According to St. Thomas Aquinas the existence of God is proved, as in Aristotle, by the argument of the unmoved mover. There are things which are only moved, and other things which both move and are moved. Whatever is moved is moved by something, and, since an endless regress is impossible, we must arrive somewhere at something which moves things without being moved. This unmoved mover is God. It might be objected that this argument involves the eternity of movement, which Catholics reject. This would be an error: it is valid on the hypothesis of the eternity of movement, but is only strengthened by the opposite hypothesis, which involves a beginning, and therefore a First Cause. Saint Thomas Aquinas (1225 - March 7, 1274) was a Catholic philosopher and theologian in the scholastic tradition, who gave birth to the Thomistic school of philosophy, which was long the primary philosophical approach of the Roman Catholic Church. ... Aristotle, marble copy of bronze by Lysippos. ...

In the Summa Theologiae, five proofs, the famous quinquae viae (five ways), of God’s existence are given.

• i. First, the argument of the unmoved mover as explained above;
• ii. Second, the argument of the First Cause
• iii. Third, that there must be an ultimate source of all necessity; this is much the same as the second argument;
• iv. Fourth, that we find various perfections in the world, and that these must have their source in something completely perfect.;
• v. Fifth, that we find even lifeless things serving a purpose, which must be that of something outside them, since only living things can have an internal purpose.