The Effect of Knowledge on Belief: Conditioning, Specificity and the Lottery Paradox in Default Reasoning

How should what one knows about an individual a ect default conclusions about that individual This paper contrasts two views of knowledge in default reasoning systems The rst is the traditional view that one knows the logical consequences of one s knowledge base It is shown how under this interpretation having to know an excep tion is too strong for default reasoning It is argued that we need to distinguish background and contingent knowledge in order to be able to handle speci city and that this is a natural distinction The second view of knowledge is what is contingently known about the world under consideration Using this view of knowledge a notion of conditioning that seems like a minimal property of a default is de ned Finally a qualitative version of the lottery paradox is given if we want to be able to say that individuals that are typical in every respect do not exist we should not expect to conclude the conjunction of our default conclusions This paper expands on work in the proceedings of the First Inter national Conference on Principles of Knowledge Representation and Reasoning