moves and changes it. Now it is not possible that the same thing should be at once in act and potency in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot, though it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, that is, that it should move itself. Therefore, whatever is moved must be moved by another. If that by which it is moved