(CG 1/012). Of the Opinion of those who say that the Existence of God is ... (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

WE will put first the reasons by which Aristotle proceeds to prove the existence of God from the consideration of motion as follows.

Everything that is in motion is put and kept in motion by some other thing. It is evident to sense that there are beings in motion. A thing is in motion because something else puts and keeps it in motion. That mover therefore either is itself in motion or not. If it is not in motion, our point is gained which we proposed to prove, namely, that we must posit something which moves other things without being itself in motion, and this we call God. But if the mover is itself in motion, then it is moved by some other mover. Either then we have to go on to infinity, or we must come to some mover which is motionless; but it is impossible to go on to infinity, therefore we must posit some motionless prime mover. In this argument there are two propositions to be proved: that everything which is in motion is put and kept in motion by something else; and that in the series of movers and things moved it is impossible to go on to infinity.

The Philosopher also goes about in another way to show that it is impossible to proceed to infinity in the series of efficient causes, but we must come to one first cause, and this we call God. The way is more or less as follows. In every series of efficient causes, the first term is cause of the intermediate, and the intermediate is cause of the last. But if in efficient causes there is a process to infinity, none of the causes will be the first: therefore all the others will be taken away which are intermediate. But that is manifestly not the case; therefore we must posit the existence of some first efficient cause, which is God.

Another argument is brought by St John Damascene (De Fid. Orthod. I, 3), thus: It is impossible for things contrary and discordant to fall into one harmonious order always or for the most part, except under some one guidance, assigning to each and all a tendency to a fixed end. But in the world we see things of different natures falling into harmonious order, not rarely and fortuitously, but always or for the most part. Therefore there must be some Power by whose providence the world is governed; and that we call God.