Justification and Defeat

This paper exhibits some problematic cases of defeasible or nonmonotonic reasoning that tend to be handled incorrectly by all of the theories of defeasible and nonmonotonic reasoning in the current literature. The paper focuses particularly on default logic, circumscription, and the author’s own argument-based approach to defeasible reasoning. A proposal is made for how to deal with these problematic cases. The paper closes with a demonstration that the proposed solution is able to differentiate, in a congenial way, between cases having the structure of the lottery paradox and cases having the structure of the paradox of the preface. The algorithm proposed for computing justificational status has been implemented in the automated defeasible reasoner OSCAR.