BELIEF AS DEFEASIBLE KNOWLEDGE

We investigate the relation between the notions of knowledge and belie f. Contrary to the well-known slogan about knowledge being ''justified , true belief'', we propose that belief be viewed as defeasible knowle dge. We offer several related definitions of belief as knowledge-relat ive-to-assumptions, and provide complete axiomatic systems for the res ulting notions of belief. We also show a close tie between our definit ions and the literature on nonmonotonic reasoning. Our definitions of belief have several advantages. First, they are short. Second, we do n ot need to add anything to the logic of knowledge: the ''right'' prope rties of belief fall out of our definitions and the properties of know ledge. Third, the connection between knowledge and belief is derived f rom one fundamental principle. Finally, a major attraction of logics o f knowledge in computer science has been the concrete grounding of the mental notion in objective phenomena; by reducing belief to knowledge we obtain this grounding for a notion of belief.