On Decision-Theoretic Foundations for Defaults

In recent years considerable effort has gone into understanding default reasoning Most of this effort concentrated on the question of en tailment, 1 e , what conclusions are warranted by a knowledge-base of defaults Surprisingly few works formally examine the general role of defaults We argue that an examination of this role is necessary in order to understand defaults, and suggest a concrete role for defaults Defaults simplify our derision-making process allowing us to make fast, approximately optimal decisions by ignoring certain possible states In order to formalize this approach, we examine decision making in the framework of decision theory We use probability and utility to measure the impact of possible states on the decision making process We accept a default if it ignores states with small impact according to our measure We motivate our choice of measures and show that the resulting formalization of defaults satisfies desired properties of defaults, namely cumulative reasoning Finally, we compare our approach with Poole's decision-theoretic defaults and show how both can be combined to form an attractive framework for reasoning about decisions.