A Modal Logic for Nonmonotonic Reasoning

In this paper a formal framework is proposed in which various informative actions are combined, corresponding to the di erent ways in which rational agents can acquire information. In order to solve the various con icts that could possibly occur when acquiring information from di erent sources, we propose a classi cation of the information that an agent possesses according to credibility. Based on this classi cation, we formalize what it means for agents to have seen or heard something, or to believe something by default. We present a formalization of observations, communication actions, and the attempted jumps to conclusions that constitutes default reasoning. To implement these informative actions we use a general belief revision action which satis es the AGM postulates; dependent on the credibility of the incoming information this revision action acts on one or more parts of the classi ed belief sets of the agents. The abilities of agents formalize both the limited capacities of agents to acquire information, and the preference of one kind of information acquisition to another. The framework presented in this paper might be seen as an attempt to formalize the behavior of intelligent information agents which will facilitate life in the information age. Content Areas: Formalizing Rational Agents, Belief Revision, Reasoning about Action, Knowledge Representation.