the founder of the Atomic Theory in Greek philosophy. The period in which he lived is unknown, but is generally supposed to have been in the fifth century before the Christian era. Elis, Abdera, and Miletus, have been severally assigned by different authors as his birthplace. According to Diogenes Laertius, he was the teacher of Democritus. By others he has been called the disciple of Pythagoras, of Melissus, and of Zeno the Eleatic. Aristotle, however, is not alone in asserting that the first principles of the atomic theory, though evolved by Leucippus, were particularly expounded by Democritus. In explaining the creation of the universe, they supposed that the two great principles in nature were a vacuum and a plenum. The plenum consisted of atoms infinite in number, and, as their name implies, indivisible. As a necessary consequence of their being many and not one, the atoms floated in the midst of the vacuum. Possessing, also, as their essential qualities, motion, solidity, and form,—in virtue of the first they began a rotatory movement; in virtue of the second they were prevented from merging into one simple body; and in virtue of the third, those of similar figure attracted each other. The atoms that were spherical, and consequently more movable, meeting together formed fire; those that were not spherical formed, in their aggregations, the other three elements, air, earth, and water. The subtlest and most refined of the fire-atoms constituted the soul. Life, too, having been identified with respiration, was explained as being merely the flux and reflux of these spherical atoms.

Here Leucippus seems to have stopped; but Democritus, following out these principles to their natural results, arrived at his system of Psychology, and at his theory of the First Cause, which he rather obscurely calls Destiny. The opinions afterwards became identified with the doctrines of Epicurus. See Epicurean Philosophy.