On the standard version of political liberalism, the exercise of political power is legitimate only if it is justifiable to all reasonable persons. Correspondingly, reasonable disagreement about the moral doctrines underlying a law makes that law not justifiable to all reasonable persons. In this paper, I argue that political liberals are committed to understanding reasonable disagreement as being rational, rather than praiseworthy disagreement between morally reasonable person because other conceptions of reasonableness commit them to an incoherent triad of claims.