The Summa Contra Gentiles (Illustrated)

And if this again passes from potentiality to actuality, it must be preceded by something else, whereby it can be brought from potentiality to actuality. But we cannot go on thus to infinity. Therefore we must come to something that is wholly actual and nowise potential. And this we call God.

This, in a certain respect, is not in contradiction with the arguments of Aristotle; for it makes no difference whether with Plato we come to a first mover that moves itself, or with Aristotle to something first which is altogether immovable.

For if movement had a beginning, it must have had its beginning from some mover. And if this mover had a beginning, it had its beginning from some agent. And thus either we shall go on to infinity, or we shall come to something without a beginning.

For matter, such as it is, is in potentiality.

Now every necessary thing either has a cause of its necessity from without, or has no such cause, but is necessary of itself. But we cannot go on to infinity in necessary things that have causes of their necessity from without. Therefore we must suppose some first necessary thing which is necessary of itself: and this is God, since He is the first

Therefore if one of its parts is at rest, it follows that the whole is at rest. For if, while one part is at rest, another of its parts were in motion, the whole itself would not be moved primarily, but its part which is in motion while another is at rest. Now nothing that is at rest while another is at rest, is moved by itself: for that which is a

Because the force of the argument lies in this, that if a thing moves itself primarily and of itself, not by reason of its parts, it follows that its being moved does not depend on some thing; whereas with a divisible thing, being moved, like being, depends on its parts, so that it cannot move itself primarily and of itself. Therefore the truth of

Nothing is at the same time in act and in potentiality in respect of the same thing. Now whatever is in motion, as such, is in potentiality, because motion is the act of that which is in potentiality, as such. Whereas whatever moves, as such, is in act, for nothing acts except in so far as it is in act. Therefore nothing is both mover and moved in

Only things which are moved are measured by time: because time is the measure of movement, as stated in 4 Phys. Now God is absolutely without movement, as we have already proved. Therefore we cannot mark before and after in Him. Therefore in Him there is not being after non-being, nor can He have non-being after being, nor is it possible to find an