Severe Withdrawal (and Recovery)
Hans Rott, Maurice Pagnucco

The problem of how to remove information from an agent's stock of beliefs is 
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 doubt or 
the agent may simply wish to entertain other possiblities. In the prominent 
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 has generalised this
work by introducing the notion of a withdrawal operation.
Underlying the account proffered by AGM is the idea of rational belief change. 
A belief change operation should be guided by certain principles or integrity 
constraints in order to characterise change by a rational agent. One 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 should be taken 
into account. Furthermore, this weakening points toward a Principle of 
Indifference. This motivates the introduction of a new belief removal 
operation that we call severe withdrawal. We provide rationality postulates 
for severe withdrawal and explore its relationship with AGM contraction. 
Moreover, we furnish possible worlds and epistemic entrenchment semantics for 
severe withdrawals.

Key words: AGM, belief change, belief contraction, epistemic entrenchment, 
severe withdrawal, systems of spheres.