His most important work was 11Epi cuo ew (De natura), of which considerable fragments are extant (chiefly in Simplicius); it is possible that he wrote also Against the Sophists and On the Nature of Man, to which the well-known fragment about the veins would belong; possibly these discussions were subdivisions of his great work.
From the Greek sophists they borrowed ingenious ways of playing off one duty against another, or duty in general against self-interest - leaving the doubter in the alternative of neglecting the one and being a knave, or neglecting the other and being a fool.
Of these the best known were: the Kolakes, in which he pilloried the spendthrift Callias, who wasted his substance on sophists and parasites; Maricas, an attack on Hyperbolus, the successor of Cleon, under a fictitious name; the Baptae, against Alcibiades and his clubs, at which profligate foreign rites were practised.
The Sophists and the Sceptics, Plato and Aristotle, the Stoics and the Epicureans took up the question, and from the time of Locke and Kant it has been prominent in modern philosophy.
This seems to have been interpreted by its author and by the Sophists in general in a subjective sense, with the result that it became the motto of a sceptical and individualistic movement in contemporary philosophy and ethics.