St. Thomas Aquinas’s Five Ways are well-known paths we can take to prove that God exists. The first way of the five argues from motion.
Imagine a stone that’s being moved by a stick in my hand. There’s nothing about the stone itself that explains its motion. It’s in motion only because the stick is moving it.
Likewise, there’s nothing about the stick itself that explains its action of moving the stone. The stick is only doing that because of my hand—and not just as it begins to move the stone, but at every moment it moves the stone.
So we have a thing being moved—a stone—and a series of movers simultaneously working together to move it: a stick, my hand, and me.
Now let’s ask a question: can every mover in this series be like the stick? This may seem like a random question, but our pursuit of the answer is going to guide us to the heart of St. Thomas’s argument.
Remember there’s nothing about the stick itself that explains its action of moving the stone. Philosophers would say there’s nothing about the stick itself (its nature) that explains the actuality of it acting as a mover. (By actuality we simply mean the stick is actually acting as a mover rather than only having the potential to act as a mover.) But if nothing about the stick itself explains its actuality as a mover, then the stick must receive that actuality from something else—another mover.
Likewise, if every mover were like the stick, then no mover would have of itself the actuality needed to act as a mover. Each one—the stick, my hand, me—would need to receive such actuality from another mover. And if that’s the case, then the stick would never actually act as a mover, nor my hand, nor I, and so on, because there’d ultimately be nothing for any mover to receive its actuality from.
But the stick is actually acting as a mover, since we posited that at the start. So we’ve answered the question, “Can every mover in this series be like the stick?” with a definitive no.
Therefore, there must be a different kind of mover, something unlike me, my hand, and the stick, that has the actuality needed to act as a mover simply by nature, and not because it receives that actuality from something else.[1] Such a mover is pure act. Otherwise, as we’ve seen, there’s no explanation for why anything else has the actuality needed to act as a mover at all.
Such a being, for Aquinas, is what we call God.
[1] You might think that I, as a mover, do have the actuality of myself to act as a mover. But you’d be mistaken. Although I do have a principle in virtue of which I can act as a mover—namely, my will—the actuality needed to act as a mover still must be derived from something outside me. The reason is because I move from not actually willing to act as a mover (in which case I am only potentially willing) to actually willing to act as a mover. This being the case, my nature, considered of itself, is merely in potential to receiving the actuality needed to act as a mover. And this is why I, as a mover, am like the stick, metaphysically speaking.