 In the early 1980s, several systems of defeasible reasoning were proposed by others in the field of artificial intelligence: Ray Reiter’s default logic (Reiter 1980; Etherington and Reiter 1983, 104–108), McDermott and Doyle’s Non-Monotonic Logic I (McDermott and Doyle, 1982), Robert C. Moore’s Autoepistemic Logic (Moore 1985), and Hector Levesque’s formalization of the “all I know” operator (Levesque 1990). These early proposals involved the search for a kind of fixed point or cognitive equilibrium. Special rules (called default rules by Reiter) permit drawing certain conclusions so long as these conclusions are consistent with what one knows, including all that one knows on the basis of these very default rules. In some cases, no such fixed point exists, and, in others, there are multiple, mutually inconsistent fixed points. In addition, these systems were procedural or computational in nature, in contrast to the semantic characterization of warranted conclusions (in terms of preferred models) in McCarthy’s circumscription system. Later work in artificial intelligence has tended to follow McCarthy’s lead in this respect.