How does Aquinas’s argument from motion lead to the idea of a prime mover?

St. Thomas Aquinas’s argument from motion, the first of his Five Ways, is a cosmological argument for the existence of God. It is grounded in the observation that everything in the natural world that is in motion is moved by something else.1

Aquinas drew upon Aristotle’s principle that an infinite regress of movers is impossible. If every object were moved by another, there would be no initial source of that motion, and thus no motion would exist at all. Consequently, logic dictates that the series of movers must terminate in a prime mover—an entity that initiates motion but is not itself moved or acted upon by any other agent.12

Aquinas identified this unmoved prime mover as the Christian God. By moving from the finite, changing nature of the world to the necessity of an unconditioned, unchanging source, he sought to demonstrate that God serves as the ultimate foundation for all causation and existence.13