This book is a summary of the author’s 40 years of research in the fields of civil law and the philosophy of law. The main focus is on the two main tasks in the doctrinal study of law: the interpretation and systematisation of legal norms. In this regard, Professor Aarnio deals with the theory of argumentation as well as with its foundations - i.e., with the ontology, epistemology and methodology of legal thinking - and develops the ideas that were first presented in The Rational as Reasonable (Kluwer 1987) in all of these dimensions. The work includes an updated discussion on the writings of Robert Alexy, Jûrgen Habermas, Ronald Dworkin and Alf Ross. A focal point of view concerns the distinction between positivism and non-positivism, in which the core of the criticism focuses on Scandinavian realism.