dlm@allegra.att.COM (05/12/89)
Date: Monday, 5/22/89 Time: 10:00 am Place: AT&T Bell Laboratories - Murray Hill 3D436 Title: Reasoning with Defaults Speaker: Hector Geffner Cognitive Systems Laboratory - Department of Computer Science, UCLA Defaults play a central role in commonsense reasoning, permitting the generation of useful predictions in the absence of complete information. These predictions are nonmonotonic, in the sense that they often need to be revised in light of new information. A number of extensions to classical logics have been proposed which successfully accommodate this non-monotonic behavior. Recent work in defeasible inheritance, however, has shown that there are additional issues, beyond non-monotonicity, which also need to be addressed in order to capture the defaults intended meaning. I will present two alternative formalizations which address these issues. In the first part I will discuss a qualitative inference system that results from interpreting defaults as high conditional probability statements. The system is characterized by a core of five rules of inference which permit derivations to be constructed in the style of natural deduction systems and which capture the context-sensitivity of defaults. A sixth rule is then introduced which extends the core with assumptions about conditional independence. In the second part of the talk, I will present a model theoretic account which provides an alternative validation of both the core rules and the conditional independence assumptions. This account appeals to a preference relation among models hinted by the probabilistic interpretation. We will then present some examples and discuss ideas about implementation. Sponsor: David Etherington - ether@research.att.com