In 1976, Chaim Perelman published, in French, his Logigue juridique ¹ ("Juridical logic"). This book was an application of the general theory of argumentation the author had developed some twenty years before, in Traité l" Argumentation (1958 — English translation: 1968).² But we shall see that law is not just an application of the Treatise: on the contrary, Perelman affirms that, for him, law has to replace mathematics in the privileged status they have had throughout the history of philosophy, say from Plato to Husserl. I shall try to show whether or not Perelman's jurisprudence attains the target: is law to become the major discipline if we want to understand practical reasoning and to introduce rationality in the realm of ethical choices?