At the core of political liberalism is the claim that political institutions must be publicly justified or justifiable to be legitimate. What explains the significance of public justification? The main argument that defenders of political liberalism present is an argument from disagreement: the irreducible pluralism that is characteristic of democratic societies requires a mode of justification that lies in between a narrowly political solution based on actual acceptance and a traditional moral solution based on justification from the third-person perspective. But why should we take disagreements seriously? This – epistemic question – has not received the attention it deserves so far. I argue that the significance of public justification can be explained through the possibility of reasonable disagreement. In a reasonable disagreement, the parties hold mutually incompatible beliefs, but each is justified to hold the belief they do. I shall use the notion of a reasonable disagreement to explain the possibility of an irreducible pluralism of moral and religious doctrines and, on that basis, why the justification of political institutions has to be public.