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{{no footnotes, date=October 2016 In nonmonotonic reasoning, the Nixon diamond is a scenario in which default assumptions lead to mutually inconsistent conclusions. The scenario is: * usually, Quakers are pacifist * usually, Republicans are not pacifist * Richard Nixon is both a Quaker and a Republican Since Nixon is a Quaker, one could assume that he is a pacifist; since he is Republican, however, one could also assume he is not a pacifist. The problem is how a formal logic of nonmonotonic reasoning should deal with such cases. Two approaches can be adopted: ; skeptical : since Nixon can neither be proved to be a pacifist nor the contrary, no conclusion is drawn; ; credulous : since Nixon can be proved to be a pacifist in at least one case, he is believed to be a pacifist; however, since he can also be proved not to be a pacifist, he is also believed not to be a pacifist. The credulous approach can allow proving both something and its contrary. For this reason, the skeptical approach is often preferred. Another solution to this problem is to attach priorities to default assumptions; for example, the fact that “usually, Republicans are not pacifist” can be assumed more likely than “usually, Quakers are pacifist”, leading to the conclusion that Nixon is not pacifist. The name ''diamond'' comes from the fact that such a scenario, when expressed as a
belief network A Bayesian network (also known as a Bayes network, Bayes net, belief network, or decision network) is a probabilistic graphical model that represents a set of variables and their conditional dependencies via a directed acyclic graph (DAG). Bay ...
, forms a
diamond shape In plane Euclidean geometry, a rhombus (plural rhombi or rhombuses) is a quadrilateral whose four sides all have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal in length. T ...
. This example is mentioned for the first time by Reiter and Criscuolo in a slightly different form where the person that is both a Republican and a Quaker is a John instead of Richard Nixon.


See also

*
Default logic Default logic is a non-monotonic logic proposed by Raymond Reiter to formalize reasoning with default assumptions. Default logic can express facts like “by default, something is true”; by contrast, standard logic can only express that something ...
*
Multiple Inheritance Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object or ...


References

* W. Marek and M. Truszczynski (1993). ''Nonmonotonic Logics: Context-Dependent Reasoning''. Springer. * R. Reiter and G. Criscuolo (1981). On interacting defaults. In '' Proceedings of the Seventh International Joint Conference on Artificial Intelligence (IJCAI'81)'', pages 94–100.


External links


Review of John Horty paper on Floating Conclusions
Non-classical logic