On Decision-Theoretic Foundations for Defaults
R. I. Brafman and N. Friedman.
International Conf. on Artificial Intelligence (IJCAI95).
In recent years, considerable effort has gone into understanding
default reasoning. Most of this effort concentrated on the question of
entailment, i.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 decision-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.
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