Born: about 480 BC in (possibly) Miletus, Asia Minor
Died: about 420 BC

Leucippus of Miletus carried on the scientific philosophy which had begun to become associated with Miletus. We know little of his life but it is thought that he founder the School at Abdera on the coast of Thrace near the mouth of the Nestos River. Today the town is in Greece and is called Avdhira. At the time that Leucippus would have lived in Abdera it was a prosperous town which politically was a member of the Delian League.

The philosopher Protagoras was born in Abdera and he was a contemporary of Leucippus but Protagoras, the first of the Sophists, spent most of his life in Athens and may have left Abdera before Leucippus arrived there. Although now there seems little doubt that Leucippus existed, it is worth remarking that Epicurus, at the end of the fourth century BC, actually believed that Leucippus had never existed since so little was known of him. However we now know enough in the way of independent evidence to be sure that Leucippus did exist.

Aristotle refers to Leucippus as a philosopher with rather different views to those of Parmenides. Aristotle refers to him several times and quotes from his works on a number of occasions. For example in De caelo Aristotle writes:-

... of those who have maintained the existence of indivisibles, some, as for example Leucippus and Democritus, believe in indivisible bodies, others, like Xenocrates, in indivisible lines.

Unfortunately Aristotle is not entirely consistent in his references to Leucippus. Some quotes suggest that atomism began with Leucippus, other quotes such as the one above bracket Leucippus and Democritus, while in a few places Aristotle seems to imply that Democritus alone invented atomism.

Certainly it seems that Leucippus was much influenced in his thinking by Zeno of Elea and by Parmenides, but it seems relatively unlikely that there is any truth in the later claim that he was a pupil of Zeno of Elea. More likely here is that later writers realised that Leucippus followed Zeno's ideas and 'pupil' was intended in this sense.

It is thought that Democritus was a pupil of Leucippus, where this time 'pupil' really does have its standard meaning. Together they are considered as the joint founders of atomic theory. Leucippus stated that atoms are [7]:-

... imperceptible, individual particles that differ only in shape and position.

The mixing of these particles gives rise to the world we experience. The reason that some early writers did not believe in the existence of Leucippus seems to be because his views and those of Democritus became completely entwined. Quite soon the whole became attributed to Democritus who was the more famous of the pair. It seems likely that Democritus as a pupil of Leucippus, developed the ideas of his teacher but it is quite beyond us to disentangle the contributions of each to this important doctrine.

Two works, almost certainly written by Leucippus, are The Great World System and On the Mind. The first of these is attributed to Leucippus by Theophrastus. Theophrastus (about 372 BC - 287 BC) was a pupil of Aristotle who had studied at Athens under Aristotle. Theophrastus became head of the Lyceum in Athens after Aristotle in 323 BC. He was in a position to be able to distinguish the works of Leucippus from those of Democritus and we shall describe his views on this matter.

Theophrastus claimed that the basic ideas of atomism were present in the philosophy of Leucippus according to which [1]:-

Both matter and void have real existence. The constituents of matter are elements infinite in number and always in motion, with an infinite variety of shapes, completely solid in composition.

According to Diogenes Laertius, the cosmology put forward by Leucippus in The Great World System is a creation of worlds by agglomerations of atoms by chance collisions. There is then differentiation with the smaller atoms being sent off into the infinity of space while the rest form into a spherical structure with the larger atoms at the centre and the smaller atoms further away from the centre.

From the treatise On the Mind we have the only quotation of the words of Leucippus which have survived. In this work he writes (see for example [8]):-

Nothing happens in vain, but everything from reason and of necessity.

1. G B Kerferd, Biography in Dictionary of Scientific Biography (New York 1970-1990). 
   http://www.encyclopedia.com/topic/Leucippus.aspx
2. Biography in Encyclopaedia Britannica. 
   http://www.britannica.com/eb/article-9047937/Leucippus

Books:

1. C Bailey, The Greek Atomists and Epicurus (Oxford, 1928).
2. J Barnes, Early Greek Philosophy (1987).
3. W K C Guthrie, History of Greek Philosophy II (Cambridge, 1965).
4. G S Kirk and J E Raven, The Presocratic Philosophers (Cambridge, 1957).

Articles:

1. J Barnes, Reason and necessity in Leucippus, in Proc. First International Congress on Democritus (Xanthi, 1984), 141-158.
2. E Craig (ed.), Routledge Encyclopedia of Philosophy 5 (London-New York, 1998), 578-579.
3. B Russell, History of Western Philosophy (London, 1961), 82-90.