the stars, Aristotle argued that they would have to be spherical, as this shape, which is the most perfect, allows them to retain their positions. (De Caelo, Book II, chapter 11) By Aristotle's time, Empedocles' view that there are four basic elements - earth, air, fire and water - had been generally accepted. Aristotle, however, in addition to this, postulated a fifth element called aether, which he believed to be the main constituent of the celestial bodies. This divine element, he believed, is uncompounded, ungenerated, eternal, unalterable, and neither heavy nor light. It can be found in its purest form in the celestial regions, but becomes adulterated in the area below the moon. (De Caelo, Book I, chapter 2) Aristotle's view of the universe was hierarchical, and he made a sharp distinction between the sublunar world of change, and the eternal and immutable heavens. Aristotle, like Eudoxus and Callippus before him, believed that each planet followed the path laid out by a certain number of spheres. Callippus had postulated 33 spheres in all, 4 each for Saturn and Jupiter, 5 each for Mars, Venus, Mercury, the sun and the moon. The problem with this model, however, was that, according to Aristotle, it did not explain how the motion of the outer spheres was to be prevented from interfering with the motion of the inner spheres. Aristotle therefore attempted a mechanical explanation, and postulated 22 counteracting spheres, which would set things in balance. It is generally held that Aristotle's addition of these counteracting spheres complicated rather than cleared up the problem of planetary motion. Aristotle's many-faceted theory of motion was a fundamental part of his world picture. The complexity of this theory is evidenced in the numerous interpretations offered by modern scholars. Here only the bare bones of it will be presented. According to Aristotle, there were three kinds of motion: rectilinear, circular and mixed. The four...
