This article is a contribution to a critical exploration of the liberal project of normatively justifying basic political principles. The specific focus is John Rawls's use of the idea of public reason. After briefly discussing the evolution of Rawls's ideas from A Theory of Justice to his most recent writings, the key components of his conception of public reason are set out. Two principal lines of criticism are developed. The first is that the criteria of legitimacy Rawls establishes for a democratic procedure are unworkably demanding. The second is that there is no reason to think that resort to the idea of public reason will significantly constrain the scope of substantive political disagreement within a constitutional democracy. The article concludes with a few speculative reflections about the relevance of the limitations of Rawls's account of public reason for the project of liberal justification more generally.