Leucippus in Harpers Dictionary of Classical Antiquities
A Grecian philosopher, the founder of the atomic theory of philosophy, which was more fully developed by Democritus (Diog. Laert. ix. 30, 34). His date is uncertain. See Democritus; Epicurus.
Leucippus in Wikipedia
Leucippus or Leukippos (Greek: Λεύκιππος, first half of 5th century BC) was one of the earliest Greeks to develop the theory of atomism - the idea that everything is composed entirely of various imperishable, indivisible elements called atoms - which was elaborated in far greater detail by his pupil and successor, Democritus. Another possible earlier candidate for atomism is Moschus of Sidon, from the Trojan War era(13th or 12th century BCE)
He was most likely born in Miletus, although Abdera and Elea are also mentioned as possible birth-places.
Leucippus was a shadowy figure, as his dates are not recorded and he is often mentioned in conjunction with his more well-known pupil Democritus, who replaced indeterminism with determinism as the ontological cause of atomic movement. It is therefore difficult to determine which contributions come from Democritus and which come from Leucippus.
In his Corpus Democriteum, Thrasyllus of Alexandria, an astrologer and writer living under the emperor Tiberius (14-37 CE) compiled a list of writings traditionally attributed to Democritus to the exclusion of Leucippus.
Leucippus was an Ionian Greek (Ionia, at present Turkey) as was Anaxagoras, and a contemporary of Zeno of Elea and Empedocles (Magna Graecia, at present Italy). Belonging to the same Ionian School of naturalistic philosophy as Thales, Anaximander, Anaximenes, he was interested in reality and not ideality as the Italic Eleatics were. The legend about the influence of Zeno is totally false, as modern studies established, also because the ontological conception of being of the Eleatics is static, monistic and deterministic, while Leucippus' is dynamic, pluralistic and indeterministic.
Around 440 B.C. or 430 B.C. Leucippus founded a school at Abdera, which his pupil, Democritus, was closely associated with. His fame was so completely overshadowed by that of Democritus, who systematized his views on atoms, that Epicurus doubted his very existence.
However, Aristotle and Theophrastus explicitly credit Leucippus with the invention of Atomism. Leucippus agreed with the Eleatic argument that true being does not admit of vacuum, and there can be no movement in the absence of vacuum. Leucippus contended that since movement exists, there has to be vacuum. However, he concludes that vacuum is identified with non-being, since it cannot really be. Leucippus differed from the Eleatics in not being encumbered by the conceptual intermingling of being and non-being. Plato made the necessary distinction between grades of being and types of negation.
The most famous among Leucippus' lost works were titled Megas Diakosmos (The Great Order of the Universe or The great world-system) and Peri Nou (On mind).
A single fragment of Leucippus survives:
Nothing happens at random (maten), but everything from reason (ek logou) and by necessity.
-Leucippus, Diels-Kranz 67 B2