Atomism (from Ancient Greek ἄτομον (atomon) 'uncuttable, indivisible') is a natural philosophy proposing that the physical universe is composed of fundamental indivisible components known as atoms.

References to the concept of atomism and its atoms appeared in both ancient Greek and ancient Indian philosophical traditions. Leucippus is the earliest figure whose commitment to atomism is well attested and he is usually credited with inventing atomism. He and other ancient Greek atomists theorized that nature consists of two fundamental principles: atom and void. Clusters of different shapes, arrangements, and positions give rise to the various macroscopic substances in the world.

Indian Buddhists, such as Dharmakirti (fl. c. 6th or 7th century) and others, developed distinctive theories of atomism, for example, involving momentary (instantaneous) atoms (kalapas) that flash in and out of existence.

The particles of chemical matter for which chemists and other natural philosophers of the early 19th century found experimental evidence were thought to be indivisible, and therefore were given by John Dalton the name "atom", long used by the atomist philosophy. Although the connection to historical atomism is at best tenuous, elementary particles have become a modern analogue of philosophical atoms.

Philosophical atomism is a reductive argument, proposing not only that everything is composed of atoms and void, but that nothing they compose really exists: the only things that really exist are atoms ricocheting off each other mechanistically in an otherwise empty void. One proponent of this theory was the Greek philosopher Democritus.

By convention sweet is sweet, by convention bitter is bitter, by convention hot is hot, by convention cold is cold, by convention color is color. But in reality there are atoms and the void.

Atomism stands in contrast to a substance theory wherein a prime material continuum remains qualitatively invariant under division (for example, the ratio of the four classical elements would be the same in any portion of a homogeneous material).

In the 5th century BC, Leucippus and his pupil Democritus proposed that all matter was composed of small indivisible particles which they called "atoms". Nothing whatsoever is known about Leucippus except that he was the teacher of Democritus. Democritus, by contrast, wrote prolifically, producing over eighty known treatises, none of which have survived to the present day complete. However, a massive number of fragments and quotations of his writings have survived. These are the main source of information on his teachings about atoms. Democritus's argument for the existence of atoms hinged on the idea that it is impossible to keep dividing matter infinitely - and that matter must therefore be made up of extremely tiny particles. The atomistic theory aimed to remove the "distinction which the Eleatic school drew between the Absolute, or the only real existence, and the world of change around us."