Abstract: What we ought to do, according to law, isn't always what we ought to do, given the existence of law. Sometimes we need to know what a legal system says we should do, under rules prevailing in a certain time and place. And sometimes we need to know what we should actually do, in the moral circumstances this legal system presents. Many fights between positivists and natural lawyers result from muddying these two inquiries. But we have good reasons, intellectual and moral, to keep them distinct. Even if prevailing social rules have no moral force of their own, those who make claims about them still owe their audiences a moral duty of candor. And the stronger our moral commitments, the more we ought to approach existing legal systems warily. Insisting that the law already reflects good morals can blind us to some very real flaws in our prevailing rules--and to the need for some very hard work in reforming them. To this extent, common-good-constitutionalist claims too often have all "the advantages of theft over honest toil": they can lead us to wish away precisely those disagreements and failings that make social and political institutions so necessary.