Democritus

Democritus of Abdera was a Greek philosopher of the fifth and early fourth centuries BCE who, building on the ideas of his teacher Leucippus, developed the theory that everything in the world is made of tiny, indivisible particles, atoms, moving in empty space. He wrote on an enormous range of subjects, from physics and mathematics to ethics and music, but none of his books survives, and his views are known from fragments and from the reports of others. His atomism became one of the most influential scientific ideas of antiquity and was revived at the birth of modern science.

Democritus came from Abdera, a prosperous Greek city on the coast of Thrace, in what is now north-eastern Greece. He is said to have described himself as a young man when the philosopher Anaxagoras was old, and to have been forty years younger than him, which places his birth around 460 BCE and makes him a younger contemporary of Socrates. Ancient writers agree that he lived to a great age, perhaps ninety or even more than a hundred, so he probably died some time around 370 BCE.

Stories of his life were collected centuries later by Diogenes Laertius and others and are of doubtful value. He is said to have spent a large inheritance on travel, visiting Egypt, Persia and perhaps India in search of learning, and to have come to Athens, where no one recognised him. Later legend made him "the laughing philosopher", amused by human folly, in contrast to the gloomy Heraclitus. A set of letters falsely attributed to the physician Hippocrates describes how the people of Abdera, thinking Democritus mad because he laughed at everything, called Hippocrates to cure him, only for the doctor to conclude that he was the sanest man in the city. The letters are fiction from a much later period.

Democritus is always paired with Leucippus, his teacher or associate, of whom almost nothing is known; some ancient writers even doubted that he existed, though most scholars accept that he did. Aristotle and his pupil Theophrastus treated the two together, and while a few ideas can be credited to Leucippus, most reports concern Democritus, who seems to have taken over and systematised the theory.

Atomism was in large part a response to Parmenides, who had argued that what is cannot come into being, change or move, and that there is no such thing as empty space. The atomists accepted that real being is unchanging and cannot arise from nothing, but they insisted that "what is not", the void, also exists. With that step, change and motion became possible again: the world could be explained as the rearrangement of unchanging bits of being moving through emptiness.

According to Democritus, the atoms are infinite in number, too small to be seen, solid and indivisible; the Greek word atomos means "uncuttable". They have no colour, taste or warmth, and differ only in shape, size, arrangement and orientation, rather as letters of the alphabet differ in form and in their order within words. They have always been moving in the infinite void, colliding, rebounding and sometimes hooking together to form clusters. Everything we see, from stones to living bodies, is a temporary combination of atoms, and coming into being and passing away are simply their joining and separating.

From this he drew bold conclusions. Through countless collisions, whirls of atoms form whole worlds, and there are innumerable worlds, some like ours and others very different, arising and perishing across the void. The process needs no designer and no purpose: everything happens through necessity. The soul too is made of atoms, especially fine and spherical ones like those of fire, and it disperses at death.

Democritus also faced the problem of knowledge that his theory created. Since atoms have no colour or taste, such qualities must arise from the way atoms affect our senses. One fragment sets this out sharply: sweet, bitter, hot, cold and colour exist only by convention, while in reality there are atoms and void. He distinguished a "bastard" kind of knowledge from the senses and a "legitimate" kind from reason, while also recognising, in another fragment, that reason depends on the senses for its evidence. Some later readers took these remarks as a form of scepticism.

Democritus was one of the most prolific writers of antiquity. A catalogue drawn up by Thrasyllus in the first century CE lists about seventy works arranged in groups of four, covering ethics, physics, mathematics, music, literature and technical subjects such as farming, medicine and military tactics. All are lost. Around three hundred fragments survive, the great majority of them short ethical sayings preserved in later anthologies such as that of Stobaeus, and the authenticity of some is uncertain.

His ethics centred on euthymia, cheerfulness or contentment, a calm and settled state of mind. He advised moderation in pleasure, warned against envy and excessive desire, and praised measured living and the pursuit of understanding. He valued the shared life of the city, saying that a well-governed state is the greatest protection. These ideas made him a forerunner of the Hellenistic schools that treated philosophy as a guide to a tranquil life.

Plato never mentions Democritus by name, and an ancient story claimed he wanted to burn his books; whether or not that is true, Plato's ordered, purposeful cosmos is in many ways the opposite of the atomists' world of chance collisions. Aristotle took atomism seriously but rejected the void and the idea of indivisible bodies. Atomism found its great heir in Epicurus, who adapted it as the foundation of his ethical philosophy, and the Roman poet Lucretius set out the Epicurean version in verse, making it a central strand of Hellenistic and Roman philosophy.

After the rediscovery of Lucretius in the fifteenth century, atomist ideas helped shape the mechanical philosophy of the seventeenth century, influencing thinkers such as Galileo Galilei and Isaac Newton, and when the chemist John Dalton developed modern atomic theory in the early nineteenth century he borrowed the old Greek name. Modern atoms can be split and are not the solid particles Democritus imagined, but his picture of a natural world explained entirely by matter in motion, without design, remains among the boldest achievements of ancient Greek philosophy.