In order to build automated reasoning systems including defeasible reasoning capabilities, many extensions of classical logic have been proposed as models of defeasible reasoning. These proposals, even if differing in many important technical details, share a common conceptual ground, since they all rely substantially on the same conceptual model of defeasible reasoning activity. Among the most notable and classic proposals in this field we mention default logic and nonmonotonic logic.However, this conceptual model suffers from some important limitations, which severely restrict its applicability scope and prevent it (as well as the approaches grounded on it) to correctly capture and represent some very general and common cases of practical defeasible reasoning. In order to overcome these limitations, both a more general conceptual model of defeasible reasoning activity and a formalism capturing the new concepts introduced are needed.Following the track of some previous investigations in this area, this paper points out a further limitation of most known models of defeasible reasoning which (as to our knowledge) has never been highlighted before.
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