to move or change themselves; for what this always amounts to is really just one part of the animal being changed by another part. A dog “moves itself” across a room, but only insofar as the potential for motion in the dog’s legs is actualized by the flexing of the leg muscles, and their potential for being flexed is actualized by the firing of the motor neurons, and the potential for the motor neurons to fire is actualized by other neurons; and so on.)

 

Third, while act and potency are made intelligible to us in relation to each other, there is an asymmetry between them such that “absolutely speaking act is prior to potency” (SCG I.16.3). A potential is always a potential for a certain kind of actuality; for example, potential gooeyness is just the potential to be actually gooey. Furthermore, potency cannot exist on its own, but only in combination with act; hence there is no such thing as potential gooeyness existing all by itself, but only in something like an actual rubber ball. It is incoherent to speak of something as both existing and being purely potential, with no actuality whatsoever. But it is not incoherent to speak of something as being purely actual, with no potentiality at all. (Indeed, as we shall see, for Aquinas this is precisely what God is: Actus Purus or “Pure Act.”) So, while for us to understand act and potency we need to contrast them with one another, in the real world outside the mind actuality can exist on its own while potentiality cannot.

 

As will become evident from the remainder of this chapter, the distinction between act and potency forms the basis of Aquinas’s entire metaphysical system; and as will become equally evident by the end of this book, the repercussions of this fundamental distinction extend well beyond general metaphysics. It is not for nothing that the first of the famous Twenty Four Thomistic Theses has it that: “Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.” (This echoes Aquinas’s own assertion that “potency and act divide being and every kind of being” [ST I.77.1, as translated by Pegis in Basic Writings of Saint Thomas Aquinas].)

 

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The First Way

 

As presented in the Summa Theologiae, the proof from motion goes as follows. We know from experience that “some things are in motion” (“motion” in the Aristotelian sense just being change, as we saw in our discussion of Aristotle’s reply to Parmenides). Now motion or change is just the reduction of something from potentiality to actuality. But “nothing can be reduced from potentiality to actuality except by something in a state of actuality” (ST I.2.3); for instance, fire, which is actually hot, makes wood, which is otherwise only potentially hot, become actually hot. Moreover, nothing can be both potential and actual in the same respect at the same time; what is actually hot, for example, is not at the same time potentially hot, but potentially cold. In that case, though, it is impossible for anything to be at the same time and in the same respect both that which is moved or changed and that which does the moving or changing. Hence, “whatever is in motion must be put in motion by another” (ST I.2.3). By the same token, if that which puts something else in motion is itself moving, there must be yet something further moving it, and so on. But if such a series went on to infinity, then there would be no first mover; and if there were no first mover, there would be no other movers, for “subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand” (ST I.2.3). It follows that “it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God” (ST I.2.3).

 

To begin at the end, someone might immediately object to this argument that whatever else Aquinas has shown, he hasn’t really shown that such a “first mover” would be God, if by God we mean a being that can be said to be all powerful, all knowing, all good, and the like. There are two things to be said in reply. First, what Aquinas is getting at in the last line of the proof is that whatever else God is supposed to be, he is supposed to be the ultimate explanation of why things happen in the world; hence, if it can be proved that there is a being who explains this, it follows that at least to that extent it will have been proved that there is something in reality corresponding to our idea of God. And he is surely right about that much. Second, while we do of course also want to know why we should regard such a being as allpowerful, all-knowing, all-good, and so forth, as I have said before, Aquinas does in fact answer that question in great detail later on in the Summa (and elsewhere). We will see how he does so after first looking at each of the Five Ways.

 

The question for now, then, is this: does this argument really establish the existence of a first Unmoved Mover? Note first of all that the argument cannot be criticized by appealing to a variation on the standard “If everything has a cause, then what caused God?” objection. Aquinas does not say that everything is in motion, but only that “some things” are in motion; nor does he say that everything is moved by something else, but only that “whatever is in motion” is moved by something else. Hence it will not do to ask “Doesn’t that mean that God must be in motion?” or “What moves God, then?” For there is nothing in Aquinas’s premises that implies that God would have to be changing like everything else is, or that he must be moved by something else.

