Th is book analyses international legal positivists’ desire to emulate the success of the empirical methods applied in the biological and physical sciences; their wish to work with law with the certainty that natural facts started to provide as the natural sciences method developed. On commencement of this project, the results to be obtained from it were not immediately clear, but these evolved slowly through the examination of many texts and historical narratives. I traced positivist lawyers’ claim about the impossibility of knowing an objective reality and of recognizing what is objectively good and just beyond the competition between individual interests. Positivists perceived their lack of the theoretical tools to predict and even understand the behaviour of individuals or states, and shifted their focus towards empiricism. Th e aim was then to provide a valid account of the world, as natural sciences were already doing, for instance, in studies of the dynamics of atoms. In tandem with its analytical undertaking, this book may also be regarded as a history concerned with the politics of science. It is within this context that I grasped the importance of the fact that legal positivists started to argue in favour of new sources of wisdom: economics and statistics. Th e phenomenon in which legal positivism adopts a philosophy of interests that aims to explain the world as it is and which helps them to produce a formalist approach to law is termed ‘economic-positivism’ in this book. By studying the work of Lassa Oppenheim and, in particular, Hans Kelsen, I have learned to understand the value of their ambitious scientifi c and political projects. It is likely that this book, whatever its merits, would not have been produced had I not discovered the rich sources of research off ered by the work of these two authors.