In the 5th century BC, Leucippus of Miletus and his disciple Democritus of Abdera formulated the hypothesis that all matter is composed of indivisible particles (ἄτομος, atoms) in perpetual motion in the void (κενόν). Atoms differ in shape, size, and position but not in sensible qualities; their combinations and separations explain all observed diversity. The theory is strictly mechanistic and materialist: it rejects final causes, divine intervention, and the continuum of matter. It was marginalized by Aristotelian physics, which denied the void and discrete matter, and remained a minority position throughout antiquity and the medieval period. Atomism survived mainly through Epicurus and Lucretius's poem De rerum natura (1st century BC). It did not become a scientific program until Dalton formulated chemical atomic theory (1803) and 20th-century quantum physics confirmed the discrete structure of matter, although modern atoms are divisible and qualitatively distinct from one another, refuting the details but not the fundamental intuition.