Atomism emerged in ancient Greece in the 5th century BCE, primarily through the work of Leucippus and Democritus. Leucippus, a pre-Socratic philosopher, developed the theory as a response to the Eleatic school, positing that the universe consists of solid, indestructible, indivisible atoms moving through void.[3][5] Democritus, his pupil, further elaborated these ideas.[1][3][5]

Atomism posits that the universe is composed of minute, indivisible particles called atoms, which are qualitatively identical (distinct only in shape, size, and motion) and move through empty space or void, combining by juxtaposition to form all matter and explain change.[1][5] These atoms are absolutely indivisible, immutable, and eternal, with observable phenomena arising from their configurations and rearrangements rather than intrinsic qualities of wholes.[5] This mechanistic view rejects holistic explanations, attributing multiplicity and change in nature to atomic interactions, in contrast to Eleatic denial of void and change or Aristotelian potentiality-actuality transformations.[2][5]

The theory argues against infinite divisibility of matter, asserting atoms as the fundamental units whose endless motion and collisions produce the flux of the sensible world, including stability that only appears permanent.[1][2] Unlike later forms, strict ancient atomism excludes forces beyond mechanical juxtaposition and denies qualitative differences beyond atomic properties.[5]

- Leucippus (5th century BCE): Originator of atomism, pre-Socratic natural philosopher.[3][5] - Democritus (c. 460–370 BCE): Pupil of Leucippus, elaborated atomic theory extensively; main source via fragments.[1][3][5]

Ancient atomism, developed by Leucippus and Democritus, faced rejection by Aristotle, who denied void and explained change via matter's potential to actuality, leading to dominance of his views post-Classical Greece.[2][5] Plato's Timaeus presented a form of atomism with elemental corpuscles, interpreted as such by Aristotle, though distinct from Democritean strict atomism.[2] The philosophy persisted marginally but revived in the Renaissance among anti-Aristotelians like Giordano Bruno, who integrated atoms into an infinite universe theory, though deemed heretical.[2]

Atomism challenged Eleatic monism and holistic views, laying groundwork for mechanistic natural philosophy and influencing Renaissance thinkers like Bruno, who linked it to infinite worlds.[2] It prefigured modern atomic theory, though philosophical atomism differs from scientific versions post-17th century, which incorporated forces and empirical evidence.[5][6] Despite suppression, its emphasis on indivisible units and void impacted mechanical philosophy via figures like Boyle and Gassendi.[4]