A Defense of the Argument from Motion

When I was first introduced to St. Thomas Aquinas’ Five Ways, I was unim­pressed. I was a junior attending Catholic high school, and, at the time, I did not be­lieve in God. Aquinas’ arguments had some intuitive appeal, but there were too many reasons to be skeptical. The universe could simply have had an eternal past, I thought, but Aquinas’ arguments assumed that it had a beginning. Not to mention, the need for things “in motion” to be “moved by another” was hopelessly archaic, for Isaac Newton showed that inertial motion requires no ex­ternal force. I could dispense with Aquinas’ arguments by simply believing that things have always been in motion and that, since motion is relative to a particular reference frame, there is no difference between mo­tion and rest.

These objections, I now realize, miss the mark—but perhaps I should back up a bit. Who was Thomas Aquinas? St. Thomas (1225 – 1274) was one of the most promi­nent philosophers, not just in the Catholic intellectual tradition, but in all of history. His project was to systematize Christian thought using the recently recovered works of Aristotle. He is notorious today not for the comprehensive nature of his philo­sophical system, but for his Five Ways, a set of demonstrations of the existence of God located at the beginning of the sprawling Summa Theologica. The stature and cred­ibility of these arguments has, however, substantially diminished since the time of their writing. Early modern philosophers eschewed classical Aristotelian metaphys­ics, which were the philosophical medium of the medieval university, for a more mechanical conception of the world. David Hume and Immanuel Kant, by some accounts, decisively refuted Aquinas’ Five Ways in the early 18th century. Today, they seem like quaint pieces of his­tory from a simpler and less advanced era, to which New Atheist writers like Richard Dawkins and Daniel Dennett need only pay lip service.

As a consequence, some might regard the Catholic Church’s claim that the reality of God can be known through natural reason as more than a little odd.[1] Perhaps the dogma is a dated article from a credulous era, but I would disagree. Part of the problem, I will venture, is that Aquinas’ arguments are often presented as though their force (or, perhaps, lack of force) should be obvious. I hope to avoid this sentiment: regardless of the soundness of Aqui­nas’ argument, some analysis and clarification will be required for any modern reader to understand it. I will argue that Aquinas’ First Way, given a fair assessment, is eminently defensible. I will use the text of Aquinas’ argument to clarify its relevant metaphysical claims and basic structure. I will also try to probe the limited sense in which Aquinas’ argument can contribute to our understanding of God.

Aquinas’ First Way, called the argument from motion for rea­sons which will become clear, was influenced by Aristotle, who first developed it in his Metaphysics. Aquinas writes:

The first and more manifest way [to know of the existence of God] is the argument from motion. 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 when 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. 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. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that 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. Therefore it is neces­sary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.[2]

Aquinas’ argument likely seems obscure, so let’s unpack it.

Aquinas repeatedly uses the terms actuality and potentiality. A central thesis of Aquinas’ philosophy is that things in the world are composed of actuality and potentiality (also, act and potency). These terms are, I think, less abstruse than they sound. One speaks of what something actually is, as opposed to what something has the potential to be. For instance, I hold my hand out and it is actu­ally open, so to speak of something’s actuality is just to speak of its present state of affairs, what it is. To say, then, that my hand has the potential to be closed is simply to speak about what state of affairs it could obtain but is not presently actual. My hand’s potentialities depend on what it is as a hand; a tree does not have an analogous potential to be “closed.”[3]

Aquinas uses these ideas to formulate his principle of motion (or causality), “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. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.” The first important distinction to make is that for Aquinas, as for Aristotle, motion refers to what a modern reader would call change, and, as such, is neither strictly Newtonian nor relativistic, but refers to a more general phenomenon.[4] Returning to the previous example, when I close my hand into a fist, my hand has changed. Local inertial motion was part of that change, but the change was not just inertial motion; there were also changes of a chemical and conformal nature. This is why Aquinas says that “motion is nothing else than the reduction of something from po­tentiality to actuality.” Any change, such as the closing of my hand, is an instance of what was previously possible becoming actual.

So far, what Aquinas has said is not particularly controversial. This Aristotelian characterization of change even has an intuitive ring to it: a change simply is a case of what is transforming into what could have been, a shift from one state of affairs to another. There is, however, another clause in Aquinas’ principle of motion. Aquinas says that “whatever is in motion is put in motion by another.” To avoid misunder­standings, it would be prudent to translate this as, “whatever is changing is changed by anoth­er.” This latter formulation will remind us that we are referring to a more general type of change, like the shaping of clay on a pottery wheel, rather than, for instance, a baseball’s being “in motion” as a result of being hit by a bat. Furthermore, changes occur “by some­thing else in act.” In other words, changes—motions, reductions from potency to act—only occur when they are caused.

