What to Believe When Inferences are Contradicted: The Impact of Knowledge Type and Inference Rule

Simple belief-revision tasks were defined by a giving subjects a conditional premise, (p—>q), a categorical premise, (p, for a modus-ponens belief-set, or ~q, for a modus tollens belief-set), and the associated inference (q or ~p, respectively). "New" information contradicted the initial inference (~ q or p, respectively). Subjects indicated their degree of belief in the conditional premise and the categorical premise, given the contradiction. Results indicated that the choice was a function of the knowledge type expressed in the conditional form; when that knowledge type was causal, the choice was affected by the number of disabling factors associated with the causal relationship. A "possible worlds" interpretation of the data is related to formal notions such as epistemic entrenchment, used in normative models of belief revision, and to reasoning from uncertain premises, from the human deduction literature.