Leucippus and Democritus were the earliest Greek atomists. The originator of the atomic theory, Leucippus (fifth century BCE), must be considered a speculative thinker of the first order, but to Democritus (c. 460–c. 370 BCE) must go the credit for working out the detailed application of the theory and supporting it with a subtle epistemology. Moreover, the range of Democritus's researches surpassed that of any earlier philosopher, and he appears to have been an original and, for his day, advanced ethical thinker.

We have very little biographical data for Leucippus. Epicurus is even reported to have said that there was no philosopher Leucippus, but the evidence of Aristotle decisively refutes this opinion (if, indeed, Epicurus did not merely intend to deny Leucippus's philosophical importance). Leucippus was probably born at Miletus; reports associating him with Elea or Abdera should be taken as reflecting views concerning his...