This book deals with the doctrinal study of law, although the focus is on the legal reasoning in general. The topics have been chosen for a special reason. I first began to think about the value of philosophy for practical lawyers more than fifty years ago. The past five decades have shown that my curiosity has not been wasted. I am now more convinced than ever before that the old phrase “bonus theoreticus, malus practicus” does not hit the nail on the head. For this reason, the goal of my contribution is to increase the understanding of the value of philosophy for lawyers, especially for everyday research. In this, I join the Scandinavian tradition, in which the interest in jurisprudence is most often intertwined with doctrinal studies of material law, such as that of civil, penal or procedural law. Good examples are Karl Olivecrona (procedural law), Alf Ross (mainly penal law), Torstein Eckhoff (public law), Per-Olof Ekel?f (procedural law) and Aleksander Peczenik (civil law). There are, of course, many philosophers who have approached law and legal reasoning, among other things. An excellent example was my close Austrian friend Ota Weinberger. In Finland, the list is quite long and representative: Georg Henrik von Wright, Jaakko Hintikka, Ilkka Niiniluoto, Eerik Lagerspetz, Raimo Tuomela, Risto Hilpinen, and many others.