Nonmonotonic reasoning deals with the analysis and formalisation of rational conclusions. Such conclusions are characterised by the feature that they are defeasible, i.e., that they can be invalidated by new information. This is in contrast to classical logic where the conclusion of a set of premisses remains derivable even if the premisses are arbitrarily enlarged (and reasoning may reduce to triviality because of the well-known classical principle of ex falso sequitur quodlibet, which allows the derivation of any conclusion from inconsistent premisses). Rational conclusions, however, try to retain consistency.
Nonomonotonic logics have been proposed at the beginning of the Eighties of the 20. century and are an important formal underpinning for knowledge-based systems. Furthermore, there is a close relationship to certain semantics of logic programming with negation-as-failure.