David Hume famously denied this premise, saying, “As all dis­tinct ideas are separable from each other, and as the ideas of cause and effect are evidently distinct, ‘twill be easy for us to conceive any object to be non-existent this moment, and existent the next, without conjoining to it the distinct idea of a cause or productive principle.”[5] Hume denies that causes and effects are necessarily connected. First, we should note that the separability of ideas of cause and effect does not quite meet the characterization of change that Aristotle and Aquinas have provided. In formulating the prin­ciple of motion, Aquinas does not refer to events. For instance, if one hears a gunshot followed by a scream, then—though we might wonder—it is plausible to say the ideas are separable: the scream could occur without having been caused by the gunshot. But recall the paradigm case of causality in Thomism: the clay on a pottery wheel is shaped because it is acted on by the potter’s fingers. The hands and the clay can be considered as “distinct ideas,” and cer­tainly one could find other means to shape clay, but there is never­theless a disanalogy from the contingency of the gunshot and the scream. The causal relationship is one of unity: while the hands act on the clay, both of which being separate substances, skepticism about the cause of the change seems less warranted.

But let us grant, for the sake of argument, that Hume’s criti­cisms are at least relevant in some sense. Hume says that one can always imagine a change[6] occurring not by something else in act, but by nothing at all. This seems indisputable. In fact, we regularly ob­serve effects without observing causes. The problem with Hume’s argument is that these observations or imaginings do not them­selves imply that any effects lack causes.[7] Imagine, for instance, that something wildly spontaneous and implausible occurs: a large stone appears on your desk “out of nowhere.” Rather than suppose that such an event had no cause, you would try find what its cause was. You would likely countenance bizarre and far-fetched expla­nations before admitting that the stone appeared for no reason.[8] In the end, you would likely say, “I just don’t know why it appeared”—not, “It appeared for no reason.” This is not just because, as the Humean might say, our minds have been conditioned to associate cause and effect. The fact is that there is simply no valid infer­ence from an image of a stone’s appearing to the stone’s appearance lacking a cause, since such an image is indeterminate between an apparent lack of cause and an actual lack of cause, and as such tells us nothing about what is really happening.

But does Hume’s argument show that it is possible and logi­cally consistent for an effect to come about without a cause? The motivation here is that, if the negation of the principle can be held without logical contradiction, even without having any example of a genuinely uncaused event from which to draw, then the principle is not necessarily true and need not be accepted. This move—the claim that consistent conceivability implies real possibility—is du­bious, however. Aristotle, for instance, may have consistently held that the earth was at the center of the universe, but he was never­theless wrong on that point. The earth simply is not and could not be at the center of the universe; it’s impossible and would contradict what we now know about the universe. The advance of science, though, isn’t what made it impossible; it was impossible all along, whether or not anyone knew about it. An idea’s conceivability, then, does not entail its real possibility. An imaginable event, even a consistently imaginable event, like an uncaused occurrence, may be nothing but a vacuity, with no clear bearing on an otherwise sound principle.[9]

Leaving Hume aside, one might object that we already have examples of uncaused events. Physics’ description of radioactive decay, for example, has ruled out Aquinas’ principle of motion. Radioactive decay shows that some changes occur randomly. The theory cannot predict when a radioactive atom will emit a particle; it can only give an average rate of decay over time, an isotope’s half-life. Therefore, radioactive decay occurs without a cause. But the conclusion here does not follow, for it depends on an equivoca­tion as to the meaning of “cause.” The principle of motion does not require that changes occur deterministically, since indeterministic events are not necessarily uncaused.[10] Radioactive isotopes are not homogeneous substances which, as a brute, inexplicable fact, emit particles at semi-regular intervals. Rather, we know the empirically discovered patterns of radioactive decay to follow from the struc­ture of radioactive atoms—a complex topic, but a matter which does not preclude causality by a long shot. The indeterminacy of a stochastic model for radioactive decay (or similar phenomena) does not show that the phenomena they describe do not have un­derlying causes. It just reflects the fact that proximately descriptive theories do not need to make reference to causes.

