double effect, principle of See PRINCIPLE OF DOU -. BLE EFFECT. double negation. (1) The principle, also called the law of double negation, that every proposition is logically equivalent to its double negation. Thus, the proposition that Roger is a rabbit is equivalent to the proposition that Roger is not not a rabbit. The law holds in classical logic but not for certain non-classical concepts of negation. In intuitionist logic, for example, a proposition implies, but need not be implied by, its double negation. (2) The rule of inference, also called the rule of double negation, that permits one to infer the double negation of A from A, and vice versa. See also FORMAL LOGIC. G.F.S.