In Chapter 8 on frames, the kind of reasoning we saw exemplified by the inheritance of properties was actually a simple form of default reasoning, where a slot was assumed to have a certain value unless a different one was provided explicitly. In Chapter 10 on inheritance, we also considered a form of default reasoning in hierarchies. We might know, for example, that elephants are gray, but understand that there could be special kinds of elephants that are not. In this chapter, we look at this form of default reasoning in detail and in logical terms, without tying our analysis either to procedural considerations or to the topology of a network as we did earlier.


Despite the fact that FOL is an extremely ...

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