Lazy Knowledge Base Update

Knowledge base update has been given much attention in the AI literature. The best-known solution to this problem is Winslett's PMA formalism. In this paper we propose di erent intuitions standing behind knowledge base update. Roughly speaking, we consider an update formula not as an e ect of an action to be performed, but rather as an observation about dynamically changing world made by an agent. This observation, if it is consistent with the knowledge base, is assumed to be just a new piece of information about the world. Otherwise, it is assumed that a single action has been performed and is its, usually partial, e ect. We formalize our approach using Dijkstra's semantics. We also examine the properties of our update operator with respect to Katsuno and Mendelzon's postulates.