This is a Cosmological Argument for the Existence of God. This represents my current understanding of Aquinas’s First Way, the “Argument from Motion,” and it is my attempt to describe a version of it for a modern layman like myself. If I have misunderstood Aquinas, or made a mistake in reasoning, be sure to point it out in the comments!
Let’s talk about change. Let’s talk about it in the most general and fundamental sense, not just cultural change, not just behavioral, or physical, or chemical, but the very metaphysics of change. What must be true for any change whatsoever to occur?
The Change Problem
A very long time ago, a Greek philosopher named Parmenides, argued that change has to always involve annihilation and creation. When a thing changes, it transitions from an initial state to a final state. For this to occur, the thing’s final state transitions from non-being to being, from non-existence to actual. The thing’s initial state is annihilated, as it transitions from actual to non-existent. For example, as a candle changes from an unlit candle to a lit one, the unlit candle no longer exists, and the burning candle is now actual. The burning candle went from a state of non-being, or nothing, to being. But nothing is just not-anything – and from nothing, nothing comes. So, Parmenides concluded, change cannot actually occur, it’s an illusion.
His argument goes something like this: 
- Change would require being to arise out of non-being or nothingness, but
- From non-being, or nothingness, nothing can arise, so that
- Change is impossible
He further argues that there is nothing to differentiate one being from another being, since there is only non-being as another candidate. But again, non-being is just not anything at all. Therefore, he says, all that exists is one, eternally unchanging being. 
Of course, this should seem absurd. After all, to even argue against change presupposes change. To argue assumes that the listener (a different being) can change his mind, while the arguer moves from one premise to the next. Now, the self-refuting nature of the argument is enough to show that it’s wrong, but we haven’t answered why it’s wrong. After all, Parmenides seems to be right about (2): from nothing, nothing comes. To deny that, would lead to even greater absurdity. After all, if something can come from absolutely nothing, there would be no way to explain why anything and everything doesn’t just pop into being.
So we have to say change is possible, but how is it possible?
The Potential Solution
Premise 1 is the problem. As Aristotle pointed out, there are more than two categories of being. Not only can things be, and not be, but they can potentially be. So, instead of the two categories Parmenides reasons with, there are three categories: 1) Act, or actuality, 2) Potency, or potentiality, and 3) non-being, or utter non-existence. The unlit candle has the potential to be burning, and the potential to be melted. In the unlit candle, the candle’s being melted isn’t completely non-existent, even though it is not actual – it exists potentially. This “potency” is an inherent part of the unlit candle; the candle has this as a potential for change. In contrast, the candle cannot change into coffee, or a cat, or a rain cloud, since it does not have these potencies.
So, change occurs when something in a state of potentiality transitions to a state of actuality. Or to use the technical verbiage, change occurs when potency is reduced to act. This also means, that all changeable things that exist have potency and act as inherent components of themselves. 
Premise 1 – Change Occurs.
From Change to Changer
Notice, though, that potency is not act. The burning candle is not actually in the unlit candle, but only potentially (otherwise it would no longer be unlit). So the burning candle cannot do anything, or cause anything, while it only potentially exists in the unlit candle. It cannot for instance, burn my hand, because it is not yet actual. So only what is already actual can cause something to go from potentiality to actuality. Change must be caused by what is actual.
A thing cannot be in both actuality and potentiality, at the same time and in the same way. So it is impossible for something to be both the thing changing, and the thing causing the change. The unlit candle is actually unlit, and potentially burning. When a match lights the candle, the burning candle is actually lit, and potentially unlit. In technical terms, we could say that when lit by the match, the candle’s state of burning was reduced (or raised) from potency to act, while its being actually unlit became potentially so.
Then, whatever changes is changed by another. More precisely, it is impossible for something to be actualizing itself. This is true even for the candle’s melting being actualized by its burning. Since even the candle’s constituent parts are divided by act and potency. The actually burning wick is actualizing the melted-ness of the wax below it. Besides, “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.”
Premise 2 – Whatever changes is changed by another.
Now if this outside actualizer is itself changing, it too must be changed by another, and so on.
A Chain of Changers
At this point, let’s consider a series of changing changers. My hand and the air around it is heating up, and the candle wax is melting. These changes are being caused by the burning candle wick. The burning candle wick is composed of carbon reacting with oxygen, exchanging electrons in roiling chemical change, which in turn are being changed by fluctuating electromagnetic fields, and photon exchanges… How deep can we go? At each deeper level of physics, we still see change (more on this in part two). If we return the analysis to act and potency, we see that each new level of changer still has potentiality in its being.
