Original: Leucippus or Leukippos (Greek: , first half of 5th century BC) was the first Greek to develop the theory of atomism — the idea that everything is composed entirely of various imperishable, indivisible elements called atoms — which was elaborated in far greater detail by his pupil and successor, Democritus.
Simplified: Leucippus or Leukippos (Greek: , first half of 5th century BC) was among the earliest philosophers of atomism, the idea that everything is made up of many imperishable, indivisible elements called atoms.