Prime Mover: An Amateur Defense

The “Prime Mover” argument is perhaps the oldest and most significant argument for the existence of God in the history of philosophy. Originating with Aristotle himself, it was revived and reasserted by the great Thomas Aquinas, who held it as the strongest proof for the existence of God. The Prime Mover argument comprises the first of Aquinas’ famous “Five Ways,” and it is the argument he spent the greatest amount of time and space in his writings defending. Despite this argument’s impressive history and importance to the philosophical tradition, it has largely gone out of “style” (as it were) in contemporary philosophy/apologetics. Today, the “Big Three” arguments used to establish/defend God’s existence are 1) the Kalam Cosmological argument, 2) the Fine Tuning argument, and 3) the Moral Argument. These are not by any means the only or even the best arguments being used today, but even just a cursory reading of relevant material should suffice to show that these arguments appear more frequently, especially to popular “apologetic” audiences. The prominence of the first two of these arguments is due largely to advances in modern cosmology. For example, the Kalam Cosmological argument originated from Islamic thinkers (versions of it actually go back much further, but Islamic thinkers, and some Jewish philosophers, really established it) and today has been robustly reintroduced and defended by William Lane Craig, largely due to the revolutionary turn in thinking in the 20th century regarding the physical beginning of the universe, namely, that our universe did have a beginning in time at the “Big Bang.” The Fine Tuning argument likewise arose out of the discovery of modern physicists that the fundamental forces of our universe had to be finely “tuned” for life to be even possible at all, and the chances of this are so astronomically minuscule that the best explanation seems to be (according to defenders of the argument) a supernatural designer. The moral argument has not emerged due to any scientific advancement (although conversations about the relation of morality to evolutionary biology are interesting and relevant), but, I would put forward, owe’s its popularity to C. S. Lewis’ famous discussion of the argument in his classic Mere Christianity.

All three of these arguments are certainly significant and worth exploring/talking about, and I do happen to find at least parts of the arguments convincing and otherwise successful. However, they are, in many respects, only secondary arguments compared to the philosophical proofs put forward by the ancient scholastic philosophers from Aristotle to Aquinas. That is to say, these arguments should not by any means be the first arguments one comes across/studies/builds a case for theism upon, and yet that is exactly the case in today’s contemporary philosophy of religion climate. The appeal of the contemporary “Big Three” arguments is that they do make use of current scientific consensus (and for modern thinkers, “scientific” seems to be synonymous with “intellectual” or “true”), but also that these arguments are easily formulated in simple, memorable syllogisms. For example, the Kalam argument as recently put forth has only two premises and a conclusion:

  1. Whatever begins to exist has a cause
  2. The universe began to exist
  3. Therefore the universe must have a cause

In order to comprehend this argument, one only needs to have a basic understanding of contemporary scientific consensus that any high school level text book should make available. For the older, classical arguments such as the Prime Mover argument, however, in order to even begin to grasp what the terms even mean, one must have an acquaintance with the deeper, underlying metaphysical concepts/worldview, which takes time and effort and digging to accomplish. Take one of the founding premises of the Prime Mover argument: Motion is potency reduced to act. This premise isn’t going to find a very broad appreciation in a society where the field of metaphysics, especially scholastic metaphysics, is largely ignored, or, if not ignored, mostly scoffed at. It’s not simple, it’s not easy, it doesn’t come in an easy-to-memorize syllogism. It’s deep and complicated and very much misunderstood. But it is, in my view, one of the best and most important arguments in history.

The Prime Mover argument, or the “First Way,” begins as such: Our experience via our senses tells us that some objects are in motion.

This seems, at first, glance, relatively straight forward and uncontroversial. One might even say it is so obviously true as to be not even worth stating. (One might say the same for the first premise of the Kalam argument, that “everything which begins to exist has a cause for its existence,” but that has not stopped skeptics of the argument for offering ardent objections). In fact, the debate over “motion” as such is ancient and somewhat intense.

When Aristotle came on the scene in the fourth century BC, one of the major philosophical debates going on was over the issue of “motion,” (motion in the classical sense, which today is closer to the concept of change). Philosophers were generally divided over whether or not change was a real feature of the world at all, and, if it were, what exactly this change entailed. This may seem silly to modern, scientific-minded people, but in reality it is quite an important metaphysical question.

