Democritus was a philosopher who, with his teacher Leucippus, developed the atomic theory of the universe. Born in a region north-east of Greece called Thrace, Democritus traveled widely during his long life and wrote about 70 books on such diverse topics as ethics, physics, mathematics, and music. All of these have been lost. He is most famous for his physical theories, only a few fragments of which survive.
Democritus believed that the universe is made up of extremely small particles of various shapes and sizes that could not be broken apart. He called these particles atoms. Atoms would move about randomly, colliding with and sticking to one another. These groups of atoms formed all other known substances. Democritus saw the universe as infinitely large and filled with many worlds that were continuously being created and destroyed. Most other Greek philosophers, including Aristotle, believed that ours was the only world in a finite universe and that it would last forever.
In the field of ethics, Democritus argued that it was morally correct for people to pursue cheerfulness as a goal in life. This could be done
by seeking pleasure, but pleasure should be sought wisely and in moderation. According to Democritus, people should not let the fear of dying ruin their enjoyment of living. Beliefs such as this earned Democritus the nickname “the laughing philosopher.” His teachings later influenced Epicurus and Lucretius. (See also Philosophy, Greek and Hellenistic.)