 Alchourrón, Gärdenfors, and Makinson (1982) developed a formal theory of belief revision and contraction, drawing largely on Willard van Orman Quine’s model of the web of belief (Quine and Ullian 1970). The cognitive agent is modelled as believing a set of propositions that are ordered by their degree of entrenchment. This model provides the basis for a set of normative constraints on belief contraction (subtracting a belief) and belief revision (adding a new belief that is inconsistent with the original set). When a belief is added that is logically consistent with the original belief set, the agent is supposed to believe the logical closure of the original set plus the new belief. When a belief is added that is inconsistent with the original set, the agent retreats to the most entrenched of the maximal subsets of the set that are consistent with the new belief, adding the new proposition to that set and closing under logical consequence. For the axioms of the AGM model, see the following supplementary document: