Then, if on the basis of other IE commands and of inferences we find that all those sentences are believed true, then the TMS reports to the IE that a contradiction has arisen. For instance, in the ABC example the statement that either Abbott, or Babbitt, or Cabot is guilty together with the statements that Abbott is not guilty, Babbitt is not guilty, and Cabot is not guilty, form a contradiction. Support default reasoning In many situations we want, in the absence of firmer knowledge, to reason from default assumptions. If Tweety is a bird, until told otherwise, we will assume that Tweety flies and use as justification the fact that Tweety is a bird and the assumption that birds fly. Remember derivations computed previously In the process of determining what is responsible for a network problem, we may have derived, while examining the performance of a name server, that Ms.
Doe is an expert on e-mail systems.
That conclusion will not need to be derived again when the name server ceases to be the potential culprit for the problem and we examine instead the routers. Support dependency driven backtracking The justification of a sentence, as maintained by the TMS, provides the natural indication of what assumptions need to be changed if we want to invalidate that sentence. A TMS maintains a Dependency Network . The network is bipartite, with nodes representing sentences and justifications . A sentence node receives arcs from justification nodes.