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Full Idea
The supervaluationist denies bivalence but accepts excluded middle. The statement 'A or not-A' is true on each admissible interpretation, and therefore true, even if 'A' (and hence 'not-A') are true and some and false on others, so neither T nor F.
Gist of Idea
Supervaluation has excluded middle but not bivalence; 'A or not-A' is true, even when A is undecided
Source
Timothy Williamson (Vagueness [1994], 5.2)
Book Ref
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.145
A Reaction
See Ideas 21605 and 21606 for the distinction being used here. Denying bivalence allows 'A' to be neither true nor false. It seems common sense that 'he is either bald or not-bald' is true, without being sure about the disjuncts.
Related Ideas
Idea 21605 Excluded Middle is 'A or not A' in the object language [Williamson]
Idea 21606 'Bivalence' is the meta-linguistic principle that 'A' in the object language is true or false [Williamson]
Idea 21604 Supervaluation assigns truth when all the facts are respected [Williamson]
Idea 21676 Epicureans say disjunctions can be true whiile the disjuncts are not true [Epicurus, by Cicero]