Stephen E. Sachs (Harvard Law School) has posted According to Law (Harvard Journal of Law and Public Policy, Forthcoming) on SSRN.  Here is the abstract:

What we ought to do, according to law, is not always what we ought to do, given the existence of law. Sometimes we merely wish to know what an existing legal system says we should do, based on the prevailing rules in a particular time and place. And sometimes we need to know what we should actually do, in the moral circumstances that 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. Prevailing social rules might have no moral force on their own, but those who make claims about them owe their audiences a moral duty of candor. And the stronger our moral commitments, the more reason we might have to approach existing legal systems warily.

Insisting that the law already reflects good morals can blind us to some very real flaws in our social rules—and to the need for some very hard work in reforming them. In this respect, 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 necessary.

Highly recommended.