A Critical Reexamination of Default Logic, Autoepistemic Logic, and Only Knowing

Fifteen years of work on nonmonotonic logic has certainly increased our understanding of the area. However, given a problem in which nonmonotonic reasoning is called for, it is far from clear how one should go about modeling the problem using the various approaches. We explore this issue in the context on two of the best-known approaches, Reiter's default logic 1980] and Moore's autoepistemic logic 1985], as well as two related notions of \only knowing", due to Halpern and Moses 1984] and Levesque 1990]. In particular, we return to the original technical deenitions given in these papers, and examine the extent to which they capture the intuitions they were designed to capture.