Formalizing human uncertain reasoning with default rules: A Psychological conundrum and a pragmatic suggestion

Abstract

The suppression of Modus Ponens by the introduction of a second conditional is introduced as a result relevant both to psychologists and to AI researchers interested in default reasoning. Some psychological considerations on the explanation of this effect, together with (a) their tentative formalisation within the framework of default logic, and (b) recent experimental results from the present authors, lead to the conclusion that our understanding of ordinary human default reasoning would benefit from considering the existence of a specific class of conditional statements, with the pragmatic status of "preconditionals".