On the Issue of Contraposition of Defeasible Rules

The past ten years have shown a great variety of approaches for formal argumentation. An interesting question is to which extent these various formalisms correspond to the different application domains. That is, does the appropriate argumentation formalism depend on the particular domain of application, or does “one size fits all”. In this paper, we study this question from the perspective of one relatively simple design consideration: should or should there not be contrapostion of (or modus tollens) on defeasible rules. We aim to show that the answer depends on whether one is considering epistemical or constitutive reasoning, and that hence different domains require fundamentally different forms of defeasible reasoning.