Immanuel Kant (1724–1804) is widely acknowledged to be Rawls’s most important philosophical influence. There is no hope of doing justice to Kant’s philosophical arguments in this short space; we will have to be content to list the features of Kant’s practical philosophy which have been most influential for Rawls.

A crucial claim of Kant (and Rawls) is that moral and political philosophy constitutes an autonomous, specifically practical branch of inquiry, essentially independent of theoretical claims about the nature of things, both scientific and metaphysical. For Kant, ordinary moral consciousness already understands itself as bound by unconditional duties, by claims about what we ought to do, independent of empirical facts about what anyone does or the way the world is. Moral philosophy is then the clarification of the conceptual presuppositions of this commitment to unconditional duty, a clarification which aims to show that our moral beliefs are coherent. In Rawls, the analogous claim is that the citizens of constitutional democracies already possess a sense of justice which then merely needs to be clarified in reflective equilibrium to produce an ordered set of principles that can serve as the basis of political justification in a well-ordered society.