Take, for example, a pot of room-temp water that is placed on a hot stove and in turn reaches a boiling temp. If I were to ask (as indeed I have, to several of my peers) how/why this change in temperature occurs, most people today would respond something to the effect of: “The molecules in the water are affected by the energy from the stove so that they speed up, bumping into each other. The speed of the molecules is what temperature is.” For many people, this answer is all that is needed, or at least, a similar type of answer that appeals to the physics of it all. Notice, however, that this does not explain how/why the temperature actually changes, it merely describes the change in a different way; rather than referring to the outer change as a whole, it now describes it in reference to more fundamental features, namely the molecular level. But this just pushes the question back, so that we could ask how it is that the molecules undergo change from one speed to another.

Don’t think that in any way I’m belittling the science or physics. I’m not, at all. The answer in reference to the physics of the change is an important answer, and it does give us a deeper, better understanding of reality. What I’m saying is that this answer alone does not give us the full picture; it may better explain what is happening, but it does not fully explain how/why it is happening. So what does?

For Aristotle and the scholastics, change (or “motion”) is potency reduced to act. That is to say, all material beings are composites of act and potency, where “act” is something that actually exists in a present state, and “potency” is something which has the potential to exist in actuality, but does not currently do so. So, for example, a piano in an empty room in act is silent (it is actually silent), but in potency it can produce sound (it has the potential to produce sound). A fuller description of these metaphysical terms would also make reference to the form or essence/nature of the being, which has the final cause or goal/end/purpose to produce certain effects. But just for the sake of this presentation of the argument, act and potency will suffice.

So, again, the first premise: motion is potency reduced to act.

Wherever there is change, this involves the potencies of an object being actualized. The room-temp water is actually room-temp, but it is potentially boiling. It is also has the potency to be frozen and thus to become ice. But it does not have the potency to, say, eat a banana or grow hair. When it does undergo motion, when it does change from room-temp to boiling, what is happening (metaphysically) is that its potencies are being actualized; its potency is reduced to act. A door that is closed is closed in act but it is open in potency; it has the potential to become open. When someone opens the door, this potency, intrinsic to the form (essence/nature) of the door, becomes actualized. A piano in an empty room is silent in act, but it has the potency to produce sound. When someone enters the room and plays the piano, this potency becomes actualized. The potency raises to act.

Which leads us to the next premise: All potency can only be actualized by another being which is itself already actual.

In other words, for potency to be actualized requires an agent which is itself actualizing some potency. A potency cannot actualize itself, because, by virtue of being a potency, it does not actually exist, it only potentially exists. If a potency could actualize itself, there is no reason why it does not always actualize itself.

So back to the previous examples. The water changes from room-temp to boiling, because its potency for boiling is made actual by the heat of the stove, which is itself already actualized. The water doesn’t boil itself. The closed door is open by someone who actually exists and opens it. The piano likewise is played by a musician; it’s potential to produce sound is actualized by an agent which is itself actualizing its potential to play music. And so on.

Now, any number of objections might be raised against this last premise. One might think of numerous examples which seem at first glance to prove that things can in fact actualize their own potentials. One example might be of a tree, whose leaves every fall change color; thus the potential for the color change is actualized by itself, with no outside agent involved. But, in fact, this example, when closely examined, turns out to support rather than discredit the premise. For what causes the green color of a leaf in the first place is its chlorophyll, and the production of chlorophyll is related to the amount of sunlight a tree receives. In autumn, the leaves receive much less sunlight, and so they stop producing chlorophyll, which means the green color is lost as well. Here, the scientific, biological description actually helps us to see how the metaphysics works. Leaves change from green to some other color (such as brown) because the potential to stop producing chlorophyll is actualized by the actual lack of sunlight and the actual colder temperatures. Even when an animal seemingly moves itself (as philosopher Edward Feser has pointed out), the premise still holds up. Because if an animal such as a dog, say, moves its front leg forward, what really happens is that the potential for this movement is actualized by the muscle contraction, and the potential for muscle contraction is actualized by the nerve impulse from the brain, which is actualized by the firing of neurons, and so on.

