Describes any one of a family of deductive systems developed by logician Roman Suszko (1919–1979) that include an identity connective $=$ corresponding to identity between propositions. Named as a rejection of philosopher Gottlob Frege’s (1848–1925) assertion that there are only two possible referents for a sentence: truth and falsity. This principle entails that identity between propositions is captured by material equivalence, represented by the principle

• • $\phi =\psi$ is true in a model if and only if $\phi \equiv \psi$ is true in that model

where $\equiv$ is material equivalence. Although developed independently, Suszko’s non-Fregean logic bears a great deal of similarity to the situation semantics of philosophers John Perry (1943– ) and Jon Barwise (1942–2000): each was developed as a reaction to this conclusion of Frege and each invokes situations as the ultimate referent of a proposition.