 

What, then, of this key premise of the argument, that is, that “whatever is in motion must be put in motion by another”? The bulk of the proof is devoted to supporting it. Yet it has often been suggested that Aquinas’s argument for it fails. One common objection is that the activity of animals shows that the premise is simply false. For isn’t it just obvious that animals move themselves? But as we noted in chapter 2, Aquinas does not deny that there is a loose sense in which animals move themselves. Strictly speaking, though, when an animal moves this only occurs because one part of the animal moves another part, as when the legs of a dog move because of the flexing of its muscles, the muscles flex only because of the firing of certain motor neurons, and so forth. When considered in detail, then, the example of animal movement does not constitute a counterexample to the principle that “whatever is moved is moved by another.”

 

It is also sometimes alleged that Aquinas is committed to the principle that whatever causes something actually to be F must itself actually be F, and that this principle is clearly false. For he gives the example of wood being made to catch fire by something which is already on fire; but as Kenny points out, fire could also be generated instead by taking two sticks that are not already on fire, and rubbing them together. But there are two problems with this objection. First, it ignores the possibility that Aquinas is here appealing to what we called in chapter 2 the “principle of proportionate causality,” according to which whatever is in an effect must somehow be in its cause, but where this allows that the cause might have the relevant feature “virtually” or “eminently” rather than “formally.” In other words, Aquinas is not making the obviously false claim that only what is already on fire can cause fire; he is rather making the claim (perfectly defensible, as we saw in the previous chapter) that whatever causes fire must have an inherent power to cause it. Second, as many commentators have pointed out, Aquinas is probably not relying in this argument on any version of the principle in question in the first place. That is, he is not saying that “whatever causes something actually to be F must itself be F in some way,” but rather that “whatever causes something must itself be actual,” that nothing merely potential can cause anything. As Rudi te Velde has suggested, some critics place too much significance on the physical details of the examples Aquinas gives in the course of the proof, failing to see that their point is merely to illustrate certain basic metaphysical principles rather than to support broad empirical or quasi-scientific generalizations.

 

Thus understood, what Aquinas is saying here is essentially just what we have already noted him saying in developing the distinction between act and potency, namely that no potency or potential can actualize itself, precisely because it is merely potential and not actual. Hence only what is itself already actual can actualize a given potency, and therefore (given that motion is just the actualization of a potency) “whatever is moved is moved by another.” This is not some dubious conjecture based on the observation of how wood catches fire and the like; it is rather supposed to be a metaphysical certainty the denial of which would be conceptually incoherent. Indeed, the principle in question is but a variation on what we referred to in chapter 2 as the “principle of causality,” which we have seen to be eminently defensible.

 

So far, so good, then. But what about the claim that a series of movers could not go on to infinity? Isn’t Aquinas just begging the question (arguing in a circle) when he asserts that if there were no first mover then there would be no movers at all? For why could there not be an infinite series of movers, so that no matter how far back you go in the series, you could always go back to yet another mover? In that case it seems there would be an explanation for the motion of any member of the series you care to take, without having to appeal to a first mover.

 

But in fact Aquinas is not begging the question at all, and has good reason for claiming that such a series could not go on to infinity. Keep in mind first of all that the proof from motion, like all the Five Ways, is not an attempt to show that the universe had a beginning at some point in the past and that God must have caused that beginning. Aquinas is not saying that if you trace the series of movers back in time you must eventually get to some temporally first mover. As we saw in chapter 2, for Aquinas as for Aristotle, the immediate cause of an effect is simultaneous with that effect: “It is clear that when a thing moves because it is moved, the mover and the mobile object are moved simultaneously” (In Phys VII.2.892). So the series of movers he has in mind is one all of whose members exist together here and now (and at any moment we might be considering the argument), and by saying that there must be a first mover, he doesn’t mean first in order of time, but rather first in the sense of being most fundamental in the order of what exists.

