Where Aquinas Goes Wrong

By | November 8, 2021

If there’s a “poster child” of classical theism, it would have to be Thomas Aquinas (1225–1274 AD).

By the end of only the third question in the Summa Theologica, Thomas Aquinas has concluded that God necessarily exists and is the absolutely simple and purely actual first, uncaused cause of the moment-by-moment being of everything else. From that the rest of his doctrine of God pretty much falls out as a corollary. God must be absolutely immutable, impassible, timeless, etc.

Now, I think classical theism—in particular the notion that God is absolutely simple—is a disastrous doctrine. I’ve argued previously that it leads to either modal collapse such that everything is metaphysically necessary or to complete divine indifference to creation.

But it’s one thing to argue that classical theism is wrong. It’s quite another to diagnose specifically how or where it goes wrong. Where in the first three articles of the Summa does Aquinas go off the rails?

After looking closely at articles 2 and 3, I believe that I’ve identified three mistakes that Aquinas makes. They are

  1. An unwarranted assumption that all real, internal distinctions require “composition”.
  2. An unwarranted extension of act/potency metaphysics from cases of dynamic, intrinsic change to cases of static, internal composition.
  3. An unwarrantedly strong explanatory assumption to the effect that all static, real distinctions need a concurrent, external cause.

I will look at each of these in turn, but first we need to look at the argument of the First Way (ST 1a.2.3) because it lays the foundation.

Aquinas’s First Way

The argument of the First Way starts as follows:

It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. (emphasis added)

In these first few sentences, Aquinas introduces Aristotle’s act/potency distinction to explain “motion”. Initially you might think that all he has in mind is physical motion from place to place, but that would be incorrect. In the underlined sentence he tells us that by “motion” he means to include any “reduction … from potentiality to actuality”. This “reduction” talk sounds weird to me. I take him to mean that any actualization of a potency counts as “motion” as far as he is concerned.

But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another.

Here Aquinas gives us the concrete example of fire, which is actually hot, heating a piece of wood, which is potentially hot, and making it actually hot. Notice, he says that this “changes” the wood. This example is one of dynamic, intrinsic change. He then goes on to say that nothing can be both “in act” and “in potency” in the same respect at the same time. This makes sense as long as potency is understood as a potency for something to be otherwise than it actually is. Thus the heated wood is actually hot but potentially cold. A thing can’t be otherwise than what it is and be what it is at the same time. Again, this makes sense when we’re thinking about dynamic, intrinsic change, something that necessarily happens over time. At one time a thing is in potency in some respect, but not actual in that respect. At a later time, the thing is actual in that respect and (perhaps) in potency in some other respect.

But Aquinas goes further and says something more controversial: nothing can “move itself” (i.e., self-actualize). Actualization must always be effected “by another”. This seems plausible when we’re talking about inanimate objects like wood and fire. The wood’s not going to heat itself. And more generally in cases involving what Aristotle called violent motion, that is, motion contrary to nature, the idea that there’s got to be an external actualizer makes sense. A rock won’t ascend upward on its own, but it will ascend if someone throws it upwards. But there are two cases that pose problems for Aquinas’s claim. The first problem case involves natural motion. Aristotle argued that the different “elements” (earth, air, fire, water, and aether) each have a natural motion, either toward, away from, or around the center of the universe. Fire, on this view, spontaneously goes upward, away from the center of the universe without (it would seem) any external thing moving it. Aquinas is in most matters a good Aristotelian, so it’s not clear to me how he would reconcile this with his claim that all motion, all actualization of a potency, requires an external actualizer. The second, and more serious, problem case involves free agency. If some creatures have free will, then when they make a free choice aren’t they in some sense self-actualizing their own potency to choose this rather than that? Of course there may well be external reasons informing that choice, but insofar as the choice is free those reasons and whatever other causal influences may be involved can’t, by themselves, actualize the choice. The free agent must still, at least in part, be a self-actualizer.

How does Aquinas defend this claim that whatever is moved (i.e., whatever potency is actualized) is moved (i.e., actualized) by something else? He doesn’t offer any clear defense in the First Way, but in the Summa Contra Gentiles (I.13.4–9) he does give three arguments in support of the claim. I look at those arguments in the Appendix below. Suffice to say, I’m not persuaded that Aquinas has given us a good reason to accept the claim that whatever is moved is moved by something else. And viewed from the perspective of modern physics, there is good reason to reject the claim with respect to physical motion. Whereas Aristotle believed that physical motion required a continuous sustaining cause—after all, at each step a “potency” is actualized, i.e., the potency to be elsewhere a moment hence—Newton argued that unaccelerated motion doesn’t need a sustaining cause based on experiments that pointed toward the concept of inertia. According to Newton’s First Law of Motion, a body at rest or in uniform (i.e., unaccelerated) motion will continue in that state unless a net external force acts upon it. In other words, asking “why does this moving object continue to move” is asking the wrong question. What we should be asking instead is “why doesn’t this object continue in a uniform state of rest or motion?” Now there are ways of restricting Aquinas’s First Way to exclude this type of counterexample, so this isn’t by itself a refutation of the argument, but it does make an important point: Just because we can pose a well-formed explanatory question (e.g., why do things continue to move?) doesn’t mean that we are rationally obligated to answer that question because it might be the wrong question to ask. Explanatory demands need justification.

