Lecture 7 — Why the circular motion of the heaven is in one direction rather than another

274 Now there are two ways of moving along a circle, from A to B or from A to C, and we have already explained that these movements are not contrary to one another.

275 But nothing which concerns the eternal can be a matter of chance or spontaneity, and the heaven and its circular motion are eternal. We must therefore ask why this motion takes one direction and not the other.

276 Either this is itself an ultimate fact or there is an ultimate fact behind it.

277 It may seem evidence of excessive folly or excessive zeal to try to provide an explanation of some things, or of everything, admitting no exception. The criticism, however, is not always just: one should first consider what reason there is for speaking, and also what kind of certainty is looked for, whether human merely or of a more cogent kind. When any one shall succeed in finding proofs of greater precision, gratitude will be due to him for the discovery, but at present we must be content with a probable solution.

278 If nature always follows the best course possible, and, just as upward movement is the superior form of rectilinear movement, since the upper region is more divine than the lower, so forward movement is superior to backward, then front and back exhibits, like right and left, as we said before and as the difficulty just stated itself suggests, the distinction of prior and posterior, which provides a reason and so solves our difficulty. Supposing that nature is ordered in the best way possible, this may stand as the reason of the fact mentioned. For it is best to move with a movement simple and unceasing, and, further, in the superior of two possible directions.

361. After determining as to the parts of the heaven and its shape, the Philosopher here decides the question of its motion.

First he discusses the manner of its motion;

secondly, the uniformity of its motion, at we have next to show that the movement (L. 8; 288a13).

Regarding the first he does three things:

first he raises the question;

secondly, he shows the difficulty of the question, at it may seem evidence (287b28; [364]);

thirdly, he proposes a solution, at if nature always follows (288a2; [365]).

About the first he does three things:

first he mentions certain things that give rise to the question. One of these is that there are two ways in which a thing could be moved through a circle. For if we let A be a point on a circle and draw a diameter from A, and let B be a point on the upper semicircle, and G a point on the lower, there will then be two possible ways for something to move over this circle: in one way, something could go from A to B; in another, from A to G. Another thing he mentions is that those two motions are not contrary, for it has been shown in Book I that two circular motions are not contrary. For if they were contrary, they would have to belong to the natures of contrary mobile things, so that one motion would be attributed to one mobile and the other to the contrary mobile — because, as has been said, if one of two contraries exists in nature, so must the other.

362. Secondly, at but nothing (287b24) he raises the question. For it is plain from the foregoing that in things that are eternal nothing happens contingently or fortuitously, because happenings due to chance do not occur always or even nearly always.. But it has been said above that the heaven is eternal and also its circular motion. Hence it is reasonable to ask why it is that the heaven is moved in one direction rather than in the other; for example, from the east in the direction of the upper hemisphere and not of the lower.

363. Thirdly, at either this is itself an ultimate fact (287b27) he shows how such a cause must be assigned. For in the previous accounts he assigned a cause of celestial events in two ways.

First, he showed that there have to be various motions in the heaven in order that generation and corruption be possible; secondly, he showed that the figure of the heaven has to be round by assuming a prior principle, namely, that the first shape is appropriate to the first body. Thus the primacy of the body is the reason why it has the first shape. And therefore he says here that if we must assign the reason why the heaven is moved as it is and not in some other way, that reason must be assigned either on the ground that such a type of motion is the principle of some effect, or rather that this type of motion depends on some prior principle.

This can also be understood in another way. For he had said that eternal things cannot be due to chance. Yet not all eternal things have a cause, for there is an eternal thing which has no cause but is itself the first cause of other things. Since, therefore, on the grounds of the eternity of the heaven and of its motion, he had arrived at the question concerning the cause why the motion of the heaven is in one direction rather than another, lest the question appear foolish or useless, he adds that it is necessary that the manner of the heaven's movement be either the first principle of all motions (which is impossible, because all motion has a movent cause) or else there must be said to be some other principle of its motion. Consequently it is reasonable to ask why it is that the heaven is moved in this direction rather than some other.

