Legal positivists, such as H.L.A. Hart, assume that the existence of legal norms is a matter of social facts. Legal facts, in other words, have an exclusively social ontology. However, classical positivist accounts assume that the relevant social fact is a practice that is defined by only individual attitudes towards the law. They are therefore unable to explain how such a practice can give agents a reason to orient their behaviour towards the law. In order to solve this problem, recent positivist accounts have turned to theories of collective intentionality to explain the nature of legal practice. Using Scott Shapiro's “planning theory of law” as an example, I will argue that such attempts must fail as long as they understand collective intentionality to be established by a structure of interlocking participatory intentions. Using arguments from Margaret Gilbert, it can be shown that legal practices involve a stronger form of joint commitment. However, in order to not require an implausibly strong agreement of legal participants about the grounds of law, Gilbert's account must be modified to successfully describe existing legal practices. I will argue that a model that conceives of joint commitments as defeasible, context-dependent descriptions of authority is best suited as a foundation for a model of legal positivism.