Modifying Is Better Than Deleting: A New Approach To Base Revision

We present three approaches to belief base revision, which are examined also in the case in which the sentences in the base are partitioned between those which can and those which cannot be changed; the approaches are shown to be semantically equivalent. A new approach is then presented, based on the modification of individual rules, instead of deletion. The resulting base is semantically equivalent to that generated by the other approaches, in the sense that it has the same models, but the rule part alone has less models, that is, is subjected to a smaller change.