Ancilla to the Pre-Socratic Philosophers, by Kathleen Freeman, , at sacred-texts.com
Leucippus of Abdêra was in his prime about 430 B.C.
He was credited with one book, called The Great World-Order; and perhaps a monograph On Mind.
1. (Title of book: The Great World-Order, here attributed to Democritus).
1a. (Restoration of Herculanean Papyrus in which Democritus is accused of plagiarising from The Great World-Order' of Leucippus).
Certain terms of the Atomic Theory can probably be traced to Leucippus, e.g.
Rhythm (meaning 'form').
Aspect (meaning 'position').
2. (From a treatise 'On Mind'). Nothing happens at random; everything happens out of reason and by necessity.