In any case, the First Way contends that there can be no self-movers (self-actualizers). The argument continues by ruling out an infinite regress of externally moved movers, and concludes that for motion (actualization of potency) to occur, there must be a first, unmoved mover (= God) who, as unmoved, has no potency to be actualized. So God must be purely actual. With that in mind, let’s consider Aquinas’s first mistake.

Aquinas’s First Mistake: assuming that all real, internal distinctions require “composition”

In ST 1a.3 on whether God is simple, Aquinas examines in successive articles several different types of internal distinctions that might exist within a thing and asks of each whether that sort of distinction could apply to God. In each case he appeals to the act/potency distinction and to the First Way argument from ST 1a.2.3 to argue that God cannot have real, internal distinctions as the purely actual, first cause. For example, he argues that God cannot have a body (i.e., be partly “composed” of physical matter), or be “composed” of matter and form, or be “composed” of substance and accident, or be “composed” of quiddity (an essence or suchness) and subject (i.e., a principle of individuation or thisness), or be “composed” of essence and existence.

Why does Aquinas think these kinds of distinctions are important? Well, he regards these as real distinctions, not merely conceptual ones. I take it that these distinctions are “real” in the sense that one term can obtain in reality without the other. Take essence and existence, for example. My essence (humanity) doesn’t entail that I exist, or that any humans exist. Theoretically, humanity could be (in the mind of God) without any actual humans existing. Likewise, a given substance can exist without any particular set of accidents (qualities that are not part of its nature). It may need to have some accidents, but not necessarily those accidents. Similarly, when a statue is broken, its matter persists, but the form it had is gone. (Incidentally, if real distinctions require that one term can exist without the other, then that may explain how Aquinas can affirm absolute divine simplicity and be a Trinitarian: Since the Father, Son, and Spirit all necessarily coexist, the distinctions between them aren’t “real” in the relevant sense.)

Without explicitly stating it as a general assumption, it’s clear that Aquinas assumes throughout that these kinds of static, internal distinctions require “composition”. I put quotes around “composition” because this is not the standard way in which we understand composition. In normal composition, one takes already available components and puts them together (com-posing them) in a dynamic process of synthesis, as when a musical composer produces a work by stringing one note together after another. But distinctions like matter/form, substance/accident, essence/existence, etc. are static distinctions. Things have both “components” at every moment of their existence. The statue always has both matter and form. Every being always has both essence and existence. Without existence it would be at all, and without an essence it wouldn’t be any particular kind of thing; at most it would only be vague, undifferentiated “stuff” (prime matter?).

So Aquinas is stretching the concept of “composition”. I think this is a mistake. It’s not, I think, a huge mistake, but at the very least, it’s conceptually sloppy because it encourages us to think of the terms of these distinctions as “parts” that can be either put together or pulled apart. But clearly the essence and existence of a thing can’t be separable parts of it. You can’t pull the existence out of a thing and leave a free-standing “essence” behind or pull its essence out and leave free-standing “existence”. Take either existence or essence away and you destroy the whole thing. Likewise, things aren’t actually built up by taking a “substance” and then attaching a bunch of “accidents” to them like putting pins in a pin cushion.

We could drop the misleading talk of “composition” and “parts” and refer to the terms of these static, internal distinctions as “metaphysical constituents” instead. Aquinas could then ask us to consider what keeps or unifies these constituents together and try to run his arguments for classical theism along those lines. So this mistake isn’t necessarily the end of the Thomistic project. But it does facilitate the second and third mistakes.

Aquinas’s Second Mistake: overextending the act/potency distinction

In ST 1a.3 Aquinas consistently appeals to the act/potency distinction to deny of God the various static types of “composition” that he identifies. This reflects a major conceptual extension of the act/potency distinction which was introduced initially to account for dynamic, intrinsic change. Both the case of physical motion and the fire-heating-log example from the First Way are dynamic processes, ones that place over time. So it’s relatively natural to think of these as involving the actualization of a potency. The Aristotelian understanding of physical motion may have been refuted by Newton, but it’s not an unnatural way of thinking about motion. It just doesn’t agree well with experiment.

