Legal doctrine has two aims: describing and systematising the law. The description of currently valid law within a given legal system is the most visible and, from a quantitative point of view, the most important task for legal doctrine. From a qualitative point of view, however, systematisation of the law is by far its most important task. Systematisation is the construction of a conceptual framework of the law, which is a necessary basis for any legal rule and for any legal reasoning. Systematisation presupposes a description of the rules, principles, concepts, etc, which are to be systematised. But description of the rules is impossible without the conceptual framework that the systematisation of the law offers. Even the law itself is based on this conceptual framework. Codifications, such as the Code Napoléon or the German Bürgerliches Gesetzbuch, were made possible because of the centuries-long preparatory work of legal doctrine.