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Neighborhood semantics, also known as Scott–Montague semantics, is a formal semantics for modal logics. It is a generalization, developed independently by Dana Scott and Richard Montague, of the more widely known relational semantics for modal logic. Whereas a relational frame ⟨ W , R ⟩ {\displaystyle \langle W,R\rangle } consists of a set W of worlds and an accessibility relation R intended to indicate which worlds are alternatives to others, a neighborhood frame ⟨ W , N ⟩ {\displaystyle \langle W,N\rangle } still has a set W of worlds, but has instead of an accessibility relation a neighborhood function
that assigns to each element of W a set of subsets of W. Intuitively, each family of subsets assigned to a world are the propositions necessary at that world, where 'proposition' is defined as a subset of W. Specifically, if M is a model on the frame, then
where
is the truth set of A.