In one of his proofs for the existence of God, Aquinas says, "If that by which it is moved be itself moved, then this also must be moved by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover."

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Aquinas' argument is based on the idea that there must be a first mover, which cannot be moved by anything else, in order to avoid an infinite regress of motion.

In One of His Proofs for the Existence of God