Democritus was born at Abdera, about 460 BCE, although according to some 490. His father was from a noble family and of great wealth, and contributed largely towards the entertainment of the army of Xerxes on his return to Asia. As a reward for this service the Persian monarch gave and other Abderites presents and left among them several Magi. Democritus, according to Diogenes Laertius, was instructed by these Magi in astronomy and theology. After the death of his father he traveled in search of wisdom, and devoted his inheritance to this purpose, amounting to one hundred talents. He is said to have visited Egypt, Ethiopia, Persia, and India. Whether, in the course of his travels, he visited Athens or studied under Anaxagoras is uncertain. During some part of his life he was instructed in Pythagoreanism, and was a disciple of Leucippus. After several years of traveling, Democritus returned to Abdera, with no means of subsistence. His brother Damosis, however, took him in. According to the law of Abdera, whoever wasted his patrimony would be deprived of the rites of burial. Democritus, hoping to avoid this disgrace, gave public lectures. Petronius relates that he was acquainted with the virtues of herbs, plants, and stones, and that he spent his life in making experiments upon natural bodies. He acquired fame with his knowledge of natural phenomena, and predicted changes in the weather. He used this ability to make people believe that he could predict future events. They not only viewed him as something more than mortal, but even proposed to put him in control of their public affairs. He preferred a contemplative to an active life, and therefore declined these public honors and passed the remainder of his days in solitude.
Democritus expanded the atomic theory of Leucippus. He maintained the impossibility of dividing things ad infinitum. From the difficulty of assigning a beginning of time, he argued the eternity of existing nature, of void space, and of motion. He supposed the atoms, which are originally similar, to be impenetrable and have a density proportionate to their volume. All motions are the result of active and passive affection.