In the course of the last few decades Aulis Aarnio has made a major contribution to the process of laying of new foundations for the theory of practical legal argumentation and modern jurisprudence (legal theory, philosophy of law). He was influenced by the modern philosophy of language and by the late works of Ludwig Wittgenstein as well as being inspired by the analytical-hermeneutic philosophy of his teacher Georg Henrik von Wright.With his original and widely acknowledged research in law and legal systems and the rationality of the resulting scholarly interpretation which have been crowned and honoured with the Alexander von Humboldt Research Award in the field of the Humanities he has changed the face of modern legal science, above all, that of juridical or legal method, the methodology of law and the social sciences and of legal and social theory, the latter in his capacity as editor-in-chief of "Associations", a new periodical from the same publisher.The Festschrift was presented to him on his 60th birthday in the name of his friends and colleagues from all over the world in a specially hand-crafted copy. This book is a kind of handbook to his discipline and contains very profound and sophisticated responses to his magnum opus, "The Rational as Reasonable. A Treatise on Legal Justification", which has determined and significantly advanced contemporary research in the field of legal and scholarly interpretation and argumentation. It deals with nearly all the central issues in the application of law and its justification.