Lecture 8 — The regularity, or uniform velocity, of the heaven's motion shown by two arguments

279 We have next to show that the movement of the heaven is regular and not irregular.

280 This applies only to the first heaven and the first movement; for the lower spheres exhibit a composition of several movements into one.

281 If the movement is uneven, clearly there will be acceleration, maximum speed, and retardation, since these appear in all irregular motions. The maximum may occur either at the starting-point or at the goal or between the two; and we expect natural motion to reach its maximum at the goal, unnatural motion at the starting-point, and missiles midway between the two. But circular movement, having no beginning or limit or middle in the direct sense of the words, has neither whence nor whither nor middle: for in time it is eternal, and in length it returns upon itself without a break. If then its movement has no maximum, it can have no irregularity, since irregularity is produced by retardation and acceleration.

282 Further, since everything that is moved is moved by something, the cause of the irregularity of movement must lie either in the mover or in the moved or both. For if the mover moved not always with the same force, or if the moved were altered and did not remain the same, or if both were to change, the result might well be an irregular movement in the moved. But none of these possibilities can be conceived as actual in the case of the heavens. As to that which is moved, we have shown that it is primary and simple and ungenerated and indestructible and generally unchanging; and the mover has an even better right to these attributes. It is the primary that moves the primary, the simple the simple, the indestructible and ungenerated that which is indestructible and ungenerated. Since then that which is moved, being a body, is nevertheless unchanging, how should the mover, which is incorporeal, be changed? It follows then, further, that the motion cannot be irregular.

367. After assigning the reason why the heaven is moved toward one direction rather than the other, the Philosopher here discusses the uniformity of the heaven's motion.

First, he proposes what he intends;

secondly, he proves his proposition, at if the movement is uneven (288a17; [369]).

About the first he does two things:

first, he proposes what he intends (288a13), and says that after the foregoing we must cover, i.e., say something briefly about, the motion of the heaven and show that it is regular, i.e., that it always has a uniform velocity, and is never irregular so as to be at one time slower and at another swifter. And this is reasonable: for this motion is the rule and measure of all other motions. Hence, no irregularity or inequality should appear in it.

368. Secondly, at this applies only (288a14) he explains what he had said. And he says that he intends here to speak of the first heaven, i.e., the outermost sphere and of the first carrying, i.e., of the diurnal motion by which the whole heaven is revolved, though the motion of the first mobile, from east to west. Now he speaks of this motion in particular because there is in it no irregularity either in fact or in appearance. But in those things below, i.e., in the motion of the planets, several motions concur to move one body, either according to different shifting and revolving spheres, as the astronomers of Aristotle's time said, as is plain in Metaphysics XII, or according to the motions of eccentrics and epicycles according to modern astronomers. From this variety of motions is caused the irregularity which appears as to the planets, according to which they seem at one time to be moved with a forward motion, at another with a retrograde, and at still another to be at rest—although in fact no motion of the heaven is irregular. Now, the arguments which he will here adduce apply not only to the motion of the first heaven which is simple and hence gives no appearance of irregularity, but also to the motions of the planets, in which there is apparent irregularity due to the concurrence of many motions.

369. Then, at if the movement is uneven (288a17), he proves his proposition with four arguments.

The first is taken from the very form of circular motion, and proceeds thus: If the first heaven were moved in an irregular manner there would obviously have to be intension, i.e., an increase of velocity, and power, i.e., a maximum velocity, and remission, i.e., a decrease of velocity. For every irregular motion possesses all three. This is not in the sense that these three are found in each and every irregular or unequal motion, but in the sense that in any motion two of them are found. Thus "power" [a maximum] and "intension" [an increase] are found, as they are found in the natural motion of heavy and light bodies, since such motion is always increasing in velocity up to the end, when it is at its swiftest; on the other hand, when such bodies are subjected to a motion contrary to their nature, there is "power" [a maximum] and "remission" [a decrease], because in the beginning such a motion is swiftest but its velocity is continually decreased until at last the whole motion is exhausted. Thus the word "every" is taken collectively in the sense that these three things are found in all motions that are irregular but not all in each one.

