LECTURE 8 (255 a 19-256 a 2)

HE EXPLAINS HOW HEAVY AND LIGHT THINGS ARE MOVED

1029. After he has shown that heavy and light things do not move themselves, he here explains how they are moved. First he explains how they are moved. Secondly, where he says, 'If then the motion . . .' (255 b 31), he concludes to his main point.

             Concerning the first part he makes two points. First he shows that they are moved naturally by something. Secondly, where he says, 'But the fact that . . .' (255 a 30), he inquires into that by which they are moved.

             He says, therefore, first that, although heavy and light things do not move themselves, nevertheless they are moved by something. This may be made clear by distinguishing between moving causes. For just as things which are moved are moved either according to nature or outside of nature, so also in things which move some move outside of nature, for example, a staff does not naturally move a heavy body like a stone, and some move according to nature, for example, that which is actually hot moves that which according to its nature is potentially hot. And the same is true of other such things. And just as that which is in act naturally moves, so that which is in potency is naturally moved, either with respect to quality, or quantity, or place.

             He has said in Book II that those things are moved naturally whose principle of motion is in them per se and not per accidens. From this it may seem that when that which is only potentially hot becomes hot, it is not moved naturally, since it is moved by an active principle existing exterior to it. As if to refute this objection, he adds, '. . . when it contains the corresponding principle in itself and not accidentally . . .' (255 a 25). This is as if he were to say that for motion to be natural, it is sufficient if 'the corresponding principle', that is, potency, which he mentioned, be in that which is moved, per se and not per accidens. For example, a bench is potentially combustible, not insofar as it is a bench, but insofar as it is wood.

             Explaining what he means when he says, '. . . and not accidentally . . .' (255 a 25), he adds that the same subject may be both quantified and qualified, but one of these is related to the other per accidens and not per se. Therefore, that which is potentially a quality is also potentially a quantity, but per accidens.

             Since, therefore, that which is in potency is moved naturally by another which is in act, nothing can be in potency and act with respect to the same thing. It follows that neither fire nor earth nor anything else is moved by itself, but by another. Fire and earth are indeed moved by another, but through violence, when their motion is outside of their natural potency. But they are moved naturally when they are moved to their proper acts to which they are in potency according to their nature.

             1030. Next where he says, 'But the fact that . . .' (255 a 30), he explains that by which they are moved. And since that which is in potency is moved by that which is in act, he first distinguishes potency. Secondly, where he says, 'As we have said . . .' (255 b 17), he explains that by which such things are moved.

             Concerning the first part he makes three points. First he shows that it is necessary to know in how many ways a thing is said to be in potency. Secondly, where he says, 'One who is learning . . .' (255 a 32), he explains these distinctions. Thirdly, where he says, 'But, be it noted . . .' (255 b 14), he answers a certain question.

             He says, therefore, first that since being in potency is predicated in many ways, it is not clear what moves heavy and light things in natural motions, for example, fire upward and earth downward.

             1031. Next where he says, 'One who is learning . . .' (255 a 32), he distinguishes being in potency, first in the intellect, secondly in quality, where he says, 'In regard to natural bodies . . .' (255 b 5), and thirdly in local motion, where he says, 'So, too, with heavy . . .' (255 b 8).

             He says, therefore, first that the potency for science in one who is learning and does not yet have the habit of science differs from the potency for science in one who already has the habit of science but is not using it.

             A thing is reduced from first potency to second when something active is joined to its passivity. And then this passivity, through the presence of that which is active, comes to be in this kind of act, which up to this point was in potency. For example, the learner, through the act of the teacher, is reduced from potency to act, to which act is joined another potency. Thus, a thing existing in first potency comes to be in another potency. For one who has science but who is not contemplating it is in a certain way in potency to the act of science, but not in the same way as he was before he learned. Therefore he was reduced from first potency to act, to which is joined a second potency, through some agent, namely, a teacher.

             But when one possesses the habit of science, it is not necessary for him to be reduced to second act by some agent. Rather he does this immediately by his own contemplation unless something else prevents him, for example, business, or sickness, or his will. But if he is not impeded so that he cannot contemplate, then he does not have the habit of science but its contrary, namely, ignorance.

             1032. Next where he says, 'In regard to natural bodies . . .' (255 b 5), he explains the same thing with respect to qualities.

