A Unified View of Propositional Knowledge Base Updates

The semantics of revising knowledge bases represented by sets of proposi t ional sentences is analyzed f r om a model-theoretic point of view. A character izat ion of a l l revision schemes that satisfy the Gardenfors ra t iona l i ty postulates is given in terms of an order ing among interpretat ions. Propert ies of the contract ion operator tha t can be defined in terms of revision are also studied. T w o new update operators, elimination and recovery, are introduced. E l im inat ion discards a l l previous preconceptions on a set of proposi t ional letters; recovery undoes the effect of the last update. I t is shown tha t el iminat ion cannot be expressed as a contract ion, and tha t recovery is in general impossible. The existence of an invar iant par t of the knowledge base compris ing a set of in tegr i ty constraints is considered and the def in i t ion of revision and contract ion are modif ied to take integr i ty constraints in to account.