Physics (Aristotle/Wikisource translation)/Book VII
Every primary mobile is moved by an external motor. The motor is entirely disjoint from its primary mobile; they share no parts. Even automobilia are divisible into the source of the motion and the primary mobile. Can there exist a primary mobile AC that is part of the source of its own motion? No. To move as a whole requires external help; AC cannot be both motor and mobile uniformly throughout. A train does not move itself as a whole; it is divisible into a locomotive and baggage-cars.
If a motor causes a certain transformation or growth in a certain time, it will complete half the transformation in half the time. If a mobile is halved, the same motor will transform it the same amount in half the time. However, half the motor will not necessarily cause the same transformation in twice the time; it may well cause no change at all. For example, one man cannot drag a ship by himself for any distance; it is too heavy.