Chapter 4 — That God is immovable

From this it is clear that God must be immovable. For, since he is the first mover, if he were moved he would have to be moved either by himself or by another. He cannot indeed be moved by another, for then there would have to be another mover prior to him, which is against the very idea of a first mover.

If he is moved by himself, this can be conceived in two ways: either that he is mover and moved according to the same respect, or that he is a mover according to one aspect of him and is moved according to another aspect. Now the first of these alternatives is impossible. For since everything that is moved is, to that extent, in potency, and whatever moves is in act, if God were both mover and moved according to the same respect then he would have to be in potency and in act according to the same respect, which is impossible. The second alternative is also impossible. If one part were moving and another were moved, he would not be the first mover essentially, but by reason of that part of him which moves. But what is essential is prior to that which is not essential. Hence there cannot be a first mover at all, if being the first mover belongs to him by reason of one of his parts. Accordingly, the first mover must be altogether immovable.

The same thing can be investigated based on things that move and are moved. For every motion is observed to proceed from something immobile, namely, from something that is not moved according to that species of motion. Thus we see that alterations and generations and corruptions occurring in lower bodies are traced back (as to a first mover) to a heavenly body that is in no way moved according to this species of motion, since it is incapable of being generated, corrupted, or altered. Therefore, the first principle of all motion must be immoveable.