THE DYNAMICS OF DEFAULT REASONING

In this paper we study default reasoning from a dynamic, agent-oriente d, semantics-based point of view. In a formal framework used to specif y and to reason about rational agents, we introduce actions that model the (attempted) jumping to conclusions that is a fundamental part of reasoning by default. Application of such an action consists of three parts. First it is checked whether the formula that the agent tries to jump to is a default, thereafter it is checked whether the default fo rmula can consistently be incorporated by the agent, and if this is th e case the formula is included in the agent's beliefs. As for all acti ons in our framework, we define the ability and opportunity of agents to apply these actions, and the states of affairs following applicatio n. To formalise formulae being defaults, we introduce the modality of common possibility. This modality is related to, but not reducible to, the notions of common knowledge and 'everybody knows'-knowledge. To m odel the qualitative difference that exists between hard, factual know ledge and beliefs derived by default, we employ different modalities t o represent these concepts, thus combining knowledge, beliefs, and def aults in one framework. Based on the concepts used to model the defaul t reasoning of agents, we look into the dynamics of the supernormal fr agment of default logic. We show in particular that by sequences of ju mps to conclusions agents can end up with extensions in the sense of d efault logic of their belief.