While the Atomic Theory is usually identified with the name of Democritus, its real founder was Leucippus, as is attested by both Aristotle (384-322 B. C.) and his successor, Theophrastus. Diogenes Laertius, who flourished about 230 of our era, refers to Leucippus (De Vitis, ix, 46) as the author of “The Great Diakosmos” (or “The Great Order of the Universe”), in which were set forth the principles upon which he based his atomic theory. A treatise “On the Mind” is also mentioned, which appears to have contained in outline the psychology of the materialistic Atomists. Of the works of Leucippus there remains but one extant fragment, in which Leucippus lays down in clear terms this universal rule: “Nothing happens without a cause, but everything with a cause and by necessity” (Aetius, Doxographers 321 B. 10). Following in the footsteps of his master, Democritus also wrote a book “On the Order of the Universe,” adhering, it would seem, very closely to the views of Leucippus. The works of his master the great Milesian, were incorporated eventually in the collected works of Democritus. No writer, subsequent to Theophrastus (born at Lesbos about 390 B. C., died in 286 B. C.), pupil of and successor to Aristotle as head of the famous Lyceum at Athens, attempted to distinguish the teaching of Leucippus from that of his more brilliant disciple. Zeller remarks that “the work and even the name of Leucippus seems to have been pretty early forgotten by most writers in comparison with the riper and more exhaustive achievements of his disciple. The persistence with which he is ignored by Epicurus, the reviver of the Atomistic philosophy, and by most of the Epicureans, may have contributed to this.[6] Of the once numerous writings of Democritus himself, only fragments remain, but these are highly important. The chief collector of the extant fragments was Mullach,[7] who places Democritus first on the list of Greek philosophers for genius and knowledge, and thinks it probable that the great Aristotle himself may owe much of his reputation for learning to diligent perusal of the words of the sage of Abdera. Says he: “(Democritus), although in other things dissimilar, in his equal study of all the arts was most like the famous Aristotle. And I scarcely know whether the Stagirite did not owe to his reading of the works of Democritus his erudition, which surpassed that of all other philosophers.” Aristotle and some later historians make frequent references to his teachings, and according to Zeller, “with respect.” Gompertz says that Aristotle confers “a crown of eulogy” upon Democritus “at the expense of Plato.”