Much more could be said on the principle of causality,[11] but for now we can grant it and ask: how does Aquinas get from an Ar­istotelian understanding of motion to a first mover? Consider the two examples Aquinas offers. The first is the burning of wood; the second, the movement of a staff by a hand. As has been implicit in my treatment so far—and perhaps in contrast to a more modern paradigm of causality—the motions that Aquinas is referring to are both simultaneous and instrumentally dependent. What does this mean? The combustion in a piece of wood is simultaneous to its being acted on by fire, and its doing so depends entirely on the activity of the fire. Similarly, the movement of a staff is simultane­ous to the movement of the hand and depends on the movement of the hand in the sense that, without the hand, the staff would not move. Hence, “whatever is in motion is put in motion by another.” That which is changing changes only because it is being changed by something else.

The requirement that motion be imparted by something else in motion naturally suggests a causal series. Consider, for instance, a stationary line of cardboard boxes on the floor. You push them at one end, and the rest of the boxes move. The last one in the line is moved by the one adjacent to it, that one moved by the one adja­cent to it, and so on. While stationary, each box has the potential to move and actually moves only as the box next to it is pushing. The box next to it, then, is “in act.” But that box was also stationary until moved by the box next to it. Such is the case with every box; each is moved by something else that is moving simultaneously. Further­more, the motion of each is, to repeat, instrumentally dependent on that of the others; the only reason that one box has any the power to move the next is because it is being moved itself. None of the boxes in the series have an intrinsic power to act. For this reason, Aquinas would call such a series essentially ordered. The changes are not just simultaneous to each other but rely on the others’ action. The causal efficacy of one box is due only to its being acted upon, so its causal efficacy is derived from the previous box.

Perhaps we can now see where Aquinas is going: “If that by which [something in motion] is put in motion be itself put in mo­tion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; see­ing that 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” (emphasis added). Aquinas aims at a con­tradiction: each element in a series like our cardboard boxes lacks an intrinsic capacity to act; each derives its causal power from the previous box. But if we suppose an infinite chain of causes, simulta­neously existing but each having only derived causal efficacy, then there should not be any causal efficacy in the series at all. But this is absurd, for the cardboard boxes do move. So there must be some el­ement which does have intrinsic causal efficacy; call it a first mover. In order to be first, such a being could not be moved by another. It is a mover which need not move, an agent which can act on others without changing itself. This is Aristotle’s Unmoved Mover.

A metaphor might be helpful: there is an image of a grizzly bear in a mirror. You follow the reflection only to find another mirror, in which the bear is reflected yet again. You continue along a series, finding an image at every step. Each image exists, however, only because there is actually a bear at the end of the series. While each reflection is derived from the actual bear and would go away the instant the bear stepped aside, the bear has its own intrinsic capac­ity to produce an image.

Returning to the previous example, one might object, of course, that the series of cardboard boxes does not move on its own, some­one might say, but it could and does move because I push it: is not man a first mover? The response is that, in a sense, man is indeed a first mover.[12] But man is not a first mover in the sense of being an absolute source of non-derivative activity, for man moves because parts of him move other parts, none of which have primary, intrin­sic efficacy. Man still cannot act without moving, or changing him­self, and so is not a true terminus of an essentially ordered series.

We may also dispense with another common objection, encap­sulated by Richard Dawkins. Lumping together Aquinas’ First, Second, and Third Ways, he claims, “All three of these arguments rely upon the idea of a regress and invoke God to terminate it. They make the entirely unwarranted assumption that God himself is immune to the regress.”[13] The accusation is one of special plead­ing: why arbitrarily exempt God from the principle that everything has a cause? But Thomas Aquinas does not make such a claim in the first place. The principle of motion states that every change has a cause. The premise allows that that which does not change (if such a thing exists) need not have a cause. Further, it is misleading to say that God has been “invoked,” for while Aquinas does end with, “and this everyone understands to be God,” he is aware that he has only shown that a first mover exists; the identity of the first mover with God requires further argument, which he offers in the Summa’s subsequent pages.[14]

It is to such considerations that we now turn. What follows from the existence of an Unmoved Mover? In the language we’ve been using, unmoved means unable to change, so a principally un­movable being, in Thomistic terms, must have no potentialities: it must be purely actual. Aquinas accordingly characterizes the Un­moved Mover as Pure Act. Al­ternatively, the Unmoved Mover might be referred to as Subsis­tent Being Itself: since it cannot change, it does not just exist but by its nature must exist. Unlike those of created things, the Un­moved Mover’s essence (or real definition, in Scholastic terms) just is existence, for which reason Aquinas prefaces his five demonstrations with God’s name as given in Exodus 3:14, “I AM WHO I AM.”[15]

This may seem superficial, so why else should we believe that the Unmoved Mover is the God of Abraham? The argument, of course, does not formally demonstrate that the Christian God ex­ists. It only claims to show that a being like God exists. By showing that the Unmoved Mover bears a number of the qualities com­monly attributed to God, it might become more reasonable to as­sociate the two, but we could not in principle reach a level of for­mal certainty, and so the argument does not eliminate the necessity of faith and hope.

