A Schema-Based Approach to Understanding Subjunctive Conditionals

Abstract

We present an AI approach to subjunctive conditionals based on a possible-worlds version of situation semantics, arguing that the connection between the antecedent~ and consequent 'I' of a conditional 'ifwere the case, 'I' would be' is an informational relation between types of situations of the kind that can represented in a hierarchy. We present a sound and complete logic of conditionals based on Stalnaker's [1968] logic C2. We discuss methods of evaluating conditionals concerning object types and their properties using inheritance hierarchies, and those concerning actions and events using hierarchies of plans. In the latter case, we specify how to generate the hierarchy of situations from a collection of plan schemas. Our theory explains the traditional 'paradoxes' of conditionals (failure of strengthening the antecedent, failure of transitivity, failure of contraposition) as arising naturally from the fact that our hierarchies use defaults and exceptions.