A Unified View of Propositional Knowledge Base Updates

The semantics of revising knowledge bases represented by sets of propositional sentences is analyzed from a model-theoretic point of view. A characterization of all revision schemes that satisfy the Gardenfors rationality postulates is given in terms of an ordering among interpretations. Properties of the contraction operator that can be defined in terms of revision are also studied. Two new update operators, elimination and recovery, are introduced. Elimination discards all previous preconceptions on a set of propositional letters; recovery undoes the effect of the last update. It is shown that elimination cannot be expressed as a contraction, and that recovery is in general impossible. The existence of an invariant part of the knowledge base comprising a set of integrity constraints is considered and the definition of revision and contraction are modified to take integrity constraints into account.