Aristotle, who lived at a later period, says that the First Principle is separated from (sense-objects), and that it is intelligible.[252] But when Aristotle says that He thinks himself, Aristotle degrades Him from the first rank. Aristotle also asserts the existence of other intelligible entities in a number equal to the celestial spheres, so that each one of them might have a principle of motion. About the intelligible entities, therefore, Aristotle advances a teaching different from that of Plato, and as he has no plausible reason for this change, he alleges necessity. A well-grounded objection might here be taken against him. It seems more reasonable to suppose that all the spheres co-ordinated in a single system should, all of them, stand in relation to the One and the First. About Aristotle's views this question also might be raised: do the intelligible entities depend on the One and First, or are there several principles for the intelligible entities? If the intelligible entities depend on the One, they will no doubt be arranged symmetrically, as, in the sense-sphere, are the spheres, each of which contains another, and of which a single One, exterior to the others, contains them, and dominates them all. Thus, in this case, the first intelligible entity will contain all entities up there, and will be the intelligible world. Just as the spheres are not empty, as the first is full of stars, and as each of the others also is full of them, so above their motors will contain many entities, and everything will have a more real existence. On the other hand, if each of the intelligible entities is a principle, all will be contingent. How then will they unite their action, and will they, by agreement, contribute in producing a single effect, which is the harmony of heaven? Why should sense-objects, in heaven, equal in number their intelligible motors? Again, why are there several of these, since they are incorporeal, and since no matter separates them from each other? WHAT THE PYTHAGOREANS TAUGHT ON THE SUBJECT. Among ancient philosophers, those who most faithfully followed the doctrine of Pythagoras, of his disciples, and of Pherecydes, have specially dealt with the intelligible.[253] Some of them have committed their opinions to their written works; others have set them forth only in discussions that have not been preserved in writing. There are others of them, also, who have left us nothing on the subject. TO THE THREE PRINCIPLES IN THE UNIVERSE MUST CORRESPOND THREE PRINCIPLES IN US.