Such a mover must either itself continue to push or pull or perform both these actions, or else the action must be taken up by something else and be passed on from one mover to another (the process that we described before as occurring in the case of things thrown, since the air, being divisible, is a mover in virtue of the fact that different parts of the air are moved one after another); and in either case the motion cannot be a single motion, but only a consecutive series of motions.