Why do lawyers and judges agree on what the law directs in particular cases as often as they do? Why do they sometimes disagree? And what can this agreement and disagreement teach us about the nature of law—about the sort of thing that law is, wherever and whenever it is found? This dissertation develops a version of Hartian Positivism and shows how it straightforwardly explains legal practitioners’ agreement, as well as their disagreement, with respect to the law. Indeed, Hartian Positivism better accounts for these facts than rival theories, such as Ronald Dworkin’s theory of Law as Integrity, Scott Shapiro’s Planning Theory, and Mark Greenberg’s Moral Impact Theory.