Democritus and Leucippus: Ancient Greek Atomic Theory (c. 440 BC)

Ancient Greek atomic theory was developed around 440 BC by Leucippus and his pupil Democritus of Abdera, who argued that all matter is made of tiny, indivisible particles moving through empty space. They called these particles atoms, from the Greek atomos, “uncuttable”. Though reached by reasoning rather than experiment, their theory became one of the most influential ideas in the history of science.

Background: the problem of change

Greek philosophers of the sixth and fifth centuries BC, beginning with Thales of Miletus, searched for the basic stuff of the world. Then Parmenides of Elea argued that change and motion are impossible: what exists cannot come from nothing, and empty space, being “nothing”, cannot exist.

The atomists accepted part of this argument. Each atom, like Parmenides’ being, is unchanging and cannot be created or destroyed. But they insisted that the void, empty space, is real. With many atoms moving in a void, change becomes possible: things come into being when atoms join and pass away when they separate.

Leucippus and Democritus

Almost nothing is known of Leucippus. He is variously said to have come from Miletus, Elea or Abdera and to have been active around 440 BC. A work called the Great World-System was attributed to him. According to Diogenes Laertius, Epicurus even claimed there had never been a philosopher called Leucippus, but most scholars accept that he existed.

Democritus was born in Abdera, a Greek city on the coast of Thrace, around 460 BC, and reportedly lived to a great age, dying perhaps around 370 BC. He travelled widely and wrote on physics, mathematics, ethics, music and much else; ancient catalogues list about seventy works. None survives complete, and his ideas are known from fragments and from reports by later writers, especially Aristotle.

The theory

According to the atomists, only two things truly exist: atoms and the void. Atoms are infinite in number, too small to see, solid and without internal parts. They differ in shape, size, arrangement and position. Aristotle illustrated this with letters of the alphabet: A differs from N in shape, AN from NA in order, and a letter turned on its side differs in position.

Atoms move constantly in the void, collide and become entangled through their shapes, some being hooked or barbed. In this way they form all the objects of the world, and even countless worlds beyond our own, without any divine plan.

Qualities like colour, taste and warmth are not in the atoms themselves but arise from how atoms affect our senses. A famous fragment of Democritus, preserved by Sextus Empiricus, says that sweet, bitter, hot, cold and colour exist only by convention, while in reality there are only atoms and void. Even the soul, in this view, was made of fine, round, fast-moving atoms.

Democritus also wrote on ethics. His ideal was euthymia, a state of cheerfulness and inner calm achieved through moderation, which may explain his later reputation as the “laughing philosopher”.

Critics and successors

Aristotle took atomism seriously but rejected it, denying that a void could exist and arguing that matter is continuous. Plato never mentions Democritus by name. Because Aristotle’s views dominated later thought, atomism remained a minority position.

It survived through Epicurus, who adopted and modified it at his school in Athens, the Garden of Epicurus, adding that atoms sometimes swerve unpredictably. The Roman poet Lucretius set out Epicurean atomism in his long poem On the Nature of Things in the first century BC. Rediscovered in 1417, the poem helped revive atomism in early modern Europe, where Pierre Gassendi and Robert Boyle defended corpuscular theories of matter.

Strengths and limits

Ancient atomism was remarkable for explaining the world through mechanical causes alone, without purpose or gods. It also offered an explanation of why matter can be compressed, dissolved or mixed: by rearranging atoms and the empty spaces between them.

Yet it was speculative. The atomists had no way to measure or observe atoms, and their theory made few testable predictions. Their atoms also had no fixed chemical identity, unlike the elements of modern chemistry.

Modern echoes

In 1808 the English chemist John Dalton published his atomic theory of chemistry and deliberately borrowed the ancient word “atom”. Modern atoms, however, are not indivisible: they contain electrons, protons and neutrons.

Timeline

• c. 460 BC: Democritus is born in Abdera.
• c. 440 BC: Leucippus formulates the atomic theory.
• c. 370 BC: Probable death of Democritus.
• c. 306 BC: Epicurus founds his school in Athens and teaches atomism.
• 1st century BC: Lucretius writes On the Nature of Things.
• 1417: Poggio Bracciolini rediscovers Lucretius’ poem.
• 1808: John Dalton publishes his chemical atomic theory.

Why it matters

The ancient atomists were the first to propose that the visible world can be explained entirely by tiny invisible particles following natural laws. Their theory was not science in the modern sense, but it supplied a powerful idea and a name that later scientists would turn into one of the foundations of physics and chemistry.