 

This brings us to an important distinction Aquinas and other medieval thinkers made between two kinds of series of efficient causes. On the one hand there are causal series ordered per accidens or “accidentally,” in the sense that the causal activity of any particular member of the series is not essentially dependent on that of any prior member of the series. Take, for example, the series consisting of Abraham begetting Isaac, Isaac begetting Jacob, and Jacob begetting Joseph. Once he has himself been begotten by Abraham (and then grows to maturity, of course), Isaac is fully capable of begetting Jacob on his own, even if Abraham dies in the meantime. It is true that he would not have existed had Abraham not begotten him, but the point is that once Isaac exists he has the power to beget a son all by himself, and Abraham’s continued existence or non-existence is irrelevant to his exercise of that power. The same is true of Jacob with respect to both Abraham and Isaac, and of Joseph with respect to Abraham, Isaac, and Jacob. Given that we are considering them as a series of begetters specifically, each member is independent of the others as far as its causal powers are concerned. Contrast this with a causal series ordered per se or “essentially.” Aquinas’s example from the First Way of the staff which is moved by the hand is a standard illustration, and we can add to the example by supposing that the staff is being used to move a stone, which is itself moving a fallen leaf. Here the motion of the leaf depends essentially on the motion of the stone, which in turn depends essentially on the motion of the staff, which itself depends essentially in turn on the motion of the hand. For if any member higher up in the series ceases its causal activity, the activity of the lower members will necessarily cease as well. For instance, if the staff was to slip away from the stone, the stone, and thus the leaf too, will stop moving; and of course, if the hand stops moving, the whole series, staff included, will automatically stop. In this case the causal power of the lower members derives entirely from that of the first member, the hand. In fact, strictly speaking it is not the stone which is moving the leaf and the staff which is moving the stone, but rather the hand which is moving everything else, with the stone being used by it as an instrument to move the leaf and the staff being used as an instrument to move both stone and leaf.

 

Causal series ordered per accidens are linear in character and extend through time, as in the begetting example, in which Abraham’s begetting Isaac occurs well before Isaac’s begetting Jacob, and Isaac’s begetting Jacob occurs well before Jacob’s begetting Joseph. Causal series ordered per se are paradigmatically hierarchical with their members acting simultaneously, as in the staff example where the movement of the leaf occurs precisely when the movement of the stone occurs, which is precisely when the movement of the staff occurs, which is precisely when the movement of the hand occurs. Now it is in Aquinas’s view at least theoretically possible for a causal series ordered per accidens to regress to infinity, and thus have no beginning point (ST I.46.2). (This is why Aquinas thinks it is not possible to prove via purely philosophical arguments that the world must have had a beginning in time.) For since each member of such a series has its causal power independently of the earlier members, there is no need to trace any particular member’s action back to the activity of a first member; for instance, when Jacob begets Joseph, it is precisely Jacob who begets him, and not Abraham who begets him by using Isaac and Jacob as instruments. By contrast, “in efficient causes it is impossible to proceed to infinity per se – thus, there cannot be an infinite number of causes that are per se required for a certain effect; for instance, that a stone be moved by a stick, the stick by the hand, and so on to infinity” (ST I.46.2). For “that which moves as an instrumental cause cannot move unless there be a principal moving cause” (SCG I.13.15). That is to say, since the lower members of a causal series ordered per se have no causal power on their own but derive it entirely from a first cause, which (as it were) uses them as instruments, there is no sense to be made of such a series having no first member. If a first member who is the source of the causal power of the others did not exist, the series as a whole simply would not exist, as the movement of the leaf, stone, and staff cannot occur in the absence of the hand.

 

What Aquinas is saying, then, is that it is in the very nature of causal series ordered per se to have a first member, precisely because everything else in the series only counts as a member in the first place relative to the actions of a first cause. To suggest that such a series might regress infinitely, without a first member, is therefore simply unintelligible. The leaf is “moved” by the stone only in a loose sense; strictly speaking, the leaf, stone, and staff are all really being moved by the hand. Thus to suggest that this series of purely instrumental causes might regress to infinity is incoherent, for they would not in that case be the instruments of anything at all (CT I.3). As A. D. Sertillanges put it, you might as well say “that a brush can paint by itself, provided it has a very long handle” (quoted by Garrigou-Lagrange in God: His Existence and His Nature).

 

Given their essentially instrumental character, all causes in such a series other than the first cause are referred to by Aquinas as “second causes” (“second” not in the sense of coming after the first but before the third member of the series, but rather in the sense having their causal power only in a secondary or derivative way). It is worth emphasizing that it is precisely this instrumental nature of second causes, the dependence of whatever causal power they have on the causal activity of the first cause, that is the key to the notion of a causal series per se. That the members of such a series exist simultaneously, and that the series does not regress to infinity, are of secondary importance. As Patterson Brown and John Wippel point out, even if a series of causes ordered per se could somehow be said to regress to infinity, it would remain the case, given that they are merely instrumental causes, that there must then be something outside the entire infinite series that imparts to them their causal power.

