AbstractChaim Perelman's critique of Kelsen's pure theory of law concerns mainly two elements: his theory of interpretation and his conception of court decision. Although a court decision is never formally deduced from the law, one cannot say neither that it is "arbitrary": it leans on reasons, which cannot be reduced to the judge's individual motives and whose value can thus be assessed by the "science of law." As for interpretation, it is not limited to establishing the meaning of the law's terms, but contributes also to specifying the meaning of the law itself in relation to cultural values—even if these values are not explicitly mentioned by the law. In confining the judge's role to the strict application of the law, the Kelsenian conception of judicial power defies the Kelsenian epistemological imperative, which is to describe "how the law is, not how it ought to be."