Both Stephen Toulmin and Chaim Perelman described argumentation as analogous to reasoning in law and jurisprudence, in which judgment rather than demonstration is called for and the exercise is one of justification rather than proof. This essay reviews the critiques of formal reason each theorist offers and their different bases for coming to law as the model for argumentation—jurisprudential procedure in Toulmin’s case and the rule of justice in Perelman’s. It then considers how the theory and practice of argumentation would be affected if this analogy were taken as the starting point for theory construction rather than just as a general orientation for the field. This paper was presented originally at the Third Tokyo Conference on Argumentation in 2008. It is reprinted by permission from the conference volume, Proceedings of the Third Tokyo Conference on Argumentation (Tokyo: Japan Debate Association, 2008), 299–303.