Necessity modals, disjunctions, and collectivity

Abstract

Upward monotonic semantics for necessity modals give rise to Ross’s Puzzle: they predict that □ϕ entails □(ϕ ∨ ψ), but common intuitions about arguments of this form suggest they are invalid. It is widely assumed that the intuitive judgments involved in Ross’s Puzzle can be explained in terms of the licensing of ‘Diversity’ inferences: from □(ϕ ∨ ψ), interpreters infer that the truth of each disjunct (ϕ, ψ) is compatible with the relevant set of worlds. I introduce two pieces of data that this analysis fails to explain. Analyzing this data, I argue, suggests that necessity modals with embedded disjunctions license ‘Independence’ inferences: from the truth of □(ϕ ∨ ψ), interpreters infer that ϕ-without-ψ and ψ-without-ϕ are each compatible with the relevant set of worlds. I outline a bilateral inquisitive semantics for necessity modals that predicts the validity of the Independence inferences. I then argue that the resulting theory should be understood as one on which disjunctions denote pluralities of propositions, and necessity modals behave like collective predicates applied to these pluralities.