Saint Thomas Aquinas' Five Proofs of God's Existence

St. Thomas, in his Summa Theologica, gives five separate proofs for the existence of God. Unlike St. Anselm's proof, which deals with concepts, Aquinas' proofs rely on empirical facts - what we can observe. In these proofs we can 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 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. 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 God.

4. The Proof from Degrees of Perfection

We all evaluate things and people in terms of their being more or less perfectly true, good, noble and so on. We have certain standards of how things and people should be. But we would have no such standards unless there were some being that is perfect in every way, something that is the truest, noblest, and best. That Most Perfect Being we call God.

5. The Proof from Design

As we look at the world around us, and ourselves, we see ample evidence of design--the bird's wing, designed for the purpose of flight; the human ear, designed for the purpose of hearing; the natural environment, designed to support life; and on and on. If there is design, there must be a designer. That Designer we call God.