Atria, F. Faculty of Law, Universidad de Talca, P.O. Box 747, Talca, Chile.This article deals with the relation between a theory of law and a theory of legal reasoning. Starting from a close reading of Chapter VII of H. L. A. Hart's The Concept of Law, it claims that a theory of law like Hart's requires a particular theory of legal reasoning, or at least a theory of legal reasoning with some particular characteristics. It then goes on to say that any theory of legal reasoning that satisfies those requirements is highly implausible, and tries to show that this is the reason why not only Hart, but also writers like Neil MacCormick and Joseph Raz have failed to offer a theory of legal reasoning that is compatible with legal positivism as a theory of law. They have faced a choice between an explanation of legal reasoning that is incompatible with the core of legal positivism or else strangely sceptical, insofar as it severs the link between general rules and particular decisions that purport to apply them.