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Intensionality And Extensionality

  A sentence is intensional, in philosophy, if substituting co-referring terms in it may change its truth value. Suppose that Mary is the mass murderer. Then ‘Mary’ and ‘the mass murderer’ are co-referring terms: both refer to the same thing. Further suppose that Fred admires Mary. Then the statement ‘Fred admires Mary’ is true. Substituting co-referring terms—replacing ‘Mary’ with ‘the mass murderer’—we get the sentence ‘Fred admires the mass murderer’. So substituting co-referring terms has taken us from a true sentence to one which may well be false. After all, Fred may not know that Mary is the mass murderer.

In contrast, a sentence is extensional if substituting co-referring terms in it is guaranteed to preserve its truth value. Consider the sentence ‘Mary is blonde’. Substituting co-referring terms, we get the sentence ‘the mass murderer is blonde’. If the original sentence is true, the second must be true. And if the original sentence is false, the second must be false. For the first sentence is extensional, and the second is derived from it simply by substituting co-referring terms. AJ

See also intentionality.Further reading S. Haack, Philosophy of Logic; , M. Sainsbury, Logical Forms.



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