Proceedings of the Annual Meeting of the Cognitive Science Society

Default reasoning is a mode of commonsense reas(Miing which lets us jump to plausible conclusions when there is no contrary information. A crucial operation of default reasoning systems is the checking and maintaining of consistency. However, it has been argued that default reasoning is inconsistent: Any rational agent will believe that it has some false beliefs. By doing so. the agent guarantees itself an inconsistent belief set (Israel, 1980). Perlis (1986) develops Israel's argument into an argument for the inconsist^cy of recoUective Socratic default reasoning systems. The Zoo Keeper's Paradox has been offered as a concrete example to demonstrate the inconsistency of commonsense beliefs. In this paper, w e show that Israel and Perils' arguments are not well founded. A rational agent only needs to believe that some of its beliefs are possibly or probably false. This requirement does not imply that the beliefs of rational agents are necessarily inconsistent Decision theory is used to show that concrete examples of seemingly inconsistent beliefs, such as the Zoo Keeper's Paradox, can be rational as well as consistent. These examples show that analyses of commonsense beliefs can be very misleading when utility is ignored. W e also examine the justifications of the exploratory and incredulous approaches in default reasoning, decision theoretic considerations favor the exploratory approach.