2. From his conception of motion Aristotle derives his view of the _collective universe_, as brought out in his books _De Cælo_. The most perfect motion is the circular, because this is constant, uniform, and ever returning into itself. The world as a whole is therefore conditioned by the circular motion, and being a whole complete in itself, it has a spherical form. But because the motion which returns into itself is better than every other, it follows, from the same ground, that in this spherical universe the better sphere will be in the circumference where the circular motion is most perfect, and the inferior one will arrange itself around the centre of the universal sphere. The former is heaven, the latter is earth, and between the two stand the planetary spheres. Heaven, as the place of circular motion, and the scene of unchangeable order, stands nearest the first moving cause, and is under its immediate influence; it is the place where the ancients, guided by the correct tradition of a lost wisdom, have, placed the Divine abode. Its parts, the fixed stars, are passionless and eternal essences, which have attained the best end, which must be eternally conceived in a tireless activity, and which, though not clearly cognizable, are yet much more divine than man, A lower sphere, next to that of the fixed stars, is the sphere of the planets, among which, besides the five known to the ancients, he reckons the sun and the moon. This sphere stands a little removed from the greatest perfection: instead of moving directly from right to left, as do the fixed stars, the planets move in contrary directions and in oblique orbits; they serve the fixed stars, and are ruled by their motion. Lastly, the earth is in the centre of the universe, farthest removed from the first mover, and hence partaking in the smallest degree of the Divine. There are thus three kinds of being, exhibiting three stages of perfection, and necessary for the explanation of nature; first, the absolute spirit or God, an immaterial being, who, himself unmoved, produces motion; second, the super-terrestrial region of the heavens, a being which is moved and which moves, and which, though not without matter, is eternal and unchangeable, and possesses ever a circular motion; and, lastly, in the lowest course this earth, a changeful being, which has only to play the passive part of being moved.