Sometimes what is implied is that cosmological arguments, even if they were to succeed, only suffice to show that a minimal­ist deism is reasonable. Recall though that the Unmoved Mover as it is construed here could not be quite like an Enlightenment watchmaker. We used the metaphor of a bear in a mirror: the idea is that if the bear steps aside, the image vanishes. Likewise, the idea is that if the Unmoved Mover were to stop acting on the world, change would cease, for each change right now must be caused by the Unmoved Mover right now—the argument does not allow that the universe was “set up” at some point in the past and has been running on its own ever since. The divine attributes are no small topic, so here I just hope to demonstrate the general process by which they might be established from our conclusions so far. I’ll outline two approaches that are indispensable to Aquinas’ theology.

The first is negative theology. We do not directly sense God, and the argument certainly has not given us a complete understanding of God. Such is impossible for limited intellects. We can affirm some true propositions about God, but our understanding of His nature is limited: we can know that Pure Act exists, but the nature of Pure Act is puzzling to us—what does it mean for something to be purely actual, to lack any potentiality? As such, Aquinas de­termines many of God’s traditional qualities by way of remotion, based on what we know He is not, as well as from His effects.

Such limitations also require the application of St. Thomas’s other famous doctrine of analogy. Terms can be predicated univocal­ly, equivocally, or analogically. A term is univocal when it is applied in the same way in separate instances, as when we use green to say that a leaf is green and a book is green. To use a term equivocally is for uses to lack any common meaning: a rock is hard and my last test was hard. To use words analogically is to use them with similar but related meanings, as when one says that one has come to see a proof in math­ematics. We mean that we’ve come to understand the proof, so see is not being used in the visual, perceptive sense (for the math need not be accompanied by visualization), but the use of the term in both cases is related. Something substantial is preserved through disparate uses.

Let’s consider a couple of specific cases to see how these prin­ciples apply. Christians typically hold there to be one true God, but might not there be several Unmoved Movers—a unique one for each change in the world? We would have committed a quantifier shift fallacy to assume otherwise. The stock example of a quantified shift fallacy is the inference from “Everyone has a mother” to “Ev­eryone has the same mother.” In Aquinas’ argument, every change leads us to a first cause, but does every change leads us to the same first cause? This dilemma is not fatal. For two things to be numeri­cally distinct, one must possess some attribute that the other lacks. One would need to have some potentiality relative to the other—but if the argument succeeds, then any first cause must not have any potentiality whatsoever. So any “two” first causes would in fact be one: there could be no distinguishing Pure Act from Pure Act. Here we employed negative theology: we can’t observe two given first causes to see if they’re the same, but we can consider what a first cause could not be (lacking) to determine what God is (one).

What of the doctrine of analogy? Analogy plays a substantial role in arguments for those “hallmark” attributes of God: om­nipotence, intelligence, will, and goodness. Such topics require a thorough ontological account of the attribute in question, and as such cannot be defended at length here. For instance, does our first cause have a will, or is it a lifeless cosmic principle? First, one has to know what one means by will, and only then can we discuss wheth­er will could be predicated analogically of the first cause; such a discussion would be a foray into the broad discipline of philosophy of mind. I will sketch the question of goodness as another example.

Both analogical predication and negative theology play into the way in which we call God good. Aquinas held that good is always analogically predicated.16 For instance, what it means for a cat to be “good” is different from what it means for a tree to be “good.” It might be good for a cat to eat healthily and grow muscular, while it would be bad for the cat to lose all of its hair by coming in con­tact with a nasty chemical. It’s good for a tree to grow—although, its growing manifests itself differently from that of the cat—but it might be necessary for the tree to lose its leaves in the winter. What is preserved between usages of good is the notion of thriving relative to what a thing is; a cat is good if it succeeds at being a cat, and a tree is good if it succeeds at being a tree.

The issue when considering the question of whether God (as philosophically encountered) is good crops up in the fact that we don’t know what would make God “a good God,” for God’s full nature is ineffable. We know that, however goodness is predicated of God, it must be analogical (for goodness is always analogically predicated). This is where negative theology comes in: a thing (like a cat or a tree) is bad inasmuch as it lacks what it ought to have ac­cording to what it is. A cat is bad inasmuch as some injury immo­bilizes it, but it is, rather, a good for a tree to remain firmly rooted in the ground. But from what we have said before: God is Pure Act, and God has no lacks (or potentialities). As such, it seems like we are constrained to predicate a limitless goodness of God—even if the notion, without the aid of revelation, remains murky.[17]

This account has been brief—and, no doubt, controversial. It is intended as an example of the role of analogy in determining God’s attributes. There is much more to be said on such topics and others.[18] My hope is not to convince, but to spark interest and show that some knowledge of God might be possible by natural reason—as, of course, we should expect if God created us, endowed with intellects, with the purpose of knowing Him.



