Belief Revision

Philippa, a Greek nineteen year old student at Patras University, has just discovered that Nikos and Angela are not her true parents; she was adopted when she was six months old from an orphanage in Sao Paulo. The news really shook Philippa. Much of what she used to believe all her life about herself and her family was wrong. After recovering from the initial shock she started putting her thoughts back in order: so that means that Alexandros is not really her cousin, and she did not take her brown eyes from (who she used to believe was) her grandmother, and she no longer needs to worry about developing high blood pressure because of the bad family history from both Nikos’ and Angela’s side. Moreover, she probably has siblings somewhere in Brazil, and if she really looked into it, she might be entitled to a Brazilian citizenship which could come in handy for that long trip she always wanted to make to Latin America. This is a typical (although rather dramatic) instance of a belief revision scenario: a rational agent receives new information that makes her change her beliefs. In the principal case where the new information contradicts her initial belief state, the agent needs to withdraw some of the old beliefs before she can accommodate the new information; she also needs to accept the consequences that might result from the interaction of the new information with the (remaining) old beliefs. The study of the process of belief revision, which gave rise to an exciting research area with the same name1, can be traced back to the early 1980s. The article that is widely considered to mark the birth of the field is the seminal work of Alchourron, Gardenfors, and Makinson reported in [1]. As a matter of fact, the framework that evolved from [1] – now known as the AGM paradigm (or simply AGM) after the initials of its three founders – is to this date the dominant framework in Belief Revision. Of course much has happened since 1985. The formal apparatus developed in [1] has been enhanced and thoroughly studied, new research directions have emerged from