Ronald Dworkin’s argument from theoretical disagreement remains a pressing challenge for legal positivists. In this paper, I show how positivists can answer Dworkin’s argument without having to attribute confusion or disingenuity to legal officials. I propose that the argument rests on two errors. The first is to assume that positivism requires legal officials to converge on precise grounds of law when convergence on more general grounds will do. The second is to construe judicial speech too literally. If we pay attention to the pragmatics of judicial speech, we see that judges do not disagree over what the grounds of law are; they at most disagree over how courts should proceed when agreed-upon, though imprecise, grounds of law underdetermine what the content of the law directs in the case at hand.

“A legal system exists,” Joseph Raz claims, “if and only if it is in force.” By this he means to suggest that the efficacy of law—that is, its capacity to control the population to which it applies—is necessary for its identity as such. Despite widespread recognition that efficacy is a condition of the existence of law, however, little time has been spent analyzing the notion. This article begins an attempt to make up the deficit. I make the case for efficacy as necessary for law and go on to develop and defend an account of the concept that is broadly Kelsenian in spirit. In doing so I address questions concerning the relationship between obedience and enforcement in an account of the existence of a legal system as well as relating the discussion to that concerning the ontological status of international law.