The power of each thing to change the next in the series comes from some potency in the changer reducing to actuality. In other words, each changing changer derives its power from the previous changer in the series. (Now is this true for all things that are changeable? We can and should investigate this claim more, but for now, there is at least strong evidence for it. There are no clear counter examples, and it is not obvious what sort of natural phenomenon might be a candidate, even hypothetically.)
Premise 3 – Whatever is changeable is changed in the act of changing another.
This series does not necessarily stretch back in time, but it instead stretches upward and outward in hierarchy. Even if the effects take time to manifest, so that the ultimate effect is not simultaneous in time with a previous cause, the series is still simultaneous in causality.
That being the case, this series of changers cannot go on to infinity. That would be like an infinitely long line of boxcars being pulled by even more boxcars, but no locomotive. Or it would be like a chain holding a chandelier, link after link deriving its suspending power from the next link. Could that chain continue forever without being attached to any ceiling? The train cars move only inasmuch as they are moved by the locomotive, and the chain holds the chandelier only inasmuch as the structure of the house holds the chain.
Premise 4 – This series of changing changers cannot regress infinitely.
So this series of actualizers must terminate in something which has no need to be actualized itself. This must be pure act with no potentiality in its being.
The Unmoved Mover
- Change occurs.
- Whatever changes is changed by another.
- Change is something existing potentiality becoming an actuality. Only what is already actual can cause something to go from potentiality to actuality.
- A thing cannot be in both actuality and potentiality, at the same time and in the same way.
- Whatever is changeable is changed in the act of changing another.
- Now if this changer is itself changing in order to cause this change, it too must be changed by another (2), and so on.
- This series of changers cannot regress infinitely,
- because each changing changer derives its power to change the next, from the previous changer in the series.
- Therefore, all such changers are only instrumental changers. Just as boxcars are instrumental in pulling other boxcars, with no locomotive, no boxcar would be pulled.
- Therefore, this series of changers must regress to an unchanging changer.
- By (3), this unchanging changer, or unmoved mover, must be unchangeable; pure actuality with no potentiality in its being.
So what is Pure Act?
At the very least, it is nothing material, nothing with dimension or extension, nothing with quantity or measure. It is nothing inert, but it is active – since it changes all things changeable. It could be nothing temporal, because it is utterly unchanging. There can be only one such being, even in principle, since as Parmenides showed us, being is only differentiated by potency. So pure being, or pure act, would be utterly one and undifferentiated.
Pure Act is space-less, timeless, immutable, unique, and the active first cause (in hierarchy) of all change. As Aquinas says,
This everyone understands to be God.
Wow! Next in this series, we’ll consider some objections, and see if this argument holds up. Along the way we’ll also consider just what this argument leaves us with, if successful. Check it out.
Sources and further reading:
Edward Feser, Aquinas, The Last Superstition
Norman Geisler, Systematic Theology, 24-25
Southern Evangelical Seminary – WhyDoYouBelieve.org
Thomas Aquinas, Summa Theologiae 1.2.3, Summa Contra Gentiles 13, Compendium Theologiae 3
Aristotle, Physics Book VIII, Metaphysics Book XII
 As stated by Edward Feser. Feser, Scholastic Metaphysics, editiones scholasticae: 2014, 31-32
 A view called Absolute Monism. Geisler, Systematic Theology, Bethany House: 2002, 19
 Feser, Aquinas, Oneworld Publications: 2009, 11
 I don’t think this argument has to rest on induction alone, but philosophical support for this claim is very involved. I just don’t have room to get into the weeds of this premise, for now. Aquinas, from what I can tell, assumes this to be understood in the Summa Theologiae. He says elsewhere, “For whatever is moved is divisible and a body, as is proved in the Physics [VI, 4] [Aristotle]. But every body that moves some thing moved is itself moved while moving it.” – Aquinas, Summa Contra Gentiles 13..
 Edward Feser uses this sort of “static” causal series in his version of the argument, and it would effectively replace premise 3 in my version here. We could call them “supporting causes” of change. The candle can only warm my hand because they are in close proximity. In part, this is because the candle is being held up by the table. But certainly it had the potential to be outside or on the floor, as well as the potential to be here on the table. The difference in those potentialities is, here and now anyway, that one of them is actual – the table is actualizing a potential in the candle to be near my hand. In turn, the house foundation is actualizing this potential in the table, and in turn, the Earth is upholding the house. Further still, the EM fields, strong and weak nuclear forces, and the gravitational field, each oriented in this way instead of another, are all holding the Earth together, which holds the house, which holds the table, which holds the candle close to my hand. If we can consider the necessary state of things as a potencies that are actualized by deeper levels of reality, then this also cannot continue forever. See Feser, Aquinas, 74-76
(I am not sure that I am convinced by his approach, but I’m not sure where it would be wrong, either. That being said, I’m also not sure that it’s necessary for the argument to work.)