Generics in defeasible reasoning. Exceptionality, Gradability, and Content Sensitivity

The problem of representing defeasible information is a long-standing topic of discussion in Knowledge Representation: for example, considering logic-based ontology representation languages, in Description Logics many proposals for defining defeasibility and typicality have been formalised, mostly emerging from existing approaches from the non-monotonic logic literature. On the other hand, little attention has been devoted to study the capability of these approaches in capturing the interpretation of typicality and exceptions from a formal ontological and cognitive point of view. To address this, we here consider the notion of generics as discussed in the linguistic and cognitive literature, i.e. a category of sentences about classes of individuals and admitting exceptions. We then use our analysis as a possible guide for understanding the important features of defeasible information in commonsense reasoning. In this paper we analyze different aspects of generics and we provide some initial desiderata for formalizing defeasible reasoning in DLs.