This paper is concerned with the problem of providing a semantic account for inheritance networks capable of representing both strict and defeasible information. The importance of representing defeasible information in a knowledge base—particularly, in a frame- or network-based inheritance reasoner — has been widely recognized ever since the publication of Minsky’s original paper on frames . Although early systems designed to allow defeasible inheritance reasoning, such as FRL  and NETL , were subject to semantic difficulties in their treatment of cancellation, these problems by now are essentially solved. In fact, there exist today a number of well-defined and intuitively attractive theories of defeasible inheritance, including those of Touretzky , Sandewall , and Horty et al. . The variety of these theories does not seem to indicate any kind of instability or chaos in our understanding, but instead, the presence of a range of options in the design space for defeasible inheritance reasoners; some of these options are surveyed in Touretzky et al. .
KeywordsSkeptical Theory Mixed Network Positive Path Negative Path Mixed Degree
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