According to legal positivism, legality is socially constructed. Traditionally, the view maintains that legal validity is based on social conventions rather than morality. Critics, most notably Ronald Dworkin, argue that it cannot account for theoretical disagreement over criteria of legal validity. In this article, I examine how theoretical disagreement bears on legal positivism by presenting a specific version of Dworkin’s Argument from Theoretical Disagreement (“ATD”) as applied to positivism, his primary target. I review leading positivist strategies for accommodating or avoiding the problem. I then challenge the ATD’s underlying assumption—shared by orthodox positivists—that conventional recognition of valid law requires agreement on interpretive criteria. Instead, I argue that practitioners conventionally recognize valid law in common in ordinary legal practice through closure procedures rather than interpretive consensus. Finally, I show how this account supports a more accurate reconstruction of legal reasoning, argumentation, and practice than Dworkinian and existing positivist alternatives.