Closure and Epistemic Modals

Abstract

According to a popular closure principle for epistemic justification, if one is justified in believing each of the premises in set Φ and one comes to believe that ψ on the basis of competently deducing ψ from Φ—while retaining justified beliefs in the premises—then one is justified in believing that ψ . This principle is prima facie compelling; it seems to capture the sense in which competent deduction is an epistemically secure means to extend belief. However, even the single‐premise version of this closure principle is in conflict with certain seemingly good inferences involving the epistemic possibility modal ♢. According to other compelling principles concerning competent deduction and epistemic justification, one can competently infer ¬♢ φ from ¬ φ in deliberation even though there are cases in which one can justifiably believe ¬ φ but would be unjustified in believing ¬♢ φ . Thus, as we argue, philosophers must choose between unrestricted closure for justification and the validity of these other principles.