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Opacity/Frege/Identity/opaque context/Hintikka: Frege had to do with the failure of the (substitutability in identity) ((s) that is, that the individuals may have a different names), not with the failure of existential generalization. ((s) That is, the individuals may not exist).
Hintikka: therefore we need different additional premises.
Semantics of possible worlds:
Substitutability in identity: here, for substitutability in identity, we need only the assumption that we can compare the referents of two different terms in every world.
Existential generalization: here we must compare the reference of one and the same term in all worlds.
Frege/Hintikka: it seems now that Frege could still be defended in a different way: namely, that we now quantify via world lines (as entities). ((s) This would meet Frege's Platonism.)
World lines/Hintikka: world lines are somehow "real"! Are they not somehow like the "Fregean senses"?
HintikkaVs: it is not about a contrast between world-bound individuals and world lines as individuals.
World lines/Hintikka: but we should not say that world lines are something that is "neither here nor there". To use world lines is not to reify.
Solution/Hintikka: we need world lines because without them it would not even make sense to ask whether a resident of a possible world is the same as that of another possible world ((s) cross-world identity).
World line/Hintikka: we use the world line instead of Frege's "way of giving".
HintikkaVsFrege: his mistake was to reify the "way of givenness" as "sense". They are not something that exists in the actual.
Quantification/Hintikka: therefore, we do not have to ask in this context "about what we quantify".
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