Defeasible Reasoning with Quantifiers

Human conditional reasoning is defeasible: people withdraw logically valid conclusions if they are aware of situations (i.e., exceptions) that prevent the consequent of the rule to happen although the antecedent is given. In this paper we investigate defeasible reasoning with quantified rules. In two experiments we rephrased conditionals from the literature (Experiment 1) and rules from penal code (Experiment 2) as either universal or existential rules and embedded them into Modus Ponens and Modus Tollens inference problems. We show that defeasible reasoning also exists for quantified rules. However, the kind of quantifier (universal vs. existential) did not affect inferences. This last finding conflicts with theories highlighting the importance of logic in human reasoning.