The Motion of Things

Some things subsist in this manner but some are always at rest and some are always moved. The body happens to be moved by and contrary to nature for it is of consequence with what kind of motion it may happen to be moved. Some move according to accident but others essentially according to accident indeed as terrestrial things when moved upward and fire downward again the parts of animals are frequently moved contrary to nature on account of positions and modes of motion. It seems that as in ships and things which have not a natural subsistence so likewise in animals that which moves and that which is moved are divided from each other and that the whole thus moves itself.