Reasoning by Cases in Structured Argumentation

We extend the ASPIC^+ framework for structured argumentation so as to allow applications of the reasoning by cases inference scheme for defeasible arguments. Given an argument with conclusion `A or B', an argument based on A with conclusion C, and an argument based on B with conclusion C, we allow the construction of an argument with conclusion C. We show how our framework leads to different results than other approaches in non-monotonic logic for dealing with disjunctive information, such as disjunctive default theory or approaches based on the OR-rule (which allows to derive a defeasible rule `If (A or B) then C', given two defeasible rules `If A then C' and `If B then C'). We raise new questions regarding the subtleties of reasoning defeasibly with disjunctive information, and show that its formalization is more intricate than one would presume.