In all probability, credit for the fundamental ideas of the atomic theory—Greek speculation’s greatest achievement—should go to Leucippus of Miletus rather than to his pupil, Democritus. However, almost nothing is known of Leucippus.
Rational speculation about the nature of the world began not earlier than the sixth century b.c.e. Four or five generations later it had progressed, in Democritus, to an essentially correct account of the nature of matter. This amazing fact has led to both exaggeration and underestimation of the Greek achievement. Some people conclude that science stood still until the revival of the atomic theory in the seventeenth century. However, scientists point out that modern atomic physics rests on evidence derived from careful quantitative experimentation of which the Greeks knew nothing; therefore, it is said, the ancient theory was merely a lucky guess—and the Greeks made all possible guesses. A brief review of the development of early Greek physics will show that while Democritus did not have English chemist and physicist John Dalton’s reasons for asserting that the world consists of atoms moving in the void, he nevertheless had some very good ones.