Perhaps the most famed logical proof of all for the existence of God is the so-called “argument from motion.” Although originating from Aristotle, the argument in its most popular and fully developed form comes to us from the General Doctor, St Thomas Aquinas. Unfortunately, however, it is also one of the most frequently misunderstood. For most take the argument from motion to be regarding kinetic causation. But, in truth, the argument of motion is something far more subtle.
Let us take a look at the summarized argument as put forth in the Saint’s Summma Theologicæ :
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. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, [such] 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.
Right from the onset, modern English readers face a problem—and this problem is the word “motion.” For the modern English reader, “motion” means local change, and only local change. Yet, when Aquinas uses the term “motion” (well, actually he used the Latin term “motus,” which is invariably translated into English as “motion,” due to “motion” being clearly derived from and having the same common use as “motus”), it means something more; to St Thomas, the word “motion” means any change whatsoever. And this is apparent in that he describes becoming heated as motion—although we know today that heat is actually the result of local change on the molecular level, the men of Thomas’ day did not, believing heat to be nothing more than a quality, rather than a local change. So it is evident that “motion” here does not mean local change, but any change.
Thus Aquinas goes on to use the Aristotelian definition of change: motion is nothing else than the reduction of something from potentiality to actuality . In other words, when a potential state becomes an actual one, something has changed.
Further, St Thomas notes the following: nothing can be reduced from potentiality to actuality, except by something in a state of actuality . And this is because actuality taken in regards to itself—in other words, act qua act—is being itself, which for those who are unfamiliar with the Thomistic distinction between being and essence, is the force of a thing’s existence, or that which posits an aspect of an essence into reality (ref De Ente et Essentia, or On Being and Essence, by St Thomas Aquinas for more detail). And in contrast, potential taken in regards to itself—or potency qua potency—is non-existence. And nothing which does not really exist causes anything; such would be absurd. Therefore: nothing can be reduced from potentiality to actuality, except by something in a state of actuality .*
Moreover, the above further implies that “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 it is a direct logical contradiction for something to simultaneously exist and not exist in the same manner at the same time.** And therefore it is again impossible that a non-existent thing can provide its own being, or be the cause of its own existence.
So St Thomas states the necessary conclusion: whatever is in motion must be put in motion by another . Put in a way less likely to be misunderstood: whatever changes must be put into a state of change by something else, as per the above.
But crucially in this argument Aquinas tells us: 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 . Expressed mathematically, if we count the number of causes required to amount to any given result (which itself will be a cause)—let’s say resulting cause number five, for example—within a causational system which regresses infinitely, we will run into a problem: . This means that no matter how many causes exist within an infinitely regressing series, there will never be enough to produce cause number five—or any other given cause, for that matter (). In other words, if causation regressed infinitely, nothing would be caused and nothing would change, because no number of causes could ever produce any result. Therefore, because things do change, causation in the real world does not regress infinitely.
“Therefore it is necessary to arrive at a first mover, put in motion by no other .” Now, as something can only cause another insofar as it is in act, and as anything with potential is only in act insofar as it has been caused to be in act, with the first mover being by definition not itself caused, it follows necessarily that it lacks potential, and is perpetually actual. It is therefore immutable, because of this lack of potential, which is necessary for motion (ie change). Further it is therefore eternal, neither coming to be nor going out of being (which would be change). Even more, it is immaterial, because all material has the potential for local change. And, as Occam’s razor then dictates, intelligent, as the only known immaterial actual thing is the intellect (ref “On the Soul“). The implications go further, and are enumerated on in both of St Thomas’ summas, Summa Contra Gentes, and Summa Theologicæ. When all are considered, they lead us unavoidably to his closing remark on the unmoved mover: this everyone understands to be God .
*Some object to this, saying that it appears as though matter (matter as defined in physics, not Thomism) seems to cause itself as per observations; however, in the case of matter, its being is afforded by the act of this universe’s existence (and perhaps some other donor of which we simply don’t know yet). This is evident insofar as matter does not exist except for within space—which is the four-dimensional “fabric,” so to speak, of the universe.
**Some object to this, saying that is appears as though matter (matter as defined in physics, not Thomism) seems to simultaneously exist and not exist in the same manner at the same time at the subatomic level, as per quantum superposition and the thought experiment of Schrodinger’s cat; however, as per Heisenberg’s uncertainty principle, this is merely the case of the position and velocity (or any other paired traits) of a given particle being unable to be simultaneously determined with exact accuracy. For example, if we are sure of a particle’s speed, we can only probabilistically calculate its position, and hence it is said to have a “superpostion,” because we cannot determine its position for certain. Therefore, in epistemological terms, the position can have multiple states, but, ontologically speaking, it has one state which we can only accurately measure while sacrificing the accuracy of another measure. Simply put, this objection is based on human limitation, not reality as it really is.
 Aquinas. ST I. Q2. A3. trans. Fr’s OP