The subject of my thesis is the idea of public reason which is the essential part of John Rawls's political liberalism. Idea of public reason demands that public justification of binding laws and political decisions in plural liberal democracies must be in terms of public political values of freedom, equality, fairness and that pluralism of conceotions of good should be understood as a product of free exercise of human reason under free institutions. I argue that idea of public reason is a part of conception of justice as reciprocity which means that pursue of our self - interest is constrained by terms that can be accepted by all participants in cooperation. Fair terms of cooperation are determined by substantive notion of reasonableness that includes public political values of fairness, freedom, equality and burdens judgement (reasonable pluralism). I argue that idea of public reason should be understood as constructivist political justification according to which political decisions are publicly justified if they are based on reasons all reasonable citizens can accept even though there are public reasons against this decision. Idea of public reason that I defend in this work is in opposition to alternative view of public reason according to which political decisions are justified only if citizens can justify them within their conceptions of good. I argues that understanding of public reason is wrong because it can not justify distributive politics of egalitarian liberalism and it depends on contingent features of evaluative standrds citizen actually accept. I argue that idea of public reason can justify more stronger distributive principles and that justification can be more stable despite possible changes in actual evaluative standards. Also, I argue that liberal idea of public reason demands certain ethos from liberal citizens that requires stronger political participation that strengthens bounds of mutual trust and civic friendship.