'Severe Withdrawal (and Recovery)' (Reprinted from vol 27, pg 501, 1999, with corrections)

The problem of how to remove information from an agent's stock of beliefs i s of paramount concern in the belief change literature. An inquiring agent may remove beliefs for a variety of reasons: a belief may be called into do ubt or the agent may simply wish to entertain other possibilities. In the p rominent AGM framework for belief change, upon which the work here is based , one of the three central operations, contraction, addresses this concern (the other two deal with the incorporation of new information). Makinson ha s generalised this work by introducing the notion of a withdrawal operation . Underlying the account proffered by AGM is the idea of rational belief ch ange. A belief change operation should be guided by certain principles or i ntegrity constraints in order to characterise change by a rational agent. O ne of the most noted principles within the context of AGM is the Principle of Informational Economy. However, adoption of this principle in its purest form has been rejected by AGM leading to a more relaxed interpretation. In this paper, we argue that this weakening of the Principle of Informational Economy suggests that it is only one of a number of principles which shoul d be taken into account., Furthermore, this weakening points toward a Princ iple of Indifference. This motivates the introduction of a belief removal o peration that we call severe withdrawal. We provide rationality postulates for severe withdrawal and explore its relationship with AGM contraction. Mo reover, we furnish possible worlds and epistemic entrenchment semantics for severe withdrawals.