The static distinctions of ST 1a.3, however, are not naturally thought of as actualizations of a potency. A substance’s acquiring a new accident can readily be thought of as an actualization, but not its having accidents. (The word “accident”, accidere in Latin, is unfortunate because it encourages us to think of non-essential properties as things that ac+cidere, that fall upon or happen to something. This fits in well with a dynamic gaining or losing of properties, but not with the static having of properties.) Likewise, a thing’s coming-to-be, a dynamic event, can be thought of as the actualization of a potency. It can’t be a potency in the thing that comes to be, of course, but it could be a potency in the causes of a thing’s coming-to-be, a potency for them to produce or give rise to the thing in question. But the static being of a thing isn’t naturally thought of as the actualization of a potency. While a thing exists it would seem that there’s no potency-for-its-existence left to actualize. While a substance has certain non-essential properties it would seem that there’s no potency-for-it-to-have-those-very-properties left to actualize.

I conclude, then, that Aquinas is overextending the act/potency distinction. At the very least, this extension needs justification, which he doesn’t provide. Just like composition, the act/potency distinction finds its natural home in a dynamic context. Composition is a process by which something with separable parts comes-to-be. Actualization is a process by which things come-to-be-otherwise than they were before. But if we first conceptualize static distinctions using a dynamic notion like “composition” (Aquinas’s first mistake), then we might be able to fool ourselves into thinking that it makes sense to apply the act/potency distinction to those static distinctions as well (Aquinas’s second mistake). This, in turn, leads to the third mistake.

Aquinas’s Third Mistake: explanatory overreach

If we think (plausibly) that dynamic actualizations of a potency—comings-to-be, ceasings-to-be, and intrinsic changes—require explanations, and if we then conceptualize various static distinctions as actualizations of a potency, then we might start thinking that these static distinctions need to be explained as well. In other words, we move from the limited and plausible explanatory principle

  1. All dynamic actualizations need a prior cause

to a much stronger, and thus less plausible, explanatory principle

  1. All static, real distinctions need a concurrent, external cause.

With something like (5) in hand, Aquinas can easily argue for absolute divine simplicity. Thus, if God were “composed” of essence and existence, then He would need a cause, and so wouldn’t be God. Likewise, if God had any non-essential properties then He would be “composed” of substance and accident and so wouldn’t be God. And so on, for the other types of static, internal distinctions that Aquinas discusses—God can’t have any internal “composition” without thereby losing His status as the ultimate or first cause.

But why should we accept anything like (5)? Recall the lesson from our discussion of the First Way above: explanatory demands need justification. Whatever the merits of (5), Aquinas doesn’t give us any relevant justification for it. Sure, we could try to derive (5) from an even more grandiose explanatory principle, like Leibniz’s principle of sufficient reason (PSR), but despite what Leibniz may have thought, the PSR is not rationally incumbent upon us. Again, just because we can formulate an intelligible explanatory question doesn’t mean that there has to be an answer, for it may be the wrong question to ask.

For example, an objection against libertarian conceptions of free will could run as follows:

Determinist: Why did you choose X rather than Y?
Libertarian: I chose X for these reasons …
Determinist: But why did you choose X for those reasons?
Libertarian: There is no why at that point, I made the choice for X. I privileged those reasons over whatever competing reasons I had for Y.

The determinist’s second question makes sense, but the libertarian should push back at that point. If the demand for explanation is accepted, then the libertarian is in a bind. He can’t cite his “reasons” to answer that demand because those reasons are now included in the explanandum. So what’s he going to appeal to? Randomness? No, that’s not free will. Further causes that were not considered as reasons? Well then either those cause were determining or not. If determining, then the determinist wins and the libertarian loses. If not determining, then the determinist will pose a new explanatory question: Why did you chose X for those reasons while being subject to those causes? The determinist’s goal is to force the libertarian either to resort to randomness or to admit yet further causes until for all practical purposes the choice is determined or at least not free.

Similarly, if the strong explanatory demands of the PSR or of (5) are accepted, then the classical theist basically wins. Non-classical theists should push back at this point and say no. In short, classical theists need to justify the explanatory demands they use to derive classical theism, and non-classical theists should press them on that. For example, it can be argued in a manner parallel to Newton’s First Law of Motion and in accordance with (4) that once a being exists it has a kind of “existential inertia” and thus does not need anything to keep it in being: Things that exist continue to exist as long as they aren’t internally unstable (e.g., radioactive decay) or something external acts to destroy them. Now maybe that’s not quite the right principle. Maybe we should accept a somewhat stronger (or weaker) explanatory principle instead. But you get the idea. That all static, real distinctions need a concurrent, external cause is not obvious, and there are more plausible (because more modest) alternative explanatory principles that would block the Thomistic derivation of classical theism.


