Dworkin and John Hart Ely among the scholars active after the end of Kelsen's life. As summarized by Sandrine Baume, "In 1927 [Kelsen] recognized his debt to Kantianism on this methodological point that determined much of his pure theory of law: 'Purity of method, indispensable to legal science, did not seem to me to be guaranteed by any philosopher as sharply as by Kant with his contrast between Is and Ought. Thus for me, Kantian philosophy was from the very outset the light that guided me.'" Kelsen's high praise of Kant in the absence of any specific neo-Kantians is matched among more recent scholars by John Rawls of Harvard University. Both Kelsen and Rawls also have made strong endorsements of Kant's books on Perpetual Peace (1795) and Idea for a Universal History (1784). In his book titled What is Justice?, Kelsen indicated his position concerning social justice stating, "[S]uppose that it is possible to prove that the economic situation of a people can be improved so essentially by so-called planned economy that social security is guaranteed to everybody in an equal measure; but that such an organization is possible only if all individual freedom is abolished. The answer to the question whether planned economy is preferable to free economy depends on our decision between the values of individual freedom and social security. Hence, to the question of whether individual freedom is a higher value than social security or vice versa, only a subjective answer is possible," Five principal areas of concern for Kelsen in the area of political philosophy can be identified among his many interests for their centrality and the effect which they exerted over virtually his entire lifetime. These are; (i) Sovereignty, (ii) Law-state identity theory, (iii) State-society dualism, (iv) Centralization-decentralization, and (v) Dynamic theory of law. Sovereignty The definition and redefinition of sovereignty for Kelsen in the context of twentieth century modern law became a central theme for the political philosophy of Hans Kelsen from 1920 to the end of his life. The sovereignty of the state defines the domain of jurisdiction for the laws which govern the state and its associated society. The principles of explicitly defined sovereignty became of increasing importance to Kelsen as the domain of his concerns extended more comprehensively into international law and its manifold implications following the conclusion of WWI. The very regulation of international law in the presence of asserted