But the series must stop somewhere, since the kinds of motion are limited; and if we say that the series bends back, ie that that which is causing alteration is in process of locomotion, we do no more than if we had said at the outset that that which is causing locomotion is in process of locomotion, and that one who is teaching is being taught; for it is clear that everything that is moved is also moved by the mover that is further back in the series in fact the earlier mover is that which more strictly moves it.