In “How Facts Make Law” and other recent work, Mark Greenberg has argued that legal positivists cannot explain how non-legal facts (such as social facts) can determine facts about legal content in a particular way, namely in a way that Greenberg calls “rationally determine”. In this paper, I argue that positivists can respond to this challenge by drawing on a) the basic account of concepts and conceptual analysis put forward by Frank Jackson, David Chalmers, and David Braddon-Mitchell in combination with b) the basic account of the concept LEGAL INSTITUTION (and its conceptual connections to the concept LEGAL NORM) that we get from Scott Shapiro’s Planning Theory of Law. In addition to providing a compelling reply to Greenberg’s argument, I argue that this response underscores one of the central problems facing legal antipositivism: namely, its lack of a convincing account of the nature of legal institutions.