Since there must be continuous motion in the world of things, and this is a single motion, and a single motion must be a motion of a magnitude (for that which is without magnitude cannot be in motion), and of a single magnitude moved by a single mover (for otherwise there will not be continuous motion but a consecutive series of separate motions), then if the mover is a single thing, it is either in motion or unmoved: if, then, it is in motion, it will have to keep pace with that which it moves and itself be in process of change, and it will also have to be moved by something: