He developed five proofs for the existence of God using logic. The first three were "cosmological" proofs rather than the "ontological" approach of St. Anselm. A cosmological proof deals with the natural order of the universe. Aquinas' most famous cosmological argument was that whatever is in motion (for example, us) must have been put in motion by something else (our parents). They, in turn, must have been put in motion by something else (their parents). But this sequence cannot go on to infinity. There must have been a first mover. This we call "God".