Commentary Aristotle Physics
Lecture 8
How heavy and light things are moved
It is the fact that these things also always derive their motion from something; what it is would become evident if we were to distinguish the different kinds of cause.
The above-mentioned distinctions can also be made in the case of things that cause motion:
some of them are capable of causing motion unnaturally (for instance, the lever is not naturally capable of moving the weight), others naturally (for instance, what is actually hot is naturally capable of moving what is potentially hot), and similarly in the case of all other things of this kind.
In the same way, too, what is potentially of a certain quality or of a certain quantity in a certain place is naturally movable when it has a principle of this kind in itself and not accidentally
(for the same thing may be both of a certain quality and of a certain quantity, but the one is an accidental, not an essential property of the other).
So, when fire or earth is moved by something, the motion is violent when it is unnatural, but natural when it brings to act the proper activities that they potentially possess.
But the fact that the term “potentially” is used in more than one sense is the reason that it is not evident from where such motions as the upward motion of fire and the downward motion of earth are derived.
One who is learning a science potentially knows it in a different sense from one who, while already possessing the knowledge, is not actually exercising it.
Wherever we have something capable of acting and something capable of being correspondingly acted on, if any such pair is in contact, then what is potential at times becomes actual.
For instance, the learner becomes from one potential something another potential something; for one who possesses knowledge of a science but is not actually exercising it knows the science potentially in a sense, though not in the same sense as he knew it potentially before he learned it.
And, when he is in this condition, if something does not prevent him, he actively exercises his knowledge; otherwise, he would be in the contradictory state of not knowing.
In regard also to natural bodies, the case is similar. Thus, what is cold is potentially hot and then a change takes place and it is fire, and it burns, unless something prevents and hinders it.
So, too, with heavy and light: light is generated from heavy, like air from water, for water is the first thing that is potentially light and air is actually light, and will at once realize its proper activity as such unless something prevents it.
The activity of lightness consists in the light thing being in a certain situation, namely, high up. When it is in the contrary situation, it is being prevented from rising. The case is similar also in regard to quantity and quality.
But, be it noted, this is the question we are trying to answer: how can we account for the motion of light things and heavy things to their proper situations?
The reason for it is that they have a natural tendency respectively toward a certain position, and this constitutes the essence of lightness and heaviness, the former being determined by an upward tendency, and the latter by a downward.
As we have said, a thing may be potentially light or heavy in more senses than one.
Thus, not only is a thing in a sense potentially light when it is water, but when it has become air, it may be still potentially light.
For it may be that, through some hindrance, it does not occupy an upper position, whereas, if what hinders it is removed, it realizes its activity and continues to rise higher.
The process whereby what is of a certain quality changes to a condition of active existence is similar, and thus the exercise of knowledge follows at once upon the possession of it unless something prevents it.
So, too, what is of a certain quantity extends itself over a certain space unless something prevents it.
The thing in a sense is and in a sense is not moved by one who moves what is sustaining and preventing its motion (for instance, one who pulls away a pillar from under a roof or one who removes a stone from a wineskin in the water is the accidental cause of motion); and, in the same way, the real cause of the motion of a sphere rebounding from a wall is not the wall, but the thrower.
So, it is clear that, in all these cases, the thing does not move itself, but it contains within itself the source of motion—not of moving something or of causing motion, but of suffering it.
If, then, the motion of all things that are in motion is either natural or unnatural and violent;
and if all things whose motion is by compulsion and unnatural are moved by something, and something other than themselves;
and again all things whose motion is natural are moved by something—both those that are moved by themselves and those that are not moved by themselves
(for instance, light things and heavy things, which are moved either by what brought the thing into existence as such and made it light and heavy or by what releases what was hindering and preventing it)— then all things that are in motion must be moved by something.
1029. After showing that the heavy and the light do not move themselves, he shows by what they are moved.
First, he shows by what they are moved;
second, he argues for his main intention, at “if, then, the motion” (255b31; [1036]).
About the first, he does two things:
first, he shows that they are naturally moved by something;
second, he investigates by what they are moved, at “but the fact that” (255a30; [1030]).
