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On the difference between updating a knowledge base and revising it. (English) Zbl 0765.68197
Principles of knowledge representation and reasoning, Proc. 2nd Int. Conf., Cambridge/MA (USA) 1991, 387-394 (1991).
Summary: [For the entire collection see Zbl 0747.00023.]
We explore a fundamental distinction between two kinds of modifications to a knowledge base represented by a propositional theory. The first one, update, consists of bringing the knowledge base up to date when the world described by it changes. The second type of modification, revision, is used when we are obtaining new information about a static world. C. E. Alchourrón, P. Gaerdenfors and D. Makinson [J. Symb. Logic 50, 510-530 (1985; Zbl 0578.03011)] have proposed and studied a set of rationality postulates that they argue must be satisfied by every operator for theory change. We show these postulates apply only to revision, not to update. We give a new set of postulates that apply to update operators, and characterize all operators that satisfy the postulates in terms of a set of partial orders defined among possible worlds.
A dual operation of revision is contraction, the act of giving up a belief. We propose a new operation called erasure. Erasure is to contraction as update is to revision. We show that Winslett’s Forget operator is a special case of a general operation called symmetric erasure, defined in terms of erasure.

MSC:
68T35 Theory of languages and software systems (knowledge-based systems, expert systems, etc.) for artificial intelligence
Keywords:
update; revision; erasure
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