Commentary Aristotle Physics
But, in the case of rotational motion, although the whole body comes to be in different places as distinguished by reason, nevertheless the whole body does not change its place as to subject: for the place remains ever the same as to subject, but varies only according to reason, as will be said in book 6. Nevertheless, the parts change place not only as to reason, but as to subject also. Therefore, in the case of rotational motion, there is not a succession of whole bodies in the same place, but of parts of the same body. Therefore, a rotating body does not essentially require a place according to its totality, but according to its parts.
479. But, against this, there seems to be the objection that the parts of a continuous body are neither in place nor moved in respect to place; rather, it is the whole that is both moved and in place. But it is clear that the outermost sphere is a continuous body; therefore, its parts are neither in place nor in motion in place. Consequently, it does not seem to be true that place should be attributed to the outermost sphere by reason of its parts.
The answer to this objection is that, although the parts of a continuous body are not actually in place, they are so potentially, insofar as the continuum is divisible. For a part, if it is separated, will be in the whole as in a place; hence, in this manner, the parts of a continuum are moved in place.
This is clearly evident in liquid continua that are easy to divide—for example, in the case of water, whose parts are found to be in motion within the whole water.
Consequently, because something is said of a whole by reason of its parts insofar as the parts of the outermost sphere are potentially in place, the entire outermost sphere is in place per accidens by reason of its parts; to be in place in that way is enough for rotational motion.
480. If a further objection is raised that what is in act is prior to what is in potency, and that consequently it seems improper that the first local motion be that of a body existing in place by means of its parts, which are potentially in place, the reply is that this is most fitting to the first motion. For it is necessary that the descent from the one immobile being to the diversity that is found in mobile things be made step by step. Now the variation based on parts existing in place potentially is less than that of wholes existing in place actually. Hence, the first motion, which is rotational, has less deformity and retains greater uniformity by being closer to the immobile substances. Now, it is much better to say that the outermost sphere is in place on account of its intrinsic parts, rather than on account of the center, which is entirely extrinsic to its substance. This is also more in agreement with Aristotle’s opinion, as is clear if one considers the passage following, in which Aristotle shows how the heavens are in place, at “that is why” (212a32).
481. For in regard to this, he does two things:
first, he shows how the outermost sphere is in place;
second, he draws a conclusion from what has been said, at “that is why the upper part” (212b13; [485]).
About the first, he does three things:
first, he shows that the outermost sphere is in place through its parts;
second, he shows how its parts are in place, at “as was explained” (212b3; [483]);
third, he shows how the parts make the whole to be in place, at “again, some things are” (212b7; [484]).
482. Therefore, because he had said that, if a body does not have something outside of it containing it, it is not in place per se, he concludes (212a32) that, if a body of this kind, such as the outermost sphere, be water (which will more easily illustrate what we are about to say on account of its easy divisibility), its parts will be in motion inasmuch as one part contains another, thus making it exist in place after a fashion.
But the entire water will be in motion in one sense and in another sense not. For it will not be in motion in such a way that the entire water will change its place as though being transferred to another place distinct as to subject, but it will be moved rotationally—a motion that requires place for the parts and not for the whole. And it will be moved not up and down, but circularly, for some things are moved up and down and change place in their entirety, namely, rare and dense bodies, or light and heavy things.
483. Then, at “as was explained” (212b3), he indicates how the parts of the outermost sphere exist in place, saying that, as was mentioned above, some things are actually in place, others potentially. Hence, in the case of a continuum of similar parts, the parts are in place potentially, as in the case of the outermost sphere; but when the parts are separated and merely contiguous, as occurs in a pile of stones, the parts are in place actually.
484. Then, at “again, some things are” (212b7), he shows how it follows from this that the entire sphere is in place. And he says that some things are per se in place—as any body that is per se in motion in place, whether it be in respect to local motion or increase, as was said above. But the heavens, namely, the outermost sphere, are not in place in this manner, as was said, since no body contains them; but inasmuch as they are moved rotationally, with part succeeding part, place is attributed to the parts potentially, as was said, inasmuch as one part is contiguous to—that is, is consecutive to—another.
Certain things, indeed, are in place per accidens, such as the soul and all forms; in this manner, the heaven, or the outermost sphere, is in place insofar as all its parts are in place, since each of its parts is contained under another in the rotation of the sphere. For in a non-round body, the outermost part remains uncontained and merely containing, but in a round body each part is both container and contained, although potentially. Hence, it is by reason of all its parts that a round body is in place. And this Aristotle takes to be per accidens, namely, what is true of the parts, as above when he said that the parts of a body are in motion per accidens in place.
485. Then, at “that is why the upper part” (212b13), he draws a conclusion from the foregoing. For since he had said that a body in rotational motion need not be in place in its entirety but only per accidens by reason of its parts, he concludes that the outermost body is moved only rotationally, because the whole in question is not anywhere. What is somewhere is itself something and has something outside of it by which it is contained; but there is nothing outside the whole. For this reason, all things are said to be in the heavens as in the outermost container, because the heavens are probably the containing whole.
He says probably because it has not yet been proved that there is nothing outside the heavens. It is not to be thought that the very body of the heavens is a place; rather, it is a certain final surface of it turned toward us, which is as a boundary in contact with the mobile bodies existing in it. For this reason, we say that earth is in water, which is in air, which is in ether, that is, fire, which is in the heavens, which are not in anything else.
486. However, according to the intention of Aristotle, this passage must be explained differently. For the example of water that he first adduced is not to be referred, according to him, to the outermost sphere only, but to the entire universe, which indeed is moved insofar as its parts are moved: some rotationally, as are the heavenly bodies; some up and down, as are the lower bodies. As to what he said later on, that some things are in place actually and other potentially, this is not to be referred to what he said previously, but is to be taken independently. For since he had said that some things are in place according to their parts and others according to their totality, he adds afterward that some things are in place according to act and others according to potency. Finally, he says that some things are in place per se and others per accidens.
In this connection, note that, according to Aristotle, “the heavens” are to be taken in two senses here:
first, they are taken for the entire universe of bodies, and especially of the heavenly;
second, for the outermost sphere.
He says, therefore, that those things are in place per se that are in motion according to place, whether they are in motion according to their totality or according to their parts, as are the heavens, that is, the universe; in place per accidens are the soul and the heavens, that is, the outermost sphere. For it is necessary to say that all the parts of the universe are somehow in place: the outermost sphere per accidens and other bodies per se, inasmuch as they are contained by a body outside of them. And he manifests this up to the end.