He says first (255a18) that, although the heavy and the light do not move themselves, they are nevertheless moved by something. And this can be made clear if we distinguish moving causes. For just as, in things that are moved, we must take it that some things are moved according to nature and some outside of nature, so also in movers, some move outside of nature—like the lever (that is, a stick), which is not naturally capable of moving a heavy body such as a stone—and some things move according to nature, as what is actually hot naturally moves what is according to its nature potentially hot, and similarly in other cases. And, just as what is in act causes motion naturally, so what is in potency is naturally moved, with respect to either quantity or quality or where.
And because he had said in book 2 that those things are moved naturally whose principle of motion exists in them per se and not accidentally, which might lead one to suppose that what is only potentially hot is, when it becomes hot, not moved naturally, since it is being moved by an external active principle of its motion, he now adds, as though to preclude this objection, “when it has a principle of this kind in itself and not accidentally,” as if to say that, in order that a motion be natural, it is enough that a principle of this kind—that is, the potency, about which he made mention—exist in that which is moved per se and not per accidens, as a bench is potentially combustible not precisely as bench, but as wood.
Hence, in explaining the expression not accidentally, he adds that the same subject can be quantified and qualified, but one of these is related to the other per accidens; what is potentially of such and such a quality is also potentially quantified, but per accidens.
Therefore, because what is in potency is naturally moved by something else in act, and nothing is in potency and in act with respect to the same, it follows that neither fire nor earth nor anything else is moved by itself, but by another. Fire and water are moved by another, but by force, when their motion is outside 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. Then, at “but the fact that” (255a30), he shows by what they are moved.
And because what is in potency is moved by something in act, first, he distinguishes potency;
second, from this, he shows by what such things are moved, at “as we have said” (255b17; [1035]).
About the first, he does three things:
first, he shows that it is necessary to know the ways in which something is said to be in potency;
second, he explains this, at “one who is learning” (255a33; [1031]);
third, with this, he solves a question, at “but, be it noted” (255b13; [1034]).
He says, therefore, that the reason that it is not evident what moves heavy and light things with respect to their natural motion (as fire upward and earth downward) is that the expression “being in potency” has many senses.
1031. Then, at “one who is learning” (255a33), he distinguishes being in potency:
first, in understanding;
second, in quality, at “in regard to natural bodies” (255b5; [1032]);
third, in local motion, at “so, too, with heavy” (255b8; [1033]).
He first says, therefore, that one who is learning and does not yet have the habit of science is not in potency to science in the same way as one who already has the science but is not contemplating by means of it.
But something is reduced from the first potency to the second when the active principle is united with the patient; and then, through the presence of the active principle, the patient comes to be with respect to such an act, but the patient is yet in potency.
For example, through the action of the teacher, a learner is reduced from potency to act, but when he is in this state of act, there is yet another potency present.
Consequently, the thing existing in first potency comes to be in another state of potency, because one having science and not contemplating is, in a sense, in potency to an act of science, but not in the same way as he was before he learned. Therefore, from first potency, he is reduced to an act to which is united a second potency by some agent, the teacher.
But, when he is in the state of possessing the habit of science, it is not necessary that he be reduced to second act by some agent; rather, he operates immediately by himself, just by contemplating, that is, unless he is prevented by other occupations or by sickness or by his will. On the other hand, if he were not impeded and still could not contemplate, then he would not be in the habit of science, but in its contrary, ignorance.
1032. Then, at “in regard to natural bodies” (255b5), he manifests the same thing in qualities. And he says that what was said with respect to the potency of anything in the mind applies also to natural bodies. For when a body is actually cold, it is potentially hot, just as an ignorant person is potentially a knower. But when this body has been so modified that it has the form of fire, then it is now actually fire and has the power to burn; and it acts at once and burns, unless it is prevented by something acting to the contrary or somehow preventing its acting, as by removing the combustible material. This is similar to what was said above—that, when someone, after learning, becomes a knower, he at once considers the science unless prevented by something.
