Update Semantics for Weak Necessity Modals

This paper develops an update semantics for weak necessity modals (‘ought’, ‘should’). I start with the basic approach to the weak/strong necessity modal distinction developed in Silk 2012b: Strong necessity modals are given their familiar semantics of necessity, predicating the necessity of the prejacent of the actual world (evaluation world). The apparent “weakness” of weak necessity modals derives from their bracketing the assumption that the relevant worlds in which the prejacent is necessary (deontically, epistemically, etc.) need be candidates for actuality. ‘Should φ’ can be accepted without needing to settle that the relevant considerations (norms, preferences, expectations, etc.) that actually apply, given the facts, verify the necessity of φ. I formalize these ideas within an Update with Centering framework. The meaning of ‘Should φ’ is explained, fundamentally, in terms of how its use updates attention toward possibilities in which φ is necessary. The semantics is also extended to deontic conditionals. The proposed analyses capture various contrasting logical properties and discourse properties of ‘should’ and ‘must’—e.g., in sensitivities to standing contextual assumptions, entailingness, and force—and provide an improved treatment of largely neglected data concerning information-sensitivity.