In A Theory of Justice John Rawls proposes that the two principles of justice should be realized through a four-stage sequence of institutional action that starts with a constitution agreed upon by delegates to a constitutional convention. A largely overlooked aspect of this proposal is that delegates are taken to hold conflicting opinions about justice. Their disagreement is one of the factors that determine their institutional choices. This paper employs Bernard Williams’ theory of the political value of liberty to explain and vindicate the role assigned to disagreement at the constitutional convention. Constitutional norms ought to be sensitive to the fact that the functioning of a political order, even one suitably ordered by the most reasonable conception of justice, inevitably involves loss of a precious liberty; the factoring of disagreement into the constitutional convention can fruitfully be understood as a way of modelling this requirement. This exegetical exercise enriches our understanding of the point of constitutions. At the same time it suggests that Rawls may not be as guilty of the cardinal sin of moralism that Williams famously accused him of.