A Logic for Revision and Subjunctive Queries

We present a logic for belief revision in which revision of a theory by a sentence is represented using a conditional connective. The conditional is not primitive, but rather defined using two unary modal operators. Our approach captures and extends the classic AGM model without relying on the Limit Assumption. Reasoning about counterfactual or hypothetical situations is also crucial for AI. Existing logics for such subjunctive queries are lacking in several respects, however, primarily in failing to make explicit the epistemic nature of such queries. We present a logical model for subjunctives based on our logic of revision that appeals explicitly to the Ramsey test. We discuss a framework for answering subjunctive queries, and show how integrity constraints on the revision process can be expressed.