of the International Conference on Knowledge Representation and Reasoning , Cambridge , MA

The AGM rationality postulates for belief revision play an important role in the formal-ization of subjunctive queries over knowledge bases. However, the well known triviality results by GG ardenfors in connection with the Ramsey Test make deening a query semantics corresponding to these rationality postulates a diicult task. While existing approaches avoid the triviality results by restricting the postulates in certain ways, we show that an exact translation into a query semantics is possible. The resulting semantics allows for the rst time the integration of subjunctive queries evaluated according to a revision semantics corresponding to the AGM rationality postulates with an update operator corresponding to the update semantics formalized by Katsuno, Mendelzon and Grahne. Additionally, the query semantics extends usual conditional implication by deening the concepts of possibility and necessity relative to a knowledge base.