Democritus c. 460 b.c.-c. 370 b.c.
Democritus is best known for the atomic or atomistic theory he co-developed with his teacher, Leucippus. Although a contemporary of Socrates, Democritus is considered among the last of the pre-Socratics. It is generally believed that Leucippus contributed more to atomism's founding than Democritus, but that Democritus was more responsible for the theory's refinement. Although their work was based on the earlier theories of the Milesians, the two made notable advancements, particularly in their explanation of density. The atomic theory is mechanistic: it holds that all matter is composed of an infinite number of indivisible and indestructible atoms of various shapes moving about in an infinite void, in an infinite universe, always and forever colliding with each other, and sometimes joining to form combinations. Nothing happens by chance, nor on purpose, and all can be explained in terms of mechanical principles, one thing causing another. Democritus's philosophy, including his belief that the soul itself is composed of atoms, angered some philosophers including Plato—who pointedly did not mention him— and Aristotle, who, in his writings, criticized Democritus and his theory. Most of the few hundred fragments that remain of Democritus's writings deal with ethics rather than atomism. His ethics stress moderation and the practice of that which is beneficial to society. Democritus also asserted that man's belief in divinities is due to his ignorance about nature, and his emphasis on cheerfulness as the goal for all individuals led to his nickname, the Laughing Philosopher.
Democritus was born on the northern shore of the Aegean Sea in Abdera, a city of ancient Thrace which was also home to the Sophist Protagoras. Little else is known about his life, except that he traveled on one occasion to talk with Anaxagoras, the leading scientist of Athens, but was rebuffed. It is believed that Democritus was fairly wealthy and that he traveled extensively in the East—to Egypt, Babylon, India, and Persia. He lived a long life, probably ninety years or more.
Democritus is said to have written many books (seventy, according to Diogenes Laertius), including one entitled Little Cosmology as a nod to Leucippus's Great Cosmology. The ancients report he was fascinated by all subjects, and wrote on music and on all aspects of science, including biology and astronomy. Only fragments of his work survive, however, and most of these are concerned with ethics. In these terse fragments, Democritus discusses laws and his belief that individuals will obey them because it is in their collective self-interest to do so. He explains the importance of maintaining a balance between too much and too little material wealth, and also promotes altruism.
Democritus's ideas failed to please those who attempted to explain the nature of things by looking at their function, as well as those who tried to explain the world in terms of a divine power. Socrates, Plato, and Aristotle all disagreed with him. In modern times he is credited with starting on the correct path: without modern scientific tools, Democritus could do little more than practice atomism as a philosophy. Robert L. Oldershaw points out, though, that “he had a remarkably modern understanding of concepts like the conservation of mass/energy, the indirect nature of perception, the continual formation of and destruction of physical systems, the reality of empty space, the basic theory of colours and the fundamental principles of causality and determinism.” Most scholars have acknowledged the impossibility of determining exactly where Leucippus's ideas end and Democritus's begin, but they continue to debate whether or not Democritus authored the ethical fragments often attributed to him. One group believes he was not the author since there is little similarity between the atomic theory writings and the ethical fragments, and further, they question why so many of these fairly unremarkable ethical pieces exist. The opposition maintains that Democritus was responsible for both sets of writings; that they lack similarity because they are concerned with vastly different topics; that they nonetheless do contain some similar elements; and that the Cynics may have preserved the ethical fragments, which would explain why so many of them are extant. Some scholars concentrate on Democritus's political theory, so far as it can be deduced. Eric A. Havelock investigates Democritus's views on laws intended to promote good behavior in society, while Michael Nill studies Democritus's views governing higher and lower forms of pleasure and their function in a well-run community. Jonathan Barnes explores the dilemma the atomists faced concerning belief and the imprecise nature of knowledge. There is disagreement on exactly what Democritus meant in certain instances—understandable given the dearth of surviving texts. Richard D. McKirahan takes a close look at the extant fragments of Democritus and places them alongside the work of contemporaneous and near-contemporaneous commentators in an attempt not only to explain atomism but to explain what the ancients thought it meant. C. C. W. Taylor contributes a similar effort in his study of Democritus's theological writings.