Abstract

This dissertation examines critically the assumptions of extensionalism and the traditional doctrine of substitutivity, according to which codesignativeness or coextensionality of terms should be a sufficient condition to guarantee intersubstitution of expression salva veritate. First, the discussion focuses on the traditional justifications of the extensionalist principles of substitutivity. The following alleged sources of support for extensionalism are examined: the claim that the extensionalist approach to substitutivity relies on fundamental principles outside the domain of semantics, like the Law of Indiscernibility of Identicals and Leibniz's Law of Substitutivity; the assumption that all sentences that exhibit failures of substitutivity have in common a certain structure, that they include explicit or implicit intensional operators, which cause failures of substitutivity; the presumption that the extensionalist principles of substitutivity follow from a thesis of compositionality of meaning,according to which the meaning of a complex expression is determined by the meanings of its components and the mode of composition. Second, the intuitive foundations of a theory about the contributions of terms to the assertions expressed by uses of sentences are explored. I claim that this theory provides strong reasons to reject the extensionalist approach to substitutivity