According to the argument of motion from Aquinas, why can’t there be an infinite regress of movers? Why does it have to stop at one unmoved one?

The reason philosophy avoids any “infinite regress” is that it leaves us with situations which are redundant, unanswerable, impossible, or useless. Aristotle first tackled this problem in his own philosophy, and it had a great influence on St. Thomas Aquinas’ cosmological arguments.

The reason the universe “has to” stop at an “unmoved mover” (another way to say this is a first cause) is because we would never have a real answer to one of the most important philosophical questions: “Why is there something rather than nothing?” In other words, why is it that the universe exists at all? That is the question that the first cause attempts to answer.

An infinite series of causes indicates that each cause in the series came into being because of the one before it. In other words, cause Z exists because of cause Y, cause Y exists because of cause X, etc. No matter how far you push those causes backward, you’re only delaying the inevitable question: How did it get there in the first place? No matter how far back you go, something had to begin setting things in motion, or else nothing would be in motion at all.

But what if, like the pre-classical Greeks or the Buddhists, we take the nature of the universe to be cyclical? What if all the causes in the universe are just moving in an endless cycle of living and dying, coming into existence and ceasing to exist?

The answer is that we would still have the same problem as before. There’s no place in that cycle where anything “new” is introduced–it’s a closed system. Even if the universe was a giant cycle… We know that the cycle is there, but that doesn’t answer how the cycle got there.

The laws of physics (specifically thermodynamics and entropy) also show us why an infinitely cyclical or an infinitely regressing universe is a problem: in a closed system, matter cannot sustain itself infinitely because eventually physical matter will be converted into heat and various forms of energy. Funnily enough, that is also why inventing a true “perpetual motion machine” is impossible: without new matter coming from somewhere, whatever matter is present will expend its potential energy in the form of kinetic energy until eventually no matter is left. If we take those facts to their logical conclusion, then even scientifically the idea of a universe which has always existed without any beginning doesn’t make sense.