Full Idea
Democritus claimed that one substance could not be composed from two nor two from one; for him it is the atoms that are the substances.
Gist of Idea
Democritus said that substances could never be mixed, so atoms are the substances
Source
report of Democritus (fragments/reports [c.431 BCE]) by Aristotle - Metaphysics 1039a10
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.218