Discusses the impact of the Greek discovery of the "cosmos” on man's perception of his place in the universe, describes the problems this posed, and interprets Plato's response to this discovery. Starting with the Presocratics, Gregory Vlastos describes the intellectual revolution that began with the cosmogonies of Thales, Anaximander, and Anaximenes in the sixth century BC and culminated a century later in the atomist system of Leucippus and Democritus.