The A Simili Argument: An Inferentialist Setting

The A Simili Argument draws the conclusion that a target case has a normative property Q since it shares a relevant property P with a source case. It can be seen as a complex inference constituted by three inferential steps: An abduction of the relevant property P, an induction of the class having that property, and a deduction of the target's having property Q. A major problem of this argument is the characterization of the property relevance. The standard answer refers to the notion of ratio: It is the ratio that fixes what is relevant for what. But the determination of the ratio is often a difficult and controversial task. This issue is considered here from an inferentialist point of view, claiming that the ratio and relevance are determined by the normative statuses reciprocally attributed by the speakers in the context of legal argumentation.