 

Whether or not the series of causes per accidens regresses infinitely into the past, then – and again, while Aquinas believed that it did not, he didn’t think this could be proven through philosophical arguments – a causal series per se existing here and now, and at any moment we are considering the matter, must necessarily trace back to a first member. But strictly speaking, even the hand in Aquinas’s example doesn’t count as a first mover – the example is intended merely as a first approximation to the notion of a first mover – because it is itself being moved insofar as its activity depends on the motion of the arm, the flexing of certain muscles, and so forth. To understand the way in which such a series regresses and how it does and must terminate, it is crucial to remember that for Aquinas, motion or change is just the reduction of potency to act. So when we talk about one thing being moved by another, which is moved by another, and so on, in a causal series ordered per se, this is shorthand for saying that a certain potency is reduced to act by something whose potency is itself reduced to act by something whose potency is itself reduced to act by … and so forth. (Or, to soften the technical terminology slightly, a certain potentiality is actualized by something whose potentiality is itself actualized by something whose potentiality is itself actualized by … and so on.) As should be evident, such a series can only possibly terminate in something which is not reduced to act or actualized by anything else, but which just is in act or actual, and thus “unmoving.” The potential of the hand for movement is actualized here and now by the flexing of the muscles of the hand, the potential of the muscles to flex is actualized here and now by the firing of certain motor neurons, the potential of the motor neurons to fire is actualized here and now by the firing of certain other neurons, and so forth. Eventually this regress must terminate in something which here and now actualizes potentialities without itself being actualized, an unmoved mover.

 

Now Kenny objects that the notion of an unmoved mover merely gives us something at rest, like a stationary billiard ball, and thus seems hardly relevant to proving the existence of God. But as GarrigouLagrange points out, and as should be clear from our discussion thus far, an unmoved mover of the sort we’ve been describing is not and cannot be “unmoved” in the sense of being in repose, precisely because it is that which actualizes the potencies of second causes. It is active, not “at rest.” There is still a further question, however. Even if it is granted that the First Way takes us to an unmoved mover, why should we hold (as Aquinas does) that this mover is also unmovable? As Scott MacDonald suggests, it may be that a first mover of the sort whose existence is established by Aquinas’s argument is one that is capable of motion even if, qua first mover, it does not in fact move. In other words, for all Aquinas has shown, a first mover may well have certain potencies which are not in fact being actualized, at least not insofar as it is functioning as the first mover in some series of efficient causes ordered per se. Perhaps its potencies are actualized at some other time, when it is not so functioning; or perhaps they never are. But as long as it has them, it will not be something that can be characterized as “pure act,” and thus, given Aquinas’s own commitments, it will not be identifiable with God. To get to a first mover of pure act, and thus one which is truly unmovable, would require in MacDonald’s view some further argument, in which case the argument from motion could succeed as an argument for God’s existence only by being “parasitic” on such a further argument.

 

Yet MacDonald is, I think, mistaken. Consider how the series we have been describing would have to continue beyond the point at which we left it, with the hand’s potentiality for motion actualized by the arm, the arm’s potentiality for motion actualized by the flexing of certain muscles, the muscles’ potentiality for flexing actualized by the firing of certain motor neurons, and so on and so forth, all simultaneously. All of this depends in turn on the overall state of the nervous system, which depends on its molecular structure, which depends on the atomic basis of that molecular structure, which depends on electromagnetism, gravitation, the weak and strong forces, and so on and so forth, all simultaneously, all here and now. That the molecules composing the nervous system constitute a nervous system specifically amounts to their having a certain potency which is here and now actualized, that the atoms composing the molecules constitute just those molecules amounts to their having a certain potency which is simultaneously actualized, and so on. To account for the reduction of potency to act in the case of the operations or activities of the hand, the muscles, and so on, we are led ultimately to appeal to the reduction of potency to act vis-à-vis the existence or being of ever deeper and more general features of reality; for “it is evident that anything whatever operates so far as it is a being” (QDA 19). But the only way to stop this regress and arrive at a first member of the series is with something whose very existence, and not merely its operations or activities, need not be actualized by anything else. This would just be something which, since it simply exists without being made to exist by anything, or is actual without being actualized, is pure act, with no admixture of potentiality whatsoever. For suppose it had some potency relevant to its existence (its existence being what is relevant to its status as the end of the regress as we have continued it). Then either some other thing actualizes that potency, in which case we haven’t really stopped the regress after all, contrary to hypothesis; or some already actual part of it actualizes the potency, in which case that already actual part would itself be both pure act and, properly speaking, the true first mover. Now, having no potency to actualize, such a being could not possibly change or move. Thus we have reached a first mover that is not only unmoved, but unmovable.

