Of things, therefore, which move and are moved, some move and are moved according to accident; but others essentially. According to accident indeed, such as are inherent in movers, or the things moved, and such as have the relation of a part. But such things move and are moved essentially, as move and arc moved, not because they are inherent in that which moves or is moved, nor because a certain part of them moves or is moved. Of those things, however, which are moved essentially, some are moved by themselves, and others by something else; and some by nature, but others by violence and contrary to nature: for that which is moved by itself is moved by nature, as, for instance, every animal; since an animal is moved by itself. But of such things as contain in themselves the principle of motion, of these we say, that they are moved by nature. Hence, the whole animal indeed, itself moves itself by nature; but 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, and from what clement it consist. Of those things also which are moved by something else, some are moved by nature, but others contrary to nature. Contrary to nature, 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. And it is especially obvious that a thing which is moved, is moved by something, in things which are moved contrary to nature, in consequence of the being moved by something else being evident. But after things which are moved contrary to nature, among such as arc moved according to nature, those are more manifest which are moved by themselves, as animals; for this is not immanifest, whether they are moved by some thing, but the obscurity consists in the manner in which it is necessary to distinguish the mover, and the thing moved: for 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. But it may be especially doubted concerning the remaining member of the last mentioned division; for of things which are moved by another, some we have considered as being moved contrary to nature; but others remain to be opposed, because they are moved by nature. And these last are the things which may occasion a doubt by what they are moved; as, for instance, things light and heavy; for these are moved by violence to opposite places; but to their proper places naturally, the light indeed upward, and the heavy downward. But it is no longer apparent by what they are moved, as it is when they are moved contrary to nature: for it is impossible to say that they are moved by themselves, since this is vital, and the peculiarity of animated natures. They would also be able to stop themselves. I say, for instance, if any thing is the cause to itself of walking, it will also be the cause to itself of not walking; so that since it is in the power of fire to tend upward, it is evident that it is also in its power to tend downward. It is also absurd to suppose that they should be moved with one motion only by themselves, if they themselves move themselves.
Again, how is it possible that any thing continued and connascent can move itself? For so far as it is one and continued, but not by contact, so far it is impassive. So far, however, as they are separated, so far the one is naturally adapted to act, and the other to suffer. Neither, therefore, will any one of these move itself; for they are connascent; nor can this be asserted of any thing else which is continued. But it is necessary that in every thing that which moves should be divided from that which is moved; as we see in things inanimate, when any thing animated moves them. It happens, however, that these are always moved by something; and this will become evident to those who divide causes. What is asserted also may be assumed in motive natures; for some things are motive contrary to their own nature. Thus, a lever has not naturally a power of moving a weight. But others are naturally motive. Thus, that which is hot in energy, is motive of that which is hot in capacity. The like also takes place in other things of a similar kind. Thus too, that which is moveable by nature, which in capacity possesses quality or quantity, or somewhere, when it has such a principle as this in itself, and not according to accident: for the same thing may possess quantity and quality, but the one happens to the other, and is not osscutially inherent. Fire, therefore, and earth are moved by something, by violence indeed, when they are moved contrary to nature, but natutally, when being in capacity, they are moved to their own energies. Since, however, a subsistence in capacity is multifariously predicated, this is the cause why things of this kind do not appear to be moved by any thing; as, for instance, fire upward, and earth downward. But he who learns, is in a different way in capacity, from him who is scientific, and who now possesses science, and, does not theorize. Always however, when the agent and patient subsist together, that which is in capacity becomes in energy. Thus, he who learns, from being in capacity, becomes something else in capacity: for he who possesses science, and does not theorize is in a certain respect scientific in capacity, but not as he was before he learnt. But when he thus subsists, unless something impedes, he energizes and contemplates, or he would be in contradiction and ignorance. These things also similarly take place in natural things: for that which is cold is in capacity hot; but when it is changed, it is now fire and burns, unless something prohibits and impedes. The like also happens about the heavy and the light; for that which is light becomes so from the heavy; as, for instance, air from water: for it was first this in capacity, and now is light, and immediately energizes, unless something impedes. But the energy of a light thing is to be somewhere, and to be in the upper region; but it is impeded, when it is in a contrary place. This likewise similarly happens in quantity and quality. This also may he investigated why things light and heavy are moved to their proper places. The cause of this, however, is that they are naturally somewhere, and this is the very being of what is light and heavy; the essence of the former being defined by existing in the upper, and of the latter, by existing in the downward region. But things are light and heavy, in capacity, multifariously as we have already observed: for when a thing is water, it is in a certain respect light in capacity, and when it is air, it is still in capacity; because it may happen that being impeded, it is not in the upward region; but if that which impedes is taken away, then it energizes, and always tends upward. In like manner, that which possesses quality, changes into a subsistence in energy; for he who possesses science immediately theorizes, unless something impedes. And that which possesses quantity is extended, unless something prevents. But he who removes that which supports and impedes; in a certain respect moves.and in a certain respect does not; as is the case with him who withdraws a pillar, or takes away a stone from a bladder in water: for he moves accidentally; just as a ball when rebounding, is not moved by the wall, but by him who threw it. lt is evident,therefore, that no one of these things moves itself; but possesses the principle of motion, not indeed of moving or acting, but of suffering. But if all moveable natures are either moved by nature, or contrary to nature, and by violence; and if all things which are moved by violence, and contrary to nature, are moved by something and by another: and again, if those things which are moved by nature, as well those which are moved by themselves, as those not moved by themselves, as things heavy and light, are moved by something; (for they are either moved by that which generates and makes the light and heavy, or by that which removes things that impede and prevent) — if this be the case, whatever is moved, is moved by something.
