In the Mood for S4: The Expressive Power of the Subjunctive Modal Language in Weak Background Logics

摘要

Our concern here is with the extent to which the expressive equivalence of Wehmeier's Subjunctive Modal Language (SML) and the Actuality Modal Language (AML) is sensitive to the choice of background modal logic. In particular we will show that, when we are enriching quantified modal logics weaker than S5, AML is strictly expressively stronger than SML, this result following from general considerations regarding the relationship between operators and predicate markers. This would seem to complicate arguments given in favour of SML which rely upon its being expressively equivalent to AML.