Abstract
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, especially characteristic of common-sense contexts. However, it is not easy for a modeller to choose a formalism that is a good fit for a particular domain from an ontological point of view. In this paper, we present a framework for formulating the characteristics of defeasibility and reasoning with exceptions which exploits and incorporates related fields of inquiry, including generics, ceteris paribus laws, and a truthmaking theory for generalisations. The resulting theory allows a grounded comparison of the various formalisms and reveals their ontological commitments. To illustrate and apply this framework, we compare the main systems of non-monotonic logics, showing the differences that may occur from an ontological perspective.