Now the moved must be in motion, but it need not move anything else; the instrument of motion must both move something else and be itself in motion (for it changes together with the moved, with which it is in contact and continuous, as is clear in the case of things that move other things locally, in which case the two things must up to a certain point be in contact); and the mover that is to say, that which causes motion in such a manner that it is not merely the instrument of motion must be unmoved.