way: two propositional variables and indicate that Unit A and Unit B, respectively, are still in orbit; the initial set of beliefs is (either one of the two satellites is still in orbit) and the new piece of information is (Unit A has landed, and is therefore not in orbit). The only rational result of the update is ; since the initial information that one of the two satellites had not landed yet was possibly coming from the Unit A, the position of the Unit B is not known. revision the play "Six Characters in Search of an Author" will be performed in one of the two local theatres. This information can be denoted by , where and indicates that the play will be performed at the first or at the second theatre, respectively; a further information that "Jesus Christ Superstar" will be performed at the first theatre indicates that holds. In this case, the obvious conclusion is that "Six Characters in Search of an Author" will be performed at the second but not the first theatre, which is represented in logic by . This example shows that revising the belief with the new information produces two different results and depending on whether the setting is that of update or revision. Contraction, expansion, revision, consolidation, and merging In the setting in which all beliefs refer to the same situation, a distinction between various operations that can be performed is made: contraction removal of a belief; expansion addition of a belief without checking consistency; revision addition of a belief while maintaining consistency; extraction extracting a consistent set of beliefs and/or epistemic entrenchment ordering; consolidation restoring consistency of a set of beliefs; merging fusion of two or more sets of beliefs while maintaining consistency. Revision and merging differ in that the first operation is done when the new belief to incorporate is considered more reliable than the old ones; therefore, consistency is maintained by removing some of the old beliefs. Merging is a more general operation, in that the priority among the belief sets may or may not be the same. Revision can be performed by first incorporating the new fact and then restoring consistency via consolidation. This is actually a form of merging rather than revision, as the new information is not always treated as more reliable than the old knowledge. The AGM postulates The AGM postulates (named after their proponents