Semantical Considerations on Nonmonotonic Logic

Abstract

Commonsense reasoning is "nonmonotonic" in the sense that we often draw conclusions on the basis of partial information that we retract when we are given more complete informatlon. Some of the most interesting products of the recent attempts to formalize nonmonotonic reasoning are the nonmonotonic logics of McDermott and Doyle [McDermott and Doyle, 1980] [McDermott, 1982]. These logics, however, all have peculiarities that suggest they do not quite succeed in capturing the intultions that prompted their development. In this paper we give a reconstruction of nonmonotonic logic as a model of an ideally rational agent's reasoning about his own beliefs. For the resultlng system, called autoepistemic log!~, we define an intuitively based semantics for which we can show autoepistemic logic to be both sound and complete. We then compare the McDermott and Doyle logics to autoeplstemlc logic, showing how it avoids their peculiarities.