CHAPTER 6 – Nonmonotonic Reasoning

Publisher Summary This chapter discusses the nonmonotonic reasoning. Language cannot capture all that one want to say about the world. A finite set of sentences can never be more than an approximate description of things as they really are. Any general rule that one might care to frame is subject to an unlimited number of exceptions and qualifications. An important technique involves the use of nonsound inferences of various kinds. That is, from a database A, one will allow certain inferences that do not logically follow from A. Often these inferences depend globally on all of the sentences in A rather than on a small subset. In particular, one will be introducing inference techniques the application of which depends on certain sentences not being in A. With such inference rules, if another sentence is added to A, an inference may have to be retracted. Ordinary logical inference rules, on the other hand, are monotonic because the set of theorems derivable from premises is not reduced by adding to the premises.