The law within each legal system is a function of the practices of some social group. In short, law is a kind of socially grounded norm. H.L.A Hart famously developed this view in his book, The Concept of Law, by arguing that law derives from a social rule, the so-called “rule of recognition.” But the proposition that social facts play a foundational role in producing law is a point of consensus for all modern jurisprudents in the Anglo-American tradition: not just Hart and his followers in the positivist school, most prominently Joseph Raz and Jules Coleman, but also the anti-positivist Ronald Dworkin, who argues that law necessarily synthesizes moral considerations with social facts. But which group’s practices ground each legal system? I...