Guest Editors’ Introduction

After preliminary foundational work of William Harper and Issac Levi, it was only 30 years ago when the formal study of belief change or, as it is alternatively called, theory change started. The seminal work was due to Carlos Alchourrón, Peter Gärdenfors and David Makinson, a trio of researchers that was soon widely referred to by the acronym “AGM”. During the 1980s, AGM introduced a qualitative model of belief change that acknowledged three doxastic attitudes, namely, belief, disbelief and nonbelief. The problem of belief change is how these attitudes should rationally change in response to new information. Two kinds of operations were regarded as central: Revision is the transformation of beliefs that happens if some new piece of information is to be incorporated into the body of a reasoner’s beliefs; especially relevant is the case in which the new information contradicts his or her beliefs. Contraction is what happens if some piece of information is to be discarded from the body of the reasoner’s beliefs. It seems fair to say that the AGM model has been very well corroborated as a model for belief change in the case in which information comes or goes in a single package, both at a certain instant in time and over a stretch of time. The 25th anniversary of the central paper of AGM [1] on partial meet contraction and revision has recently been celebrated in a special