In the Summa Contra Gentiles (I.13.4–9) Aquinas gives three arguments in support of the claim that whatever is moved (i.e., whatever potency is actualized) is moved (i.e., actualized) by something else. Here’s the first one:

In this proof, there are two propositions that need to be proved, namely, that everything that is moved is moved by another (emphasis added), and that in movers and things moved one cannot proceed to infinity. The first of these propositions Aristotle proves in three ways. The first way is as follows. If something moves itself, it must have within itself the principle of its own motion; otherwise, it is clearly moved by another. Furthermore, it must be primarily moved. This means that it must be moved by reason of itself, and not by reason of a part of itself, as happens when an animal is moved by the motion of its foot. For, in this sense, a whole would not be moved by itself, but a part, and one part would be moved by another. It is also necessary that a self-moving being be divisible and have parts, since, as it is proved in the Physics [VI, 4], whatever is moved is divisible.

On the basis of these suppositions Aristotle argues as follows. That which is held to be moved by itself is primarily moved. Hence, when one of its parts is at rest, the whole is then at rest. For if, while one part was at rest, another part in it were moved, then the whole itself would not be primarily moved; it would be that part in it which is moved while another part is at rest. But nothing that is at rest because something else is at rest is moved by itself; for that being whose rest follows upon the rest of another must have its motion follow upon the motion of another. It is thus not moved by itself. Therefore, that which was posited as being moved by itself is not moved by itself. Consequently, everything that is moved must be moved by another.

The first paragraph does not establish the principle in question. In only puts forth some constraints, namely, that a self-mover must (a) “have within itself the principle of its own motion”, (b) that it must be “primarily moved”, i.e., the whole thing moves, not merely one part moving another, and (c) it must be “divisible and have parts”. Of these constraints, (a) and (b) would seem to pertain to both the cases of natural motion and free agency. As for (c), Aquinas appeals to Aristotle’s authority here and doesn’t give us an argument for it. That’s unfortunate, because it’s not clear to me why we should accept (c). Couldn’t that which self-moves be a metaphysical simple, something indivisible and without “parts”? Maybe human souls are like that. In any case, without (c) the Aristotelian argument in the second paragraph doesn’t work. That argument aims to pit constraints (b) and (c) against each other. But if one of those constraints isn’t necessary—and I have suggested that (c) at least isn’t necessary—then the argument fails.

Here’s Aquinas’s second argument for the crucial assumption:

In the second way, Aristotle proves the proposition by induction [Physics VIII, 4]. Whatever is moved by accident is not moved by itself, since it is moved upon the motion of another. So, too, as is evident, what is moved by violence is not moved by itself. Nor are those beings moved by themselves that are moved by their nature as being moved from within; such is the case with animals, which evidently are moved by the soul. Nor, again, is this true of those beings, such as heavy and light bodies, which are moved through nature. For such beings are moved by the generating cause and the cause removing impediments. Now, whatever is moved is moved through itself or by accident. If it is moved through itself, then it is moved either violently or by nature; if by nature, then either through itself, as the animal, or not through itself, as heavy and light bodies. Therefore, everything that is moved is moved by another.

This argument purports to give an exhaustive breakdown of cases and to show that in each case whatever is moved is moved by something else. What I don’t find persuasive here is the claim that neither things “moved by their nature” (i.e., natural motion) nor things “moved from within” (i.e., by the soul) are self-movers. Why not? Because they are moved by “the generating cause” and “the cause removing impediments”?? I’m not buying it. The mere absence of impediments doesn’t move anything. It’s not even a “cause”, properly speaking. And I have no idea what he means here by “the generating cause”. As for animal movement (motion “through itself”) the only suggestion I can see here is that the animal is moved by “the soul”. Okay, but then what moves the soul? Isn’t the animal soul a self-mover? If not, perhaps because animals don’t have free agency, then what about free agency? Don’t we need to consider that as another case beyond what’s considered here?

Finally, here’s Aquinas’s third argument from the Summa Contra Gentiles:

In the third way, Aristotle proves the proposition as follows [VIII, 5]. The same thing cannot be at once in act and in potency with respect to the same thing. But everything that is moved is, as such, in potency. For motion is the act of something that is in potency inasmuch as it is in potency. That which moves, however, is as such in act, for nothing acts except according as it is in act. Therefore, with respect to the same motion, nothing is both mover and moved. Thus, nothing moves itself.

This is awfully brief. The first sentence is arguably true, but irrelevant. Nothing can both potentially otherwise and actually otherwise at the same time and in the same respect. I can buy that. But how does free agency require something to be both in act and potency at the same time and in the same respect? Freely actualizing my own potency to choose chocolate ice cream, say, doesn’t imply that I’m somehow already “actualized” in that respect before my choice. No, I’m only actualized in that respect after or rather by my choice. So this third argument just falls flat to me. What needed to be demonstrated is that self-motion or self-actualization require being in act and in potency in the same respect at the same time, but when I choose I’m in potency with respect to choosing chocolate before my choice and in act with respect to choosing chocolate after my choice, not at the same time.

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