On the difference between updating a knowledge base and revising it
Hirofumi Katsuno, Alberto O. Mendelzon · Cambridge University Press eBooks · 1992
INTRODUCTION Consider a knowledge base represented by a theory ψ of some logic, say propositional logic. We want to incorporate into ψ a new fact, represented by a sentence μ of the same language. What should the resulting theory be? A growing body of work (Dalal 1988, Katsuno and Mendelzon 1989, Nebel 1989, Rao and Foo 1989) takes as a departure point the rationality postulates proposed by Alchourrón, Gärdenfors and Makinson (1985). These are rules that every adequate revision operator should be expected to satisfy. For example: the new fact μ must be a consequence of the revised knowledge base. In 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.