Know Your Exceptions: Towards an Ontology of Exceptions in Knowledge Representation

Defeasible reasoning is a kind of reasoning where some generalisations
may not be valid in all circumstances, that is general conclusions may fail in some
cases. Various formalisms have been developed to model this kind of reasoning,
which is characteristic of common-sense contexts. However, it is not easy for a
modeller to choose among these systems the one that better fits its domain from an
ontological point of view.
In this paper we first propose a framework based on the notions of exceptionality
and defeasibility in order to be able to compare formalisms and reveal their ontological commitments. Then, we apply this framework to compare four systems,
showing the differences that may occur from an ontological perspective.