A Logic for Representing Default and Prototypical Properties

Abstract

The area of default reasoning may be considered as consisting of two separate but closely related subareas. The first deals with representing default information, and addresses issues such as reasoning about default statements or determining the consistency of a set of defaults. The second addresses reasoning about the default properties of an individual, given a set of default statements. Most extant work in default reasoning has concentrated on the second area. This paper addresses the first area only, as an initial investigation into the area of default reasoning. The approach is based on adding a operator to first-order logic. This operator is used to express prototypical, rather than strict, relations between entities and properties. A possible worlds semantics is provided for the logic along with a proof theory; soundness and completeness results are also provided. It is argued that the variable conditional in the logic captures common intuitions concerning defaults and prototypical properties.