This leads to the crucial distinction between essentially ordered and accidentally ordered causal series. Think (again using an example from Feser) of a genealogy. A man begets a son who begets his own son, and so forth. If a man were to die and thus cease to exist, even though he caused his son to exist, his son could still continue to exist and could produce his own children. The existence of the son is dependent not upon the continued existence of the father, but only on a single causal act which brought about his existence. This is an accidentally ordered casual sequence. For an essentially ordered causal sequence, think of a musician playing a piano. The musician, by playing the piano, causes the music. But unlike in an accidentally ordered series such as a father who begets a son, if the musician stops pressing the keys of the piano, the piano will stop producing music. The music is dependent not only on a single causal act of the musician, but on the continued sustaining power of the musician. By pressing my foot down on the gas pedal of a car, I cause the car to move. If I lift my foot off, the car will come to a stop. The movement is dependent essentially on the pressing of the pedal. It is not that I can press the pedal once and that one causal act will keep the car moving. However, to turn the car on requires only the single causal act of turning the key (although it continues to require the key, so, considered in different ways, this is both an essential and accidental causal series).

Why is this distinction important? Because there are instances where the reduction of potency to act involves an essentially ordered causal series, such as playing a piano, and these causal series cannot in principle have an infinite regress.

Take the piano: a piano going from silent to producing sound involves the potency of producing sound being raised to act by the act of a hand pressing the keys. A hand pressing the keys is itself an instance of a potency being raised to act, namely the potency to press the key being raised to act by the movement of the finger. The movement of the finger is itself the potential for movement being actualized by muscle contraction (as in the example of an animal), which is a potency actualized by nerve impulse, which is a potency actualized by neuron firings, and so on, right down to the very molecular, atomic, and subatomic layers of reality. This is an essentially ordered series because, say, if the neuron did not fire, the impulse would not be sent, and the muscle would not contract, the finger would not move, the key would not be pressed, and the piano would remain silent. This is all happening simultaneously, here and now.

But, again, this causal series cannot go back in an infinite regress. Each step of the series we examined involved motion, a potency being raised to act. As we said, all material things are composites of potency and act. All material objects are actual in some ways, but they all have potentials, and this is true right down to the very fundamental layers of physical reality. But any and every time a potency is raised to act, it must be done so by something that is already act. But if that is itself an instance of potency being raised to act, it itself must be caused by something itself already in act. At some point, we must reach something that is PURE ACT, without any potencies. Why? Because if it had potencies which were being actualized, it would need something else that is already actual. In order for motion to occur in these essentially ordered sequences, it is metaphysically necessary that some being exists which is PURE ACT. 

If you hadn’t guessed it by now, this being of Pure Act, the Prime Mover, is what we call God.

(In my next post, I’ll examine why this being of Pure Act is God, and I’ll also look at common/major objections and critiques to this argument. Feel free to comment with your own objections, and I’ll try to examine them).

Written by Harrison Jennings



I must acknowledge that for the vast majority of this post I am greatly indebted to the works of philosopher Edward Feser, who has opened my eyes on so much. While this article is in my own words, and includes my own thoughts and analyses, the central ideas come from the writings of Feser. These include his books (The Last Superstition, Aquinas, Scholastic Metaphysics) as well as his online blog.

Feser, Edward. The Last Superstition: A Refutation of the New Atheism. South Bend: St. Augustine’s Press, 2008. ebook.

Feser, Edward. Aquinas. London: Oneworld Publications, 2009.


15 thoughts on “Prime Mover: An Amateur Defense

  1. You write,

    At some point, we must reach something that is PURE ACT, without any potencies. Why? Because if it had potencies which were being actualized, it would need something else that is already actual. In order for motion to occur in these essentially ordered sequences, it is metaphysically necessary that some being exists which is PURE ACT.

    I don’t think you’ve established this. The per se series you mention (the piano player), has a first mover: the mind of the piano player. The player decided to play the piano, and that initiated the per se series you mention. The mind of the player isn’t Pure Act, and if you appeal to the conservation of the mind, you’re talking about a different causal series. True, the player is contingent, and I can see how that series gets you to Pure Act, but the piano player series doesn’t appear to get you there.

    So, it appears that not all per se causal series gets us to a metaphysically simple being. I, of course, could be mistaken, and I welcome your thoughts.


