Law in general, including constitutional rights and legal argumentation, has a dual nature. This is the underlying thesis of this collection of twenty-one chapters devoted to legal philosophy and constitutional law. Law connects a real dimension, defined by authoritative issuance and social efficacy, with an ideal dimension, defined by the claim to correctness, which essentially includes a claim to justice. The chapters of the first part of the book establish on this basis a non-positivistic concept of law. In the second part, the concept of constitutional rights is connected with proportionality analysis, explicated by principles theory and understood as a necessary condition of the rationality of the application of constitutional rights. In the third part, it is shown that rationality is possible in law because rational legal argumentation is possible. Here the basis is discourse theory. The final result is a system that brings the formal idea of legal certainty together with the substantive idea of justice.