Phusis is the ancient Greek word for "nature," cognate with the verb "to grow" (phuein); as in English, it can be used both for the natural world as a whole and for the "nature" (i.e., the essential or intrinsic characteristics) of any particular thing, which it has "by nature" (phusei). Nomos encompasses both law and unwritten, traditional social convention. The contrast between the two concepts is central to ancient sophistic thought, with roots in the pre-Socratic inquiry into the underlying natures of things.

For the Sophists, nomos and phusis are polar terms, roughly equivalent (respectively) to the socially constructed and the universally, objectively given. The contrast was most strikingly applied in relation to justice. Antiphon's On Truth argues that justice is a matter of nomos, and nomos and phusis conflict; one should observe the requirements of justice when there are witnesses, but follow...