    • Interesting point. I’m not sure I agree that the conservation of the piano player’s mind is a different causal series. The motion of the objects in the series (e.g., the piano player’s hand), as well as the causal activity of the movers in the series, both *presuppose*, and hence depend upon, the *existence* of the mind. It’s difficult to see how the series of movers could be actually distinct from the causal series upon which the mind’s existence is based, if the former directly depends upon the latter.
      But, also, this assumes that the “decision” of the mind to play the piano is something that isn’t actually “moved” by anything more ultimate. But I don’t think this is true. For Aquinas, the intellect itself is purely potential towards the objects of the understanding, which actualize it. At no point is the mind of piano player an absolute “first mover”, unmoved by anything else.


  2. Harrison,
    You write: ‘Even when an animal seemingly moves itself (as philosopher Edward Feser has pointed out), the premise still holds up. Because if an animal such as a dog, say, moves its front leg forward, what really happens is that the potential for this movement is actualized by the muscle contraction, and the potential for muscle contraction is actualized by the nerve impulse from the brain, which is actualized by the firing of neurons, and so on.’

    Where does the ‘and so on’ lead? As far as I can tell, everything Prof. Feser points to is contained within the animal; ie, a closed system, at least for the purpose of the illustration. Am I misreading? If not, what leads one to imagine that the natural world itself is not such a closed system?


    • Hi, Rodric. While not attempting to speak for Harrison, I’d like to offer my two cents. Yes, the proximate chain of movement is contained within the animal, but the animal’s motion is caused by its molecules, and the molecules’ motion is caused by its atoms, and the atoms’ motion is caused by strong/weak forces, etc., but this cannot proceed to infinity. Every time you go a step up and find something else whose existence must be accounted for in something else, you’ll never get to the first movement until you stop at something that accounts for itself—and that we know to be God. The rationally necessary stopping point is in something which has no potential to be moved—a being of Pure Act.


      • Thanks for the reply, Scalia. In the chain from the animal to the animal’s molecules to the molecules’ atoms and so on, I saw only parts of the animal itself; and so I was questioning whether there were necessarily anything outside the animal causing it to act.

        You mentioned ‘strong/weak forces’ but I don’t quite see how we can say that a force is a cause in itself. For example, an apple that falls to the ground isn’t caused to fall by gravity; it’s caused to fall by the mass of the earth. Gravity is the name we give to the pattern of masses attracting each other.

        It occurs to me that this kind of illustrates my concern. If we imagine two objects in an otherwise uninhabited corner of the universe that attract each other, each object causes the other to move. They form a closed system of movers, but neither is first. Or is there something I’m missing here?


        • Hello again, Rodric. I’m not quite certain how to proceed because I answered your question. Let me put it another way:

          Say you have a chandelier. You ask how the chandelier is suspended in from the ceiling. You’re told that a chain link is connected to the chandelier. How, then, does the chain link hold up the chandelier? Well, the link above it it holding it up, and so forth to the hook in the ceiling. How does the hook hold the chain? Well, the hook is connected to a stud in the ceiling. How does the stud hold the hook. Well, because it’s connected to the building frame. You hopefully get the idea. While the hook is indeed holding the chain which holds the chandelier, we know that the hook doesn’t ultimately account for why the chandelier is suspended from the ceiling. It cannot account for it because the hook itself is relying on something to hold it up. This is the act/potency distinction elucidated by Aristotle/Aquinas. The hook is in act, and as act can hold a chain, but it is also in potency to be held in place by the ceiling stud. Thus, the effect of suspension cannot be ultimately explained by something that is dependent (having passive potency). It can only be explained by something that is not dependent.

          The answer I gave remains: Every time you go back and find something whose cause or existence is in something else, you’ll need to keep going back. The only logical place to stop is in something which accounts for itself. That is, in something whose existence is explained by itself and not in something else. This would be Pure Act or Pure Existence. It really doesn’t matter how far up or down the chain you go. In every essentially ordered series (per se), each link is merely instrumental whereas the efficient cause initiates the series. That is, the causal efficacy is in the origin, not in any link. And since each link is a composite of act and potency (else it could not be an instrumental cause), the only place to stop is in something that has no passive potency, and that is Pure Act.

          Thus, to say that movement is wholly contained within an animal is incorrect, for the animal itself is moving via something beyond it. Again, whatever cannot account for itself is necessarily accounted for in another. And if that “other” account cannot account for its movement, then we must look beyond that to its cause. You will never get to the efficient cause until you get to the Unmoved Mover.


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