 

MacDonald might object to this that the move from accounting for the activities or operations of things to accounting for their existence or being in effect involves an appeal to something other than motion, and thus to an argument other than the argument from motion; and though (as MacDonald would acknowledge) this would not by itself show that there is anything wrong with the argument, it would leave untouched his claim that the First Way by itself is incomplete and “parasitic” for any effectiveness it has vis-à-vis proving God’s existence on some other argument. But as commentators like Norman Kretzmann and D. Q. McInerny have noted, if the point of an argument from motion is to explain motion, and to explain motion requires explaining the existence of the things doing the moving and the way in which factors outside them contribute to their ability to move, then an explanation of the existence of moving things is quite naturally going to be a part of any argument from motion. More to the point, if motion is just the reduction of potency to act, then since the existence of a thing no less than its activity involves (in everything other than that which is pure act) the reduction of potency to act, any explanation of motion must account for the existence of things and not just their activities. Far from making an argument from motion “parasitic” on some other kind of argument, the move to the explanation of the existence of moving things is a necessary part of any such argument.

 

Notice that at no point in our exposition of the argument from motion have we had to appeal to any claims from Aristotelian physics, “outmoded” or otherwise. The argument proceeds entirely in terms of such metaphysical notions as the act/potency distinction, the principle of causality, and so forth. Still, it is sometimes suggested that Newton’s principle of inertia undermines the proof from motion, for if (as that principle tells us) it is just a law of physics that a body in motion tends to stay in motion unless acted upon from outside, then (so it is claimed) Aquinas’s view that whatever is moving must here and now be moved by something else is thereby shown to be false. But there are several problems with this objection. First and most obviously, Newton’s principle concerns only “local motion” or movement from one place to another, while motion in the Aristotelian sense includes (as we have seen) not just local motion, but also changes in quality (like water becoming solid when it freezes), changes in quantity (as when a pool of water gets larger or smaller), and changes in substance (as when hydrogen and oxygen are combined to make water) (In Phys III.2.286). (There is a strict sense of “motion” within the Aristotelian tradition on which changes in substance are not counted as motions, but they are motions or changes in a loose sense; and as several commentators have noted, they do in any case count as reductions of potency to act of the sort the argument from motion seeks to account for.) At the very least, then, the defender of the First Way can say that whether or not local motion needs an explanation of the sort the argument provides, these other kinds of change do need such an explanation.

 

But in fact there is no good reason to exclude local motion from the range of that which needs explanation in terms of a first unmoved mover. After all, it is no good just to say “Well, it’s simply a law of physics that things in motion tend to stay in motion unless acted upon from outside.” For one thing, there is still the question of what puts something in motion in the first place, and in general of a thing’s acquisition or loss of momentum, and explaining these events will require just the sort of explanation the First Way tells us other instances of change do. More fundamentally, we also still need to know what it is exactly for something to be a law of physics, and why such a law holds.

 

Regarding the first question, some defenders of the First Way have suggested that Newton’s principle is nothing more than a mathematical model which is of utility in making predictions but which strictly speaking does not describe the objective nature of physical objects. One reason for adopting such an instrumentalist (as opposed to realist) interpretation of the principle of inertia is that to interpret the principle realistically would commit us (so it is argued) to the metaphysically absurd consequence that a finite cause can have an infinite effect. It is then sometimes further suggested that to explain local motion, especially of a projectile sort, we need therefore to postulate that the initial cause of a thing’s movement (the arm which throws a spear, say) imparts to it a force, “impulse,” or “impetus” which keeps it in motion, and thus passing from potency to act, as long as it does move, and where this impetus serves as an instrumental cause whose efficacy must ultimately be traced to the simultaneous activity of a first mover. Other defenders of the argument reject this “impetus” theory and would grant that Newton’s principle does tell us something about the real nature of physical objects. But they would then insist that this simply leaves us with the question of what actualizes the potential existence of things having natures of the sort described by the principle of inertia, and that to answer this question we have (for reasons already seen) to appeal to something which is pure act. In short, Newton’s principle can hardly undermine the First Way if the existence of a first unmovable mover is needed in order to explain why the principle holds in the first place.

