Two Concepts of Plausibility in Default Reasoning

In their unifying theory to model uncertainty, Friedman and Halpern (1995-2003) applied plausibility measures to default reasoning satisfying certain sets of axioms. They proposed a distinctive condition for plausibility measures that characterizes "qualitative" reasoning (as contrasted with probabilistic reasoning). A similar and similarly fundamental, but more general and thus stronger condition was independently suggested in the context of "basic" entrenchment-based belief revision by Rott (1996-2003). The present paper analyzes the relation between the two approaches to formalizing basic notions of plausibility as used in qualitative default reasoning. While neither approach is a special case of the other, translations can be found that elucidate their relationship. I argue that Rott's notion of plausibility allows for a more modular set-up and has a better philosophical motivation than that of Friedman and Halpern.