Atomism

Atomism is a natural philosophy that holds the physical universe is built from fundamental, indivisible units called atoms. This idea emerged independently in ancient Greek and ancient Indian thought. In Greece, Leucippus is the earliest figure reliably associated with atomism and is generally recognized as its inventor. He and other Greek atomists believed nature has two basic principles: atoms and the void. Different clusters of atoms, varying in shape, arrangement, and position, produce the many macroscopic substances we see. Around the same time, Kanāda of the Vaiśeṣika school proposed similar indivisible particles, known as paramāṇu. Later, Indian Buddhists like Dharmakirti developed their own atomist theories, including the concept of momentary atoms (kalapas) that flash in and out of existence.

In the early 19th century, chemists and natural philosophers found experimental evidence for particles of chemical matter they thought were indivisible. John Dalton gave these particles the name "atom," a term long used in atomist philosophy. Though the link to ancient atomism is weak, elementary particles today serve as a modern counterpart to philosophical atoms.

Philosophical atomism is a reductive argument: it claims not only that everything is made of atoms and void, but that nothing composed of them truly exists—only atoms ricocheting mechanistically in empty space are real. The Greek philosopher Democritus championed this view. Atomism contrasts with substance theory, where a continuous prime material remains qualitatively the same under division, such as the constant ratio of the four classical elements in any portion of a homogeneous material.

In 5th-century BCE Greece, Leucippus and his student Democritus proposed that all matter consists of small, indivisible particles they called atoms. Nothing is known about Leucippus except that he taught Democritus. Democritus, however, wrote prolifically—over eighty treatises—none of which survive complete, though many fragments and quotations remain. These are the main source for his atomist teachings. His argument for atoms rested on the impossibility of infinite division: matter must be made of extremely tiny particles. The theory aimed to remove the Eleatic school's distinction between absolute, real existence and the changing world around us.

Democritus developed his ideas in response to Parmenides, who argued logically that change is an illusion. Democritus rejected this, believing change is real—or at least that the illusion of change must be explained. He therefore supported the concept of the void, an infinite space where atoms move and pack or scatter differently. These different arrangements in the void create the shifting outlines and bulk of objects that organisms feel, see, eat, hear, smell, and taste. Sensations like hot or cold have no real existence; they are merely effects produced in organisms by the different packings and scatterings of atoms composing the sensed object.

Democritus held that atoms are too small for human senses to detect, infinitely many, infinitely varied in type, and have always existed. They float in the void, varying in form, order, and posture—some convex, some concave, some hook-shaped, others eye-shaped. They constantly move and collide. He wrote that only atoms and void truly exist; all other things exist only by social convention. Everyday objects are composed of many atoms united by random collisions, their forms and materials determined by which atoms make them up. Human perceptions also arise from atoms: bitterness comes from small, angular, jagged atoms passing over the tongue; sweetness from larger, smoother, rounder atoms.

Democritus's work survives only in secondhand reports, some unreliable or conflicting. The best evidence comes from Aristotle, who discussed Democritus's and Plato's contrasting views on indivisibles composing the natural world. Plato, in his *Timaeus*, described solids as composed of indivisible triangles. Different arrangements of these triangles produce the four simple bodies—fire, air, water, and earth—and rearrangements allow their transformation. Plato assigned the cube to earth for its flat base and stability, the tetrahedron to fire for its penetrating points and sharp edges, and the octahedron and icosahedron to air and water for their blunter points and edges, making them less mobile.