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Fregean sense/Hintikka: Fregean senses are special entities in Frege, in order to be able to function as referents in intensional contexts at all.
For example, to restore the inference in the example (morning star/evening star), we do not need the
identity of morning star and evening star, but the
identity of the Fregean sense of "morning star" and "evening star".
N.B.: but Frege does not reinterpret the identity in the expression morning star = evening star itself in this way. Indeed, he cannot express this because there the identity is present in an extensional context, and later in an intensional context.
Identity/Frege/Hintikka: therefore one cannot say that Frege reinterprets our normal concept of identity.
Problem: it is not even clear whether Frege can express the identity of the senses by means of an explicit sentence. For in his own formal language (in "Begriffsschrift" and "fundamental laws") there is no sentence that could do so. This is what he himself says in "About sense and meaning": to the senses of our expressions, we can only refer to by prefixing the prefix "the meaning of". But he never uses it himself._____________Explanation of symbols: Roman numerals indicate the source, arabic numerals indicate the page number. The corresponding books are indicated on the right hand side. ((s)…): Comment by the sender of the contribution.
Jaakko and Merrill B. Hintikka
The Logic of Epistemology and the Epistemology of Logic Dordrecht 1989
J. Hintikka/M. B. Hintikka
Untersuchungen zu Wittgenstein Frankfurt 1996