1033. Then, at “so, too, with heavy” (255b8), he manifests the same thing in the local motion of the heavy and the light. And he says that a light thing comes to be from a heavy, as a hot thing comes to be from the cold, as for example, when air, which is light, comes to be from water, which is heavy. Therefore, this water is first potentially light and later becomes actually light, and then it has its own activity at once, unless something prevents this. But, now, being light, it is related to a place as potency to act, for the act of the light as light is to be in some definite place, namely, above; but it is prevented from being up by the fact of being in a contrary place, namely, down, because it cannot be in two places at the same time. Hence, that which keeps a light thing down prevents it from being up. And what has been said of local motion is true also of motion with respect to quantity or quality.
1034. Then, at “but, be it noted” (255b13), he uses the foregoing to answer a question. For although the act of the light is to be up high, yet some ask why the heavy and the light are moved to their appropriate places. But the cause of this is that they have a natural aptitude for such places. For to be light is to have an aptitude for being up high, and the account of the heavy is to have an aptitude to be down. Hence, to ask why a heavy thing is moved downward is exactly the same as to ask why it is heavy. Accordingly, the very same thing that makes it heavy makes it be moved downward.
1035. Then, at “as we have said” (255b17), he uses the foregoing to show what moves the heavy and the light. And he says that, since what is in potency is moved by what is in act (as has been said), it must be considered that something is said in many senses to be potentially light or heavy.
For in one way, when something is yet water, it is in potency to lightness; in another way, when air has now been made from the water, it is still in potency to the act of what is light, which is to be up high in the same way that one having the habit of science and not considering it is said still to be in potency. For what is light can be prevented from being up.
If, therefore, that obstacle be removed, it immediately acts for the purpose of being up by ascending, as it was said with respect to quality that, when a thing is actually of such and such a quality, it immediately tends to its act, as a knower immediately considers unless he be prevented. And the same is true with respect to the motion of quantity, for from the fact that an addition of quantity has been made to a quantitative thing, extension immediately follows in an increasable body, unless something prevents it.
Accordingly, it is clear that what moves—that is, what removes the obstacle preventing and sustaining—in some sense causes motion and in other senses does not; for example, if a pillar supports something heavy, and thus keeps it from descending, the one who casts down the pillar is said somehow to move the heavy object that was supported by the pillar.
In like manner, one who removes a stopper that was preventing water from flowing out of a container is said in some sense to move the water; but he is said to move per accidens and not per se.
Also, when a sphere, a ball, rebounds from a wall, it is moved per accidens by the wall but per se by the one who first threw it. For it was not the wall that gave it the impetus for motion, but the thrower; but it was per accidens that, being prevented by the wall from continuing according to its impetus, it rebounded into a contrary motion, the original impetus remaining. In like manner, the one who casts down the pillar did not give the heavy object resting upon it the impetus or inclination to be downward, for it had that from the first generator, which gave it the form upon which that inclination follows. Consequently, the generator is the per se mover of the light and the heavy, but the remover of obstacles is a per accidens mover.
He concludes, therefore, that it is clear from the foregoing that none of these—that is, of the heavy and the light—moves itself; yet their motion is natural because they have in themselves the principle of their motion, not indeed a moving or active principle, but a passive one, which is a potency to such and such an act.
From this, it is evidently contrary to the intention of the Philosopher that, in matter, there be an active principle, which some declare is necessary for a natural motion; for a passive principle is sufficient, since it is a natural potency for act.
1036. Then, at “if, then, the motion” (255b31), he argues for the conclusion chiefly intended in the whole chapter. And he says that, if it is true that all things that are per se moved are moved either according to nature or outside their nature and by force, and if, of those that are moved by force, it is true that all are moved not only by a mover but even by an external mover that is other, and again, if, among things that are moved according to nature, some are moved by themselves—in which things it is clear that they are moved by something not extrinsic, but intrinsic—while others, such as heavy and light things are moved according to nature not by themselves, but by some mover as has been explained (for either they are moved per se by the generator that makes them be heavy and light or they are moved per accidens by whatever releases, that is, removes, what impedes them or removes their natural motion), then, if all this be true, it is accordingly clear that all things that are moved are moved by something, either by an intrinsic or an extrinsic mover, which is to be moved by something other.