In "A Theory of Justice" Rawls put at the center of his work the idea of justice. "Political Liberalism" states now that the core idea of political philosophy is the fact of reasonable pluralism. What is the reason for this shift? And what happens now to the idea of justice? It is commonly believed that at the basis of Rawls' changing his mind is the communitarian critique, which put under pressure Rawls' search for a normative conception of a justice all can agree on. This paper is meant to show that there is no antinormativist turn in Rawls' thought. Emphasis is put on the idea of obligation, by showing how the old question of legitimation is what Rawls is focusing on. With an unforeseen result: There is no way to justify the second principle of justice through the new constructivistic account of justification.