The perspective that is central to this book is that of political philosophy. Thus, the investigations presented here are not directly focused on the formal validity of law, researched by legal dogmatics, or simply on social acceptance of legal rules, researched by social sciences. Rather, we examine the area situated in between legal validity or social acceptance of law and its change: the area of reasons supplying its philosophical and political justification. In such an approach, legal change can be understood broadly, and the law that is of interest here can be of different fields (e.g. constitutional law, international law, or private law). The adopted concept of legal change embraces, therefore, formal changes made by amendments, passing new laws or issuing formal decisions and court judgments, as well as informal changes, such as those emerging from changing interpretation of the existing rules by the bodies applying the law. Theoretical distinctions, however, such as those between formal and informal change, or different fields of law, do not have a significant role to play; the research conducted in this book emerges from a belief that in all practical processes of change the justifying reasons are a point of reference for the debate, even if appearing only implicitly. It is assumed, and systematically demonstrated in this volume, that this normative background – examined by political philosophy – is inevitable for deep understanding of legal change in a democratic society.