Lecture 19 — The second difficulty of Lecture 17 is resolved

333 As to the difficulty that into the one primary motion is crowded a vast multitude of stars, while of the other stars each has been separately given special movements of its own, there is in the first place this reason for regarding the arrangement as a natural one. In thinking of the life and moving principle of the several heavens one must regard the first as far superior to the others.

334 Such a superiority would be reasonable. For this single first motion has to move many of the divine bodies, while the numerous other motions move only one each, since each single planet moves with a variety of motions. Thus, then, nature makes matters equal and establishes a certain order, giving to the single motion many bodies and to the single body many motions.

335 And there is a second reason why the other motions have each only one body, in that each of them except the last, i.e. that which contains the one star, is really moving many bodies. For this last sphere moves with many others, to which it is fixed, each sphere being actually a body; so that its movement will be a joint product. Each sphere, in fact, has its particular natural motion, to which the general movement is, as it were, added. But the force of any limited body is only adequate to moving a limited body.

336 The characteristics of the stars which move with a circular motion, in respect of substance and shape, movement and order, have now been sufficiently explained.

472. Having given a solution to the first problem, he [the Philosopher] here solves the second one, which asks why it is that, whereas there are innumerable stars in the sphere of the first motion, there is but one in each of the other lower spheres.

473. To this he gives three answers.

The first of these is based on the excellence of the first sphere as compared to the others. And he says (292b25) that to the question why it is that a multitude of stars is found to be connected with the motion of the first sphere, which is one, while in the other lower spheres of the planets each star has separately its own motions (so that the motions of Saturn are distinct from those of Jupiter, and so on for all the others, whereas all the fixed stars are located according to one motion), to this the answer must be given that this is reasonable first of all for one reason, namely, because we must understand that the first sphere has a great excellence in comparison to the other spheres, both in point of life, since it has a nobler life as having a nobler soul, and in point of being a principle of each, since, namely, universal causality is truer of the first sphere than of any of the others. Now this excellence can be considered from three facts: first, it [the first sphere] is more immediately ordered to the first mover; secondly, it contains and revolves all the other spheres; and thirdly, it has the simplest and the swiftest motion. It is plain that that which is noblest and more acting in the heavenly bodies is the star, and this is proved by its luminosity. And therefore, it is fitting that the first sphere should abound with a multitude of stars as compared with the other spheres.

474. But if we suppose that the sphere of the fixed stars is not the outermost sphere but that there is another above it, in which there is no star, our proposition is not affected. For the motion of the sphere exists only for the motion of the star, as is said in Metaphysics XII—hence, that motion of the supreme sphere that lacks stars is ordained to the movement of the fixed stars, just as, according to the ancient astronomers, each planet has many spheres lacking a star but ordained to the motion of the star fixed in the last of them. Therefore, according to this, so far as the coordination of motion is concerned, that first sphere falls into the same order as the sphere of the fixed stars. For this reason also Aristotle significantly says that there are many stars connected with the first carrying, but not connected with the first sphere, since the carryings are determined with respect to the stars, for the sake of carrying which along the spheres are moved, and not with respect to the spheres. This only implies, then, that the motion of the fixed stars will not be wholly simple, as Aristotle supposes, but composed of two motions.

475. The second argument is at such a superiority would be (292b30) and it is based on the proportion of the multitude of stars to the multitude of motions. And he says that that about which the problem is raised occurs according to reason. For the first carrying along, although it is one, nevertheless many of the heavenly bodies (which he here calls divine bodies, because of their perpetuity), are moved with it, whereas in the case of the lower carryings, many of them move one body alone, because each of the wandering stars, i.e., the planets, is moved with several motions, as has been said above. In this way, then, nature makes a certain equality of proportion between the fixed stars and the planets, and disposes them in good order, in the sense that, to the one first motion it assigns many bodies, i.e., many stars, but conversely, as to the planets, to one body, i.e., to one star, it assigns many motions. And this distribution [by nature] is reasonable. For the planets are as though certain instruments of the supreme sphere, which is as though the principal agent with respect to bodies, insofar as, through the medium of the planets, there are in a certain way transmitted and applied to these lower bodies the manifold powers of the fixed stars. Now an instrument acts insofar as it is moved, while a principal agent acts according to its own form and proper power. Consequently, it is fitting that the supreme sphere abound with a multitude of stars, in which diverse active powers are rooted, while the planets abound in a multitude of motions.

This explanation is valid even in the light of the suppositions of present-day astronomers. For even though the sphere of the fixed stars has two motions, it shares in a minimum way in the second, since it is most slow in it.

476. He gives the third explanation at and there is a second reason (293a4) and it is based on the multitude of spheres moving each of the planets, according to the theory of the ancient astronomers. And he says that the reason why in each apparent motion of the planets there is found but one stellar body in motion, is that there are many spherical bodies moving the star, so arranged that those which are first, i.e., the higher, are movers of that sphere which is at the end and which contains the star. For the star is moved as fixed in the last of many spheres ordered to moving one sphere. Or one can understand that the last of the spheres is in some way connected with the higher spheres and is moved according to their motion. Now it is evident that each of these spheres is a certain body. Thus, therefore, the common work of all the spheres revolving a given planet belongs to it, i.e., to the sphere which is supreme in that system and which revolves all the lower—for the motion of the lowest sphere, in which the planet exists, is the proper and natural motion of that planet, while the motions of the higher spheres are, so to speak, added to correct the irregularity that appears in the planet's motion, namely, of swiftness and slowness, retrogression, advancing and standing still. Accordingly, it is plain that, since the higher sphere moves all the lower that are ordered to the motion of the same one planet, then, if in addition to that it had to move several stars, it would be laborious for it—each body's strength being finite in comparison to another body, it having been shown in Physics VIII that there is not infinite power in a finite magnitude.

477. It should not be thought that this difficulty would derive from there being weight in the stars or from anything that resists motion, but rather it is because there is required an excess of the mover over the mobile. But there could not be an excess of the superior sphere in power if in the lower things there was together with a multitude of spheres also a multitude of stars, because in the bodies of stars the power of the heavenly bodies most abounds.

It should also be diligently noted that he posits a finite proportion between the movent sphere and the moved bodies on the ground that the movent sphere is a body. This indicates that the separated mover, which is an incorporeal and immaterial substance, does not, according to the intention of Aristotle, finitely exceed the body which is moved by it, but infinitely, as existing outside the whole body of magnitude and not determined by matter. This reveals that Averroes is in error in his commentary when he says that the addition of the first mover over the power of the moved is not infinite except in infinite time. But how, If the power of the separated mover is infinite, it does not move with infinite speed, i.e., in an instant, and how, if the power of a body is finite, that body can endure for an infinite time, has been shown in Physics VIII.

It should be noted that this third argument does not apply to the position of the modern astronomers, who do not assign to the planets a number of spheres, one of which moves all the others, as the ancient astronomers did; nevertheless, these latter did hold that the many fixed stars were not moved except by one sphere.

478. In summing up (293a11), he says we have stated of the stars, which are moved with a circular motion, what they are according to the substance of their nature and according to their shape. Moreover, we have discussed their motion and order.