Mathieu Beirlaen: Reasoning by Cases in Formal Argumentation

Given a premise ‘P or Q’, an argument for R on the basis of P, and an argument for R on the basis of Q, the reasoning by cases inference scheme allows us to conclude that R. If each of P and Q classically entail R, then the inference to R given ‘P or Q’ is deductively valid. In cases where the argument from P to R and the argument from Q to R hold only tentatively, the conclusion R may be derivable tentatively as well. We are interested here in this more tentative, defeasible variant of the reasoning by cases scheme, the formalization of which is well-known to cause trouble in many rule-based systems of non-monotonic logic, such as default logic [1]. Continue reading....