A contradiction sat at the centre of pre-Socratic thought: Parmenides had proved by argument that what is must be indivisible, changeless and motionless, while the senses showed change everywhere. Leucippus and his student Democritus resolved it in a single move. They kept Parmenides's indivisible being but, instead of one vast sphere, divided it into infinitely many small particles: *atomos*, uncuttable. Between them they placed what Parmenides had denied — the void. Change is then not creation but displacement: atoms move through the void, combine and separate.

The system is strikingly consistent. Atoms differ in shape, position and arrangement, and the qualities of things arise from those arrangements. 'Sweet and bitter exist by convention; in reality there are only atoms and void,' Democritus says — the first statement of the distinction between primary and secondary qualities, running on to Galileo and Locke. The soul too is made of atoms and is therefore mortal; the universe holds infinitely many worlds, none of them arranged for a purpose.

That last point decided the theory's fate. Once the purposive nature of Plato and Aristotle dominated antiquity, atomism remained a minority view; it continued through Epicurus and Lucretius but was condemned in the Christian Middle Ages for resting on chance and purposelessness. Not one of Democritus's writings survives whole — what we know comes from quotations. Revived by Gassendi in the seventeenth century, atomism was placed on experimental ground by Dalton's chemical atomic theory in 1808. That a prediction confirmed twenty-two centuries later was drawn, without observation, from a problem in logic is among the most discussed cases in the history of science.