Abstract: This thesis attempts to uncover how an American debate about legal unity is at the origins of the international investment regime. Although it is impossible to claim a univocal continuum from more than a century of professional experience in international law, this thesis attempts to show that there are continuities with today’s current debate on the constitutionalization of international law and, particularly, of the regime of international investment. Taking systems theory as its point of departure, this research adopts a concept of constitution that is the meaningful articulation of a prohibition of denial of justice. The procedural line that is activated by the articulation of the prohibition of the denial of justice is marked by a series of decisions that were empowered by legal norms, all of them loosely coupled to one another and to other social systems, making it possible to understand them in their historical context. The historical analysis begins, thus, with the very first moment where the concept of the prohibition of denial of justice emerged, and it explores the link between this concept and international law. In developing the development of federalism, the American Constitution created incentives for the Supreme Court to solve conflicts by establishing new empowering norms. Later on, this experience proved to be fundamental for the articulation, now on the international scene, of a concept of “denial of justice.” Finally, in light of this specific interpretation of constitutional norms within and beyond the states, the thesis claims that it is the principle, not a norm, of denial of justice that is at the heart of the current regime of international investment as a specific program designed by states to guarantee, in the transnational space, the structural coupling of law and economics— that is, property. By stressing that the concept of constitutionalism in the international scene can only be manifested through loose couplings, the very limits of this specific regime comes to light. International investment law is not necessarily a novelty within legal theory, which can account for its unity even in a pluralist setting, but this unity, as only loosely coupled with politics, is less open to inclusionary practices.