The article addresses three distinct, although related, questions: 1. A theoretical question: is the law a system of norms? 2. A question about the method of legal science: at what conditions is it convenient to speak of the law as being a system of norms? 3. A question of sociology of law and history of legal thought: what cultural and institutional conditions made it possible to speak of the law as a normative system? In order to explain in what sense the law is a system of norms, it is necessary to identify the cultural and institutional preconditions and the methodological and epistemological consequences of the concept of law as normative system. The article holds that the methodological consequences are of the utmost importance: to conceive of the law as a system of norms allows us to perform certain practical operations and theoretical observations which are essential to our legal experience. If the law is conceived as a normative system, a legal method can emerge, and a specifically legal form of reasoning can develop.