substitutivity salva veritate a condition met by two expressions when one is substitutable for the other at a certain occurrence in a sentence and the truth-value (truth or falsity) of the sentence is necessarily unchanged when the substitution is made. In such a case the two expressions are said to exhibit substitutivity or substitutability salva veritate (literally, ‘with truth saved’) with respect to one another in that context. The expressions are also said to be interchangeable or intersubstitutable salva veritate in that context. Where it is obvious from a given discussion that it is the truth-value that is to be preserved, it may be said that the one expression is substitutable for the other or exhibits substitutability with respect to the other at that place. Leibniz proposed to use the universal interchangeability salva veritate of two terms in every ‘proposition’ in which they occur as a necessary and sufficient condition for identity – presumably for the identity of the things denoted by the terms. There are apparent exceptions to this criterion, as Leibniz himself noted. If a sentence occurs in a context governed by a psychological verb such as ‘believe’ or ‘desire’, by an expression conveying modality (e.g., ‘necessarily’, ‘possibly’), or by certain temporal expressions (such as ‘it will soon be the case that’), then two terms may denote the same thing but not be interchangeable within such a sentence. Occurrences of expressions within quotation marks or where the expressions are both mentioned and used (cf. Quine’s example, ‘Giorgione was so-called because of his size’) also exhibit failure of substitutivity. Frege urged that such failures are to be explained by the fact that within such contexts an expression does not have its ordinary denotation but denotes instead either its usual sense or the expression itself. See also QUANTIFYING IN, REFERENTIALLY TRANSPAREN. C.A.A.