Leucippus was born at Elea, but some say at Abdera and others at Miletus. He was a pupil of Zeno
. These were his beliefs.
Being is unlimited in form but they all change into one another. The Body of being includes both Being and Nothingness. Worlds are formed when being is atomized and becomes entangled with Nothingness; And from the, as it were, Aristippian motion of the entanglement, the phenomenona of the physical universe arise. The sun revolves in a larger circle around the moon. The earth rides steady in its revolution, its shape being tympanic. Leucippus was the first to set up atoms as first principles. This gives a general summary of his basic beliefs; His particular beliefs were as follows.
As stated, Being is unlimited but part of Being is immanent and part is dimension, and these he calls stoichoi. Out of these arise the worlds without number and those same worlds dissolve back into them. This is the way of the Cosmos. In a given region of space, a given number of atoms are being resolved from the stoichoi into dimensional form. These collect together and attain a unity of form by their motion, combination, and recombination in the allowable formations, seperating into disparate and like forms, the like joining with like. And the atoms are so numerous that their motion becomes irregular and they pass into dimension/nothingness, like chaff winnowed from wheat; those in smooth motion form stable spherical systems of atoms, shell within shell, composing every kind of thing; and as these whirl around, their motion is resisted by the center and the enclosing shell becomes thinner, the enclosing shell becomes thinner and thinner as the adjacent atoms continually dissolve into the unity of the composed form.
In this manner our celestial spheres are formed. And again, even the outer shell grows larger by the influx of atoms from the outside, and, as it is brought into the unity add to itself whatever atoms it touches. And of these some portions are locked together and form a mass, at first damp lumpy, but when they have dried and revolve in the celestial spheres the afterwards ignite and form the stars.
The orbit of the sun is outermost and moon is closer the earth; everything else is between these two spheres. All the stars are ignited by the speed of their motion; this also contributes to the burning of the sun but only slightly to that of the moon.
As the world is born, so too it grows, decays and perishes, in virtue of some necessity, the nature of which he does not specify.
My translation of Diogenes Laertius
IX, Chapter 6, with liberties taken and a slight excision of material on the nature of eclipses and weather.