45. The Being that is Pure Act. Proofs of its Existence. -- Above all changing things, which are mixtures of act and potency, there reigns supreme one immovable Being, which is Pure Actuality (to ti en einai to prôton).

The principal proof of the existence of God is based on the existence of change (motus). Movement, though eternal, is unintelligible without a Prime Mover, itself immovable. For, as nothing can pass, of itself, from potency to act (44, 3), all movement or change necessarily supposes a mover; and unless we grant some one prime mover, itself entirely beyond and free from the influence of all change whatsoever, we are forced to admit -- for the explanation of actual change -- that there exists an infinite series of moving causes: which is absurd. Aristotle adds to this a second proof, based on the order, harmony and unity of the world, -- known afterwards in philosophy as the teleological proof.