In the course of our argument directed to this end we established the fact that everything that is in motion is moved by something, and that the mover is either unmoved or in motion, and that, if it is in motion, it is moved at each stage either by itself or by something else; and so we proceeded to the position that of things that are moved, the principle of things that are in motion is that which moves itself, and the principle of the whole series is the unmoved.