Indiscernibility of identicals (Q1091378)

This is an excellent solution of the traditional problems of ''intensional vs. non-intensional context'' with respect to the existential import and to the premise ''the f \(=\) the g'', where f, g are definitely descriptive terms. It is shown that neither Frege's contextualism nor Russel's denying the self-contained meaning of descriptive terms, are plausible solutions, and the sophisticated apparatus of partial type theory [\textit{P. Tichý}, Rep. Math. Logic 14, 59-72 (1982; Zbl 0488.03007)] together with Tichý's transparent intensional logic is applied to prove that no ''ad hoc'' solution is necessary. Tichý's general solution (Theorems 10.1 and 11.1) is proved by means of ''natural deduction for partial type theory'' (section 9); the first part of the paper contains an intuitive exposition of the problems and their solutions. A stimulative theory of languages is given (section 7) as well as of ''linguistic attitudes'' (section 8, where a definite solution of the puzzle of ''omniscience'' is found).