The inadequacy of the neighbourhood semantics for modal logic

Abstract

We present two finitely axiomatized modal propositional logics, one between T and S4 and the other an extension of S4, which are incomplete with respect to the neighbourhood or Scott-Montague semantics.Throughout this paper we are referring to logics which contain all the classical connectives and only one modal connective □ (unary), no propositional constants, all classical tautologies, and which are closed under the rules of modus ponens (MP), substitution, and the rule RE (from A ↔ B infer αA ↔ □B). Such logics are called classical by Segerberg [6]. Classical logics which contain the formula □p ∧ □q → □(p ∧ q) (denoted by K) and its “converse,” □{p ∧ q)→ □p ∧ □q (denoted by R) are called regular; regular logics which are closed under the rule of necessitation, RN (from A infer □A), are called normal. The logics that we are particularly concerned with are all normal, although some of our results will be true for all regular or all classical logics. It is well known that K and R and closure under RN imply closure under RE and also that normal logics are also those logics closed under RN and containing □{p → q) → {□p → □q).