The intuition animating this seemingly new concern is actually no different from the guiding idea of A Theory of Justice, namely that the fundamental charter of a society must rely only on principles, arguments and reasons that cannot be reasonably rejected by the citizens whose lives will be limited by its social, legal, and political circumscriptions. In other words, the legitimacy of a law is contingent upon its justification being impossible to reasonably reject. This old insight took on a new shape, however, when Rawls realized that its application must extend to the deep justification of Justice as Fairness itself, which he had presented in terms of a reasonably rejectable (Kantian) conception of human flourishing as the free development of autonomous moral agency.
The core of Political Liberalism, accordingly, is its insistence that, in order to retain its legitimacy, the liberal state must commit itself to the "ideal of public reason." This roughly means that citizens in their public capacity must engage one another only in terms of reasons whose status as reasons is shared between them.
