Man of Mystery. Certainly the most original fifth-century contribution to the debate on the nature of change was that of Leucippus (Leukippos) of Miletus. Little is known about his life, except that he lived around 430 B.C.E., wrote two books (On Mind and The Great World System), and was the teacher of the famous Democritus of Abdera. It is through his pupil that Leucippus's revolutionary insights into the world of nature have mainly been preserved. In turn, the list of works attributed to Democritus runs to more than seventy titles, and includes studies on such diverse subjects as mathematics, farming, medicine, grammar, ethics, and literature. In what remains of his writings on physics or natural science, the atomic theory that Leucippus proposed and Democritus further developed marks a highpoint in ancient speculation.

The Atom. The main features of this theory...