  This picture requires that it be possible to share a subject matter without sharing truths that define it. Interpretivism about law implies the possibility of disagreement about the grounds of law, because it makes law’s constitutive explanation a matter of substance—specifically, a matter of the moral justification of the role of institutional history in the determination of rights and obligations. Notice that the possibility of disagreement about grounds is implied not only by the controversial nature of morality, but already by the substantive (therefore potentially controversial) character of the grounds. If the question of grounds is substantive, we can disagree about what they are without changing the subject. Unsurprisingly, this is the issue with which Dworkin begins Law’s Empire (1986). 