             He says that what was said above about the potency for science in the soul also applies to natural bodies. For when a body is actually cold, it is potentially hot, just as one who is in ignorance is in potency for knowing. But when it has been changed so that it has the form of fire, then it is fire in act, having the power of acting. And it operates immediately by burning, unless something prevents it by a contrary action or unless it is in some other way impeded, for example, by a withdrawal of the combustible object, just as it was said that after someone has become a knower by learning, he immediately contemplates, unless something prevents him.

             1033. Next where he says, 'So, too, with heavy . . .' (255 b 8), he explains the same thing with reference to the local motion of heavy and light things.

             He says that a heavy thing becomes light in a way similar to that in which a cold thing becomes hot. For example, air, which is light, comes from water, which is heavy. The water, therefore, is first in potency to become light, and afterwards it becomes light in act, and then it immediately possesses its operation, unless something prevents it. But the now existing light thing is compared to place as potency to act (for the act of a light thing, as such, is to be in some determined place, namely, up). But it is prevented from being up because it is in the contrary place, namely, down. For it cannot be in two places at once. Hence that which holds the light thing down prevents it from being up. And what is said about local motion must also be said of motion in respect to quantity or quality.

             1034. Next where he says, 'But, be it noted . . .' (255 b 14), he answers a certain question about the foregoing.

             Granted that the act of a light thing is to be up, nevertheless some ask why heavy and light things are moved in their proper places. The reason for this is that they have a natural aptitude for such places. For to be light is to have an aptitude for that which is up. And the nature [ratio] of the heavy is to have an aptitude for that which is down. Hence, to ask why a heavy thing is moved downward is nothing other than to ask why it is heavy. The same thing which makes it heavy also makes it to be moved downward.

             1035. Next where he says, 'As we have said . . .' (255 b 17), he shows from the foregoing what moves heavy and light things.

             He says that since that which is in potency is moved by that which is in act, as was asserted, it must be realized that a thing is said to be potentially light or heavy in many ways.

             In one way, while it is still water, it is in potency to become light. In another way, when air has already been made from water, it is still in potency to the act of the light, which is to be up, just as one who has the habit of science but is not contemplating it is still said to be in potency. For it happens that what is light may be prevented from being up.

             But if that impediment is removed, it rises immediately so that it may be up. As was said with respect to quality, when a quality is in act, it strives immediately toward its own operation, just as he who is a knower immediately contemplates, unless something prevents him. And the same applies to the motion of quantity. For when an addition of quantity has been made to quantity, extension follows immediately in the body which can be increased, unless something prevents it.

             Therefore, it is clear that that which removes that which prevents and restrains in a certain sense moves and in another sense does not. For example, if a column supports something heavy, and so prevents it from falling, he who destroys the column in a certain sense is said to move the weight supported by the column. Similarly, one who removes a stone which stops water from flowing out of a vessel in a certain sense is said to move the water. He is said to move per accidens, and not per se. For example, if a sphere, that is, a ball, rebounds from a wall, it is moved by the wall per accidens and not per se. It is moved per se by the initial thrower. For the wall did not give it any impetus toward motion, but the thrower did. It is per accidens because, when the ball was impeded by the wall, it did not receive a second impetus. Rather because of the same remaining impetus it rebounded with an opposite motion. And similarly, one who destroys a column does not give to the supported weight an impetus or inclination downward. For it has this from its first generator which gave to it the form which such an inclination follows. Therefore, the generator is the per se mover of heavy and light things. But that which removes an obstacle is a per accidens mover.

             He concludes, therefore, that it is clear from what has been said that no heavy or light thing moves itself. Nevertheless the motion of these things is natural because they have a principle of motion within themselves, not, indeed, a motive or active principle, but a passive principle, which is the potency for such act.

             From this it is clearly contrary to the Philosopher's intention to say that there is an active principle in matter, which some maintain is necessary for natural motion. This passive principle, which is the natural potency for act, is sufficient.

             1036. Next where he says, 'If, then, the motion . . .' (255 b 31), he arrives at the conclusion principally intended in this whole chapter.

             He says that if it is true that everything that is moved is moved according to nature or outside of nature and by violence, then it is clear that all things which are moved by violence are moved not only by some mover but by some other extrinsic mover. And further of things which are moved according to nature, some are moved by themselves, in which it is clear that they are moved by something, not indeed, extrinsic, but intrinsic, and some are moved according to nature but not by themselves, as heavy and light things. These latter are also moved by something, as was shown (because either they are moved per se by the generator which makes them heavy and light, or they are moved per accidens by that which removes what impedes or prevents their natural motion). Therefore, it is clear that whatever is moved is moved by some mover, either intrinsic or extrinsic, which he calls 'being moved by another'.