  1. Vatican Council I, Dei Filius 2: DS 3004; Vatican Council II, Dei Verbum 6. See also Catechism of the Catholic Church. 2nd ed. 32-33. Vatican: Libreria Editrice. Vatican 1995.
  2. Summa Theologica, Thomas Aquinas, Ia, Q. 2, Art 3, http://www.newadvent.org/summa/1002.htm, last accessed 11/11/13.
  3. Oderberg, David, Real Essentialism, ch. 4, New York, NY: Routledge, 2007.
  4. Feser, “The medieval principle of motion and the modern principle of inertia,” p. 4-17, in Proceedings of the Society for Medieval Logic and Metaphysics, 10, 2012.
  5. Hume, David, A Treatise of Human Nature, p. 79f, ed. L. A. Selby-Bigge, Clarendon Press: Oxford, 1978.
  6. Hume is speaking more specifically of things coming into existence, with post-Scholastic thinkers like Leibniz in mind in particular, but one might suppose that his claim aims at change from one state of actuality to another as well.
  7. Elizabeth Anscombe, “Hume on causality: introductory”
  8. Davies, Brian, The Reality of God and the Problem of Evil, p. 37-38, London: Continuum International Publishing Group, 2006.
  9. For a critique of modal metaphysics, see Ross, James, Thought and World: The Hidden Necessities, ch. 1-3, 8, Notre Dame, Ind.: University of Notre Dame Press, 2008.
  10. Suppose radioactive decay is indeterministic. Then events can still be caused by radioactive decay. For example, setting aside the considerations specific to the interpretation of quantum physics, the death of Schrodinger’s cat would be caused by a random emission of a particle. Though its death would be in­deterministic, it is still caused. But then, it does not seem that indeterminism and acausality are connected in the way they would need to be for such an objection.
  11. Oderberg, David, ‘“Whatever is Changing is Being Changed by Something Else”: A Reappraisal of Premise One of the First Way’, in J. Cottingham and P. Hacker (eds) Mind, Meth­od and Morality: Essays in Honour of Anthony Kenny, Oxford: Oxford University Press, 2010: 140-64. See also Pruss, Alex­ander, The Principle of Sufficient Reason, New York, NY: Cam­bridge University Press, 2006, for a consideration of the prin­ciple of sufficient reason, which claims that every contingent fact has an explanation.
  12. Immanent locomotion, or self-movement, is the defining, sa­lient property of life for Thomas Aquinas. For a contempo­rary treatment, see Oderberg, David, ‘Synthetic Life and the Bruteness of Immanent Causation’, in E. Feser (ed.) Aristotle on Method and Metaphysics, p. 206-235, Basingstoke: Palgrave Macmillan, 2013.
  13. Dawkins, Richard, The God Delusion, p. 100-101, London: Bantam, 2006.
  14. Feser, Edward, “The New Atheists and the Cosmological Argument,” p. 154-177, in Midwest Studies in Philosophy, 37, 2013.
  15. Gilson, Etienne. Elements of Christian Philosophy, New York: Doubleday, 1960.
  16. Summa Theologica, Thomas Aquinas, Ia, Q. 5, Art 1, http://www.newadvent.org/summa/1005.htm, last accessed 11/20/2013.
  17. A closer examination of God’s goodness, with relation to the problem of evil, God’s causality, and analogical predication can be found in Davies, Brian, The Reality of God and the Problem of Evil.
  18. Further reading: Feser, Edward, Aquinas: A Beginner’s Guide, Oxford: Oneworld Publishers, 2009. Davies, Brian, Thinking About God, Eugene, OR: Wipf and Stock Publishers, 2010. Garrigou-Lagrange, Réginald, Reality: A Synthesis of Thomis­tic Thought, Ex Fontibus Co., 2012, reprint. Oderberg, David, “The Cosmological Argument,” p. 341-350, in C. Meister and P. Copan (eds) The Routledge Companion to Philosophy of Reli­gion, London: Routledge, 2007.



Greg Brown is from Durham, CT, and is plan­ning to major in Mathematics. He enjoys reading, hiking, and playing cards.

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