Metaphysics

​But whether we must assume one such substance or more than one, and how many, must not escape our notice; rather, we must also remember the assertions of others, that they have said nothing about the number which is even clear to state. For the hypothesis of Ideas contains no peculiar inquiry of its own (since those who speak of Ideas say that the Ideas are numbers, but about the numbers they speak at one time as if they were infinite, and at another time as if they were limited up to ten; but as to the cause why the number of numbers is just so many, nothing is said with demonstrative zeal). But we must speak from our own assumptions and distinctions. For the principle and the first of beings is immovable both in itself and accidentally, and it moves the first eternal and single movement. But since that which is moved must be moved by something, and the first mover must be immovable in itself, and the eternal movement must be moved by an eternal being, and the single movement by a single being, and since we see that besides the simple locomotion of the universe, which we say is moved by the first and immovable substance, there are other eternal locomotions, namely those of the planets (for the body which moves in a circle is eternal and restless; as has been shown in the physical treatises concerning these), it is necessary that each of these locomotions also be moved by a substance which is itself immovable in itself and eternal. For the nature of the stars is an eternal substance, and that which moves is eternal and prior to that which is moved, and that which is prior to a substance must be a substance. It is clear, then, that there must be just so many substances, eternal in their nature and immovable in themselves, and without magnitude for the reason given before. That, therefore, there are substances, and of these one is first and another second in the same order as the locomotions of the stars, is clear. But as to the number of these locomotions, we must now study it from that mathematical science which is most akin to philosophy, namely astronomy; for this science makes its inquiry about a substance which is sensible indeed but eternal, whereas the others, such as arithmetic and geometry, are about no substance at all. That, then, the locomotions are more than the moving bodies, is clear to those who have even moderately touched upon the subject (for each of the planets is carried through more than one locomotion). But how many these happen to be, we now say for the sake of having a conception what some of the mathematicians say, so that there may be some definite number for our intellect to grasp; for the rest, we must partly search ourselves and partly learn from those who search, and if anything appears to those who busy themselves with these matters contrary to what is now said, we must love both indeed, but obey the more accurate.—Now Eudoxus assumed that the locomotion of the sun and of the moon each was in three spheres, of which the first was that of the fixed stars, the second along the circle through the middle of the zodiac, and the third along the circle which is inclined across the width of the zodiac (and that along which the moon is carried is inclined in a greater width than that along which the sun is carried); and that of the planets each was in four spheres, and of these the first and second were the same as those (for the sphere of the fixed stars was that which carries all of them, and that which is placed under this and has its locomotion along the circle through the middle of the zodiac is common to all of them), and the poles of the third sphere of all of them were in the circle through the middle of the zodiac, and the locomotion of the fourth was along the circle inclined to the middle of this third sphere; and that the poles of the third sphere were peculiar to each of the others, but the same for Venus and Mercury. And Callippus assumed the same position of the spheres as Eudoxus, but as for their number, he assigned the same as he did to Jupiter and Saturn, but for the sun and the moon he thought that two more spheres had to be added, if one is to account for the phenomena, and for each of the remaining planets one.