Neighborhood semantics
# Neighborhood semantics

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 $\langle W,R\rangle$ consists of a set W of worlds (or states) and an accessibility relation R intended to indicate which worlds are alternatives to (or, accessible from) others, a neighborhood frame $\langle W,N\rangle$ still has a set W of worlds, but has instead of an accessibility relation a neighborhood function

    $N:W\to 2^{2^{W}}$

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 (i.e. the set of worlds at which the proposition is true). Specifically, if M is a model on the frame, then

    $M,w\models \square \varphi \Longleftrightarrow (\varphi )^{M}\in N(w),$

where

    $(\varphi )^{M}=\{u\in W\mid M,u\models \varphi \}$

is the truth set of $\varphi$.

Neighborhood semantics is used for the classical modal logics that are strictly weaker than the normal modal logic K.

## Correspondence between relational and neighborhood models

To every relational model M = (W, R, V) there corresponds an equivalent (in the sense of having pointwise-identical modal theories) neighborhood model M' = (W, N, V) defined by

    $N(w)=\{(\varphi )^{M}\mid M,w\models \Box \varphi \}.$

The fact that the converse fails gives a precise sense to the remark that neighborhood models are a generalization of relational ones. Another (perhaps more natural) generalization of relational structures are general frames.

