this paper, we argue that no such set of postulates will be adequate for every application. In particular, we make a fundamental distinction between two kinds of modifications to a knowledge base. The first one, update, consists of bringing the knowledge base up to date when the world described by it changes. For example, most database updates are of this variety, e.g. "increase Joe's salary by 5%". Another example is the incorporation into the knowledge base of changes caused in the world by the actions of a robot (Ginsberg and Smith 1987, Winslett 1988, Winslett 1990) . We show that the AGM postulates must be drastically modified to describe update. The second type of modification, revision, is used when we are obtaining new information about a static world. For example, we may be trying to diagnose a faulty circuit and want to incorporate into the knowledge base the results of successive tests, where newer results may contradict old ones. We claim the AGM postulates describe only revision.

We study the problem of combining updates ---a special instance of theory change--- and counterfactual conditionals in propositional knowledgebases. Intuitively, an update means that the world described by the knowledgebase has changed. This is opposed to revisions ---another instance of theory change--- where our knowledge about a static world changes. A counterfactual implication is a statement of the form `If A were the case, then B would also be the case', where the negation of A may be derivable from our current knowledge. We present a decidable logic, called VCU 2 , that has both update and counterfactual implication as connectives in the object language. Our update operator is a generalization of operators previously proposed and studied in the literature. We show that our operator satisfies certain postulates set forth for any reasonable update. The logic VCU 2 is an extension of D. K. Lewis' logic VCU for counterfactual conditionals. The semantics of VCU 2 is that of a m...

Abstract. We offer a solution to the triviality result by Gärdenfors. His result claims that there are no non-trivial belief revision systems which satisfy the AGM postulates and are compatible with the Ramsey Test. We show that this result does no longer apply if we restrict some of the AGM postulates to deal only with the non-conditional part of epistemic states. 1

This paper is intended to motivate research in paraconsistent logic by presenting a test case for paraconsistent logicians to apply their wares. The paper introduces

Riassumiamo brevemente la nostra attività di ricerca nel campo delle logiche non-classiche iniziata negli anni ’90. In particolare, descriviamo la nostra ricerca riguardante l’applicazione delle logiche non-classiche alla rappresentazione della conoscenza e lo sviluppo di metodi di prova per logiche non-monotone e condizionali. We briefly outline our research activity in the field of nonclassical logics started in the 90s. In particular, we describe our research in the application of non-classical logics to knowledge representation and in the development of proof methods for non-monotonic and conditional logics. Keywords: Non-classical logics, knowledge representation, proof methods 1