What real evidence can be supplied for God's existence? St. Thomas, in his Summa Theologica, sets forth five separate proofs for the existence of God, Unlike St. Anselm's proof, which deals with pure concepts, St. Thomas' proofs rely on the world of our experience-what we can see around us. In these proofs we can easily see the influence of Aristotle and his doctrine of the Four Causes.
l) The Proof from Motion. We observe motion all around us. Whatever is in motion now was at rest until moved by something else, and that by something else, and so on. But if there were an infinite series of movers, all waiting to be moved by something else, then actual motion could never have got started, and there would be no motion now. But there is motion now. So there must be a First Mover which is itself unmoved. This First Mover we call God.
2) The Proof from Efficient Cause. Everything in the world has its efficient cause--its maker--and that maker has its maker, and so on. The coffee table was made by the carpenter, the carpenter by his or her parents, and on and on. But if there were just an infinite series of such makers, the series could never have got started, and therefore be nothing now. But there is something everything there is! So there must have been a First Maker, that was not itself made, and that First Maker we call God.
3) The Proof from Necessary vs. Possible Being. Possible, or contingent, beings are those, such as cars and trees and you and I, whose existence is not necessary. For all such beings there is a time before they come to be when they are not yet, and a time after they cease to be when they are no more. If everything were merely possible, there would have been a time, long ago, when nothing had yet come to be. Nothing comes from nothing, so in that case there would be nothing now! But there is something now-the world and everything in it-so there must be at least one necessary being. This Necessary Being we call