  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.
