physis Greek term for nature, primarily used to refer to the nature or essence of a living thing (Aristotle, Metaphysics V.4). Physis is defined by Aristotle in Physics II.1 as a source of movement and rest that belongs to something in virtue of itself, and identified by him primarily with the form, rather than the matter, of the thing. The term is also used to refer to the natural world as a whole. Physis is often contrasted with techne, art; in ethics it is also contrasted with nomos, convention, e.g. by Callicles in Plato’s Gorgias (482e ff.), who distinguishes natural from conventional justice. See also ARISTOTLE , PLATO, TECHN. W.J.P.