A contrario reasoning (or ‘a contrario argument’ or ‘argument a contrario’) is traditionally understood as an appeal to the deliberate silence of the legislator: because a legal rule does not mention case X specifically, the rule is not applicable to it. Modern perspectives on legal reasoning often apply this label to a broader concept of reasoning, namely the reasoning by which a legal rule is not applied because of the differences between the case at hand and the one(s) mentioned in (...) the legal rule. This article first explains how the broader concept could have come into being, and then argues that from an argumentation theoretical point of view the modern concept makes no sense as a category of argumentation. Furthermore it is shown under which conditions the traditional concept can be sound. (shrink)