364. Then, at it may seem evidence (287b28), he shows the difficulty of this question. And he says that the very desire to attentively set forth difficult and occult things and give their cause, and to inquire into all the aspects, without omitting anything, will perhaps be seen as a sign either of a deep-rooted stupidity, causing one to be unable to distinguish between what is easy and what is difficult, or else as a sign of great promptitude, i.e., of great presumption, causing one not to know the measure of his ability with respect to the search for truth. And although some deserve rebuke on this point, it is not a just thing to condemn all investigators indiscriminately. Rather we should first have regard to two things.

First, we must look for the motive which induces a man to speak of such things: Is he doing it out of love for the truth or in order to show off his wisdom?

Secondly, we must consider how one is in assenting to the things he asserts: does he have a weak certitude about them like the common run of mankind, or does he know them more firmly, i.e., above the general run? When, therefore, a person can attain to a knowledge of necessary causes with greater certitude than the general run of man, he who finds such necessary reasons deserves our thanks rather than a rebuke.

365. Then, at if nature always follows (288a2), he solves the question he raised. And he says that if thanks should be rendered to those who discover more certain necessary things, as to the present question it is enough to say how the matter seems to us, even though it is not so certain. He says, therefore, that among things that come to be produced, nature always does what is best, as moved and directed by the first principle, which is the essence of goodness, Now, we observe that a local motion is more noble to the extent that it tends toward a nobler direction—for the species of a motion is determined by its terminus. Thus with regard to straight local motions, a local motion toward a higher place is more honorable and more noble than one to a nether place, since the place which is above is of more worth than one which is down. This fact is also plain with regard to man: for the head, which is above, is nobler than the feet, which are down. Likewise, the front of a man is nobler than the back, and the right than the left, as we have said above, and as is plain also with respect to animals. Therefore the question under discussion testifies to the fact that in the heaven there is a before and an after, i.e., a front and a back, which things he did not mention previously. Now it is this, namely, the distinction between front and back in the heaven, that solves the question at hand. For if the motion of the heaven is the best possible, as has been maintained, then the cause in the aforesaid question is this: It is best for the heaven to be moved with a simple motion, i.e., always toward the same direction and one without any interruption, i.e., without the interpolation of rest (which would have to occur if the heaven were now moving in this direction and now in that direction); furthermore, it is best that it be moved toward the nobler part of the direction, and the front part is the more noble. And that is why the heaven is moved from the east toward its front, i.e., toward the upper hemisphere and not toward the lower, which is the back of the heaven.

366. But this argument does not seem appropriate. For above he had explained the distinction of these parts in the heaven from the beginning of motion, namely, from the fact that the motion of the heaven seems to begin from one direction and not from another. But here he assigns the reason why the heaven is moved the way it is and not some other way, from the distinction of the parts of the heaven. Consequently, he seems to be arguing in a circle. To this it should be answered that the distinction among the parts of the heaven is the reason why the motion of the heaven begins whence it does and not elsewhere; and not conversely. But the fact that the motion does begin whence it does and not somewhere else is a sign of the distinction of the parts of the heaven. Now the cause of the distinction of these parts is the power of the soul moving the heaven, or of some intellectual substance differently applied to the different parts of the heaven. There is nothing wrong, when asking "Whether something is," should one prove with a sign; but when one is dealing with the "reason why something is," the sign must be reduced to the cause. For example, we might prove that the heart is moved by the motion of the pulsating vein; but if we should ask what is the cause of the motion of the pulsating vein, it will be said that it is because of the heart's movement. In like manner, Aristotle, from the fact that the motion of the heaven begins at a certain part, proved, as though from a sign, that there is such-and-such a distinction of parts in the heaven. Nevertheless he reduces the beginning of the motion to the difference in the parts of the heaven, as to the cause. Now the front and rear of the heaven are distinguished, not naturally, namely, in terms of some specific part of the heavenly body (since one and the same part of the heavenly body which is now in the upper hemisphere will later be in the lower), but in terms of position, just as was stated above concerning the difference between the right and left [sides of the heaven].