 AGM belief revision theory can be used as the basis for a system of defeasible reasoning or nonmonotonic logic, as Gärdenfors and Makinson have recognized (Makinson and Gärdenfors 1991). If \(K\) is an epistemic state, then a nonmonotonic consequence relation \(\dproves\) can be defined as follows: \(A \dproves B\) iff \(B \in K * A\). Unlike Pollock’s system or semantic inheritance networks, this defeasible consequence relation depends upon a background epistemic state. Thus, the belief revision approach gives rise, not to a single nonmonotonic consequence relation, but to family of relations. Each background state \(K\) gives rise to its own characteristic consequence relation.