Logic for Bel ief Revis ion

Using G£rdenfors's notion of epistemic entrenchment, we develop the semantics of a logic which accounts for the following points. It explains why we may generally infer If -~A then B if all we know is AVB while must not generally infer If -~A then B if a~ we know is {AVB,A}. More generally, it explains the nonmonotonic nature of the consequence relation governing languages which contain conditionals, and it explains how we can deduce conditionals from premise sets without conditionals. Depending on the language at hand, our logic provides different ways of keeping the Kamsey test and getting round the G£rdenfors triviality theorem. We indicate that consistent additions of new items of belief are not to be performed by transitions to logical expansions. 1 I n t r o d u c t i o n 1.1 A n e x a m p l e Imagine tha t you are walking along a long and lonely beach. It is a beautiful night. Still you feel somewhat uncomfortable. You are hungry. But you know that at the end of the beach there are two restaurants, one of t h e m run *University of Stuttgart, Institute for Computational Linguistics, Keplerstrat3e 17, D-7000 Stuttgart 1, West Germany