Modelling Defeasibility in Law: Logic or Procedure?

This paper investigates whether current nonmonotonic logics are suitable for formalising the defeasibility of legal reasoning. It does so by studying the role of burden of proof in legal argument, in particular how allocations of burden of proof determine the required strength of counterarguments. It is argued that the two currently available modelling approaches both have some shortcomings. On the one hand, techniques for modelling burden of proof in nonmonotonic logics do not allow for shifts of the burden of proof from one party to the other. On the other hand, current procedural models of legal argument are too rigid, in that every counterargument induces a shift of proof burdens; this fails to respect that in legal reasoning burden shifts only occur in some cases. It is then shown how current dialectical models of defeasible reasoning can be adapted to overcome these shortcomings.