republicanism, classical See CLASSICAL REPUBLI -. CANIS. rerum natura (Latin, ‘the nature of things’), metaphysics. The phrase can also be used more narrowly to mean the nature of physical reality, and often it presupposes a naturalistic view of all reality. Lucretius’s epic poem De rerum natura is an Epicurean physics, designed to underpin the Epicurean morality. A.P.M.