 

But it may be that even these general points concede too much force to the objection, for things are much less conceptually clear cut here than it might at first appear. For example, if, as is standardly done, we think of Newtonian inertial motion as a “state” rather than a process, then we need to get clear on exactly how such “motion” could be motion in the Aristotelian sense (i.e. a genuine change), in which case it also needs to be made clear exactly how Newton’s principle is supposed to conflict with the Aristotelian principle that what is in motion (that is, changing) requires a mover. Or if inertial motion is change of some sort, then we need to get clear on the sense in which such motion can be said to be a “state.” It should also be kept in mind that in the physical universe as it actually exists, no object undergoing local motion is ever unaffected by outside forces, given for example the constant gravitational attraction every body exerts on every other. Hence at every moment at which an object is moving through space, and not merely at its initial acquisition of momentum, its motion is being affected in a way that requires explanation in terms of something outside it. But in that case, even with respect to the explanation of local motion, the principle of inertia seems practically moot. The conceptual waters here are deep, and reflect difficulties for interpreting modern physics that arise whatever its relationship to Aristotelian metaphysics. The point is that those who assert a conflict between Aquinas and Newton simply have not made their case until they have worked out these crucial details. It will not do lazily to assert, without addressing these issues, that modern physics has somehow “explained” local motion in such a way that reference to a first mover is unnecessary.

 

Another objection sometimes raised against the First Way is that anything moving something else, including a first mover, would have to be undergoing motion itself, as for example the hand of our example moves even as it is moving the staff. Therefore (the objection continues) the very notion of an unmoved mover is incoherent. But this objection begs the question. The argument from motion claims to prove that no motion, including the motion of moved movers, would be possible at all unless there is a first mover which is pure act and thus unmovable. So, given that the premises of this argument are true and that the conclusion follows logically from them, it follows that the conclusion is true and therefore coherent. Accordingly, it won’t do simply to insist that the conclusion must be false; one has to show specifically either that one of the premises is false or that the conclusion does not follow. Otherwise, one ought to admit that the argument shows precisely that an unmoved mover really is possible (since actual) after all.

 

Besides, it is hardly as if the notion of an unmoved mover were anything like as problematic as that of (say) an “immortal mortal.” An “immortal mortal” would be something that both dies and does not die, which is self-contradictory. But an unmoved mover is something that makes other things move without itself undergoing motion, and there is no obvious self-contradiction in that. Furthermore, as G. H. Joyce argues, the reason that the movers of our experience are themselves moving even as they move other things is precisely because they are limited in the various ways entailed by being composites of act and potency. (For example, because an arm is actually at one point in space and only potentially at another, its potential to be at some other point in space has to be actualized by something else if it is to get the staff to that other point in space.) But something which is pure act, devoid of all potency, would have no such limitations, and thus not need to be moved itself as it is moving other things. Moreover, it would (as we shall see later) be outside of time, and indeed that which creates time, so that to the extent that the objection in question implicitly assumes that the first mover goes from not acting at one moment in time to acting at another moment in time, the objection simply misconceives the nature of the first mover’s activity (In Phys VIII.2.989). Finally, as Garrigou-Lagrange points out, given that (as we will see a little later on) our knowledge of the first mover is necessarily largely negative, it should not be surprising if it is harder for us to get our minds around it than it is for us to understand the more mundane movers of our experience.

 

We have devoted a good deal of space to the First Way, partly because of its intrinsic importance and partly because Aquinas himself put so much emphasis on it. (He famously regarded it as the “more manifest way” [ST I.2.3] and presented versions of the argument from motion again and again in his writings, as the citations given above indicate.) Moreover, many of the issues that arise in the discussion of the First Way, such as the impossibility of an infinite regress of causes ordered per se, also arise in discussion of the other ways. Hence our fairly detailed discussion of the First Way helps to set the stage for our treatment of the others. Most importantly, it has also (hopefully) shown that the objections commonly raised against the argument are hardly as conclusive as they are sometimes assumed to be, and that it is, accordingly, as worthy of consideration today as it was in Aquinas’s day.

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