Cosmopolitan Constitutionalism is a specific proposal in the international legal debate, the goal of which is the application of constitutional principles at the global level to achieve the universal guarantee of human rights. The author proposes that if we want to respond to the question of whether this project is possible and desirable, we need to analyse whether this is a plausible proposal, considering the distinctive features of law in the transnational sphere. In this light, the principal aim of this work is to show the principal challenges that Cosmopolitan Constitutionalism presents for the classic debates of legal philosophy, considering the current conditions of the international sphere. In this paper, the three topics that are considered are the debates between morality and law, law and power, and law and unity. The topics are problematized from the perspective of two contemporary scholars of Cosmopolitan Constitutionalism: Luigi Ferrajoli and Jurgen Habermas.