The paper examines the origins of the distinction between physis, “nature,” and nomos, “norm,” and the uses made of it during the period of the Sophists. The two terms did not originally lend themselves to being contrasted, but the contrast becomes natural in light of two mid-fifth-century developments: a growing interest in the different customs of different societies and a proliferation of accounts of the origins of human civilization. While the contrast is employed by others, such as Herodotus and the medical writers, it is the Sophists themselves, above all, who exploit it for sociological and philosophical purposes. Some, such as Protagoras, see nomos as building on physis – that is, on tendencies in human nature; others see an opposition between the two, and suggest that we would be better off ignoring nomos and attending to what our natures dictate. The contrast is also applied to religion, which some Sophists treat as nomos.