This essay explores the treatment of the relation between nature (phusis) and norm or convention (nomos) in Democritus and in certain Platonic dialogues. In his physical theory Democritus draws a sharp contrast between the real nature of things and their representation via human conventions, but in his political and ethical theory he maintains that moral conventions are grounded in the reality of human nature. Plato builds on that insight in the account of the nature of morality in the myth in the Protagoras. That provides material for a defense of morality against the attacks by Callicles in the Gorgias and Thrasymachus and Glaucon in the Republic, all of whom seek to use the nature-convention contrast to devalue morality.