Democritus (460?-370? bc), Greek philosopher, who developed the atomic theory of the universe, which had been originated by his mentor, the philosopher Leucippus.

Democritus was born in Abdera, Thrace. He wrote extensively, but only fragments of his works remain. According to his exposition of the atomic theory of matter, all things are composed of minute, invisible, indestructible particles of pure matter (atoma,”indivisibles”), which move about eternally in infinite empty space (kenon,”the void”). Although atoms are made up of precisely the same matter, they differ in shape, size, weight, sequence, and position. Qualitative differences in what the senses perceive and the birth, decay, and disappearance of things are the results not of characteristics inherent in atoms but of quantitative arrangements of atoms. Democritus viewed the creation of worlds as the natural consequence of the ceaseless whirling motion of atoms in space. Atoms collide and spin, forming larger aggregations of matter.

Democritus also wrote on ethics, proposing happiness, or “cheerfulness,” as the highest good—a condition to be achieved through moderation, tranquillity, and freedom from fear. In later histories, Democritus was known as the Laughing Philosopher, in contrast to the more somber and pessimistic Heraclitus, the Weeping Philosopher. His atomic theory anticipated the modern principles of the conservation of energy and the irreducibility of matter.