Harmony and Complexity

Abstract

In Chaps. 4–6 of Book II, Aristotle argued that the motions of the heavenly spheres which surround the Earth are regular and eternal. That is, they rotate forever at a constant rate without acceleration or retardation. This must be the case because (i) their motion at present is seen to be regular, and (ii) any irregularity in their motion must have a discernible cause. After all, why would a cause of irregular motion suddenly arise, after being previously dormant? And similarly, why would a cause of irregular motion suddenly pass away, after having been previously active. Such unaccounted for causes can only be referred to chance or spontaneity, which Aristotle rejects as a valid mode of explanation for the motion of the heavens. So now, having discussed the substance (æther), size (large but finite), number (one), shape (spherical) and motion (regular) of the universe, Aristotle turns his attention to the composition, shape and motion of the bodies which populate the heavenly spheres. He begins by considering the fixed stars, then goes on to consider the complex motion of the wandering stars—the planets.

Nature is no wanton or random creator.

—Aristotle