I offer a new argument against the legal positivist view that non-normative social facts can themselves determine the content of the law. I argue that the nature of the determination relation in law is rational determination: the contribution of law-determining practices to the content of the law must be based on reasons. That is why it must be possible in principle to explain what makes the law have the content that it does. It follows, I argue, that non-normative facts about statutes, judicial decisions, and other practices cannot themselves determine the content of the law. A full account must appeal to considerations independent of the practices that determine the relevance of the practices to the content of the law. Normative facts are the best candidates.