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Individual/well-defined/Hintikka: an individual is well-defined if it can be picked out by a name at a node of the world line.
World line: a world line can link non-existent embodiments of individuals as long as they are well-defined, for all worlds in which a node of the world line is localizable.
Truth conditions are then simple: (Ex) p (x) is true in world w iff. there is an individual, e.g. with the name z such that p (z) is true in w.
Individual/possible worlds/existence/Hintikka: how can an individual exist in several worlds? (By being in different worlds in different relations to its environment))?
World line/Hintikka: we must distinguish two ways, how a world line cannot be drawn.
Case 1: our criteria of cross-world identification work with individual i and still fail in world w, which leads us to say that i does not exist in w.
Case 2: more radical: the criteria fail even in the sense that they cannot tell us what i is at all, then we cannot decide whether i exists in w or not. (well-defined).
Well-defined/Existence/Hintikka: N.B.: we can now say: thesis: that well-defined objects are in a certain sense in the actual world. This is the best rational reconstruction.
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