370. Then he shows where the maximum speed is found in an irregular motion. And he says that the power of a motion, i.e.., its maximum speed, is found either whence it is brought forth, at the terminus a quo, or whither it is borne, i.e., at the terminus ad quem, or near the middle. Thus, in things that are being naturally moved with a straight motion the greatest velocity is toward the terminus toward which they are borne, since a natural motion is intensified at the end, as was had in Book I; while in things that are moved contrary to nature, the greatest velocity is found whence, i.e., toward the terminus a quo, since a violent motion is intense in the beginning and slackens at the end, as was had in Book I. But in things projected, the maximum velocity is found near the middle.

371. Now there is a question about what the Philosopher here calls things projected. For things that are projected are moved either according to a natural motion, as when a stone is thrown downward, in which case it is seen that the motion is increased at the end, or they are moved with compulsion, as when a stone is thrown upward, in which case its motion must be most intense at the beginning, and not in the middle.

But Simplicius says that by things projected the Philosopher here means the bodies of animals, whose soul does not move them directly upward or downward but, as it were, laterally, after the manner of an arrow and other projectiles, and that is why Aristotle here calls them things projected. Now it is plain [Simplicius continues], that in the case of the motions of animals, the maximum velocity is found neither in the beginning, when animals in a sense gradually depute their members to motion, or at the end, when their members are now tired, but near the middle, when they are in the very onrush of their motion. However, this seems to be a forced explanation. For this reason, Alexander says that "middle" here should refer to the place, and not to the time. For the motion of an arrow and of other things similarly projected is neither upward nor downward, but in what is intermediate to both these, and it is in this intermediate area that the maximum velocity of these motions is reached. But we cay say that even if we take "middle" as referring to time, such projectiles still move more swiftly near the middle. For the motion of such projectiles is caused from the impulsion of the deferent medium which more easily receives the impression of the mover than does the heavy body that is projected, as is plain in Physics VIII. Consequently, after much air has now been set in motion, the motion of projection is swifter in the middle than at the beginning, when still only little air was being set in motion, or at the end, when the impulsion of the one projecting has already begun to fade. And a sign of this is that such projectiles do not strike with as much force an object which is entirely near or which is very remote as they do one which is at a mediate distance.

372. Thus it is plain, therefore, that the maximum velocity of any irregular motion is either at the beginning, or in the middle, or at the end. But these three are not to be found in the circular movement of the heavenly body — neither with respect to time (since it is eternal according to his opinion) nor with respect to length, i.e., with respect to the shape of the place, which is along a circular line drawn around, i.e., as though always returning circularly to itself, and unbreakable, i.e., not divided in act so as to allow an actual beginning and an end to be designated in it. Therefore in the revolution of the heaven there is no power, i.e., maximum velocity, discernible at any part, and, as a consequence, no irregularity, which is due increase and remission.

373. He gives at further, since everything (288a27) the second argument which is taken at once on the part of the mover and mobile. For it has been shown in Physics VII and VIII that whatever is moved is moved by some movent. Consequently, if there is irregularity in a motion, it must be due either to the movent, or to the thing moved, or to both. For if the movent does not move always and with equal power, but sometimes with more and sometimes with less, the motion will be sometimes slower, sometimes swifter. For the velocity of a motion results from the fact that the power of the movent, on account of its size, greatly dominates the mobile. And similarly, if the body being subjected to motion, by virtue of some alteration, does not maintain the same state, it will not be at all times equally subject to the power of the movent, and so there will not be an equal speed of motion. Finally, if both are modified, namely, the movent and the mobile, the motion could be irregular. But none of these possibilities can be verified with regard to the heaven. For it has been shown above that the mobile body here involved is first and simple, since it is moved by a first and simple motion; moreover, that it is ungenerated, indestructible and utterly unchangeable {i.e., with a change causing a variance in substance or strength). Now there is all the more reason for the mover of such a body to be of such a condition. For since the mover is more powerful than the moved, if the body which is moved is a primary and simple body and subject neither to generation nor corruption, so much the more so will its mover be. It was also shown in Physics VIII that the mover of the heaven is incorporeal and without any magnitude. If then the heaven, which is a body, is not changed in the state of its substance and strength, much less will its mover, which is incorporeal, be changed. From this it is plain that it is impossible for the motion of the heaven to be irregular.