The Epistemic Relative Necessity Account for Modal Inferences About Modus Ponens Problems

An epistemic relative necessity account is proposed to treat nonmodal deductive reasoning as modal reasoning. It assumes that an epistemically valid conclusion from the factual premises is pragmatically necessary relative to the premises. Three studies on modal Modus Ponens problems (with the form: given the premises of p and if p then q, individuals were asked to judge whether the conclusion is "necessarily q," "q," or "possibly q") revealed (1) Participants generally defaulted to interpreting arbitrary conditionals "if p then q" as "if p then must q." (2) Modal MP problems without retrievable counterexamples to conditionals tended to elicit inferences "necessarily q" rather than "q." (3) The influence of level of relevance in conditionals (arbitrary vs. causal conditionals) on modal inferences was modulated by whether causal conditionals had retrievable counterexamples: Causal conditionals with retrievable counterexamples elicited more "possibly q" inferences (belief bias responses) and fewer "necessarily q" inferences than arbitrary and causal conditionals without retrievable counterexamples. The overall response pattern favors only the epistemic relative necessity account, indicating that a mentally valid nonmodal deductive inference can be transformed into a modal inference including the modal word "necessary" in the conclusion. Our research bridges linguistic and psychological research on epistemic necessity.