In this paper, I deploy an argument that I have developed in a number of recent papers to show that the most influential version of legal positivism - that associated with H.L.A. Hart - fails. The argument's engine is a requirement that a constitutive account of legal facts must meet. According to this rational-relation requirement, it is not enough for a constitutive account of legal facts to specify non-legal facts that modally determine the legal facts. The constitutive determinants of legal facts must provide reasons for the obtaining of the legal facts (in a sense of "reason" that I develop). I show that the Hartian account is unable to meet this requirement. That officials accept a rule of recognition does not by itself constitute a r...