The problem of legal knowledge: Kant’s practical postulates and contemporary legal theory

The purpose of Immanuel Kant’s philosophy of law is generally misunderstood by contemporary philosophers and legal theorists. In this article we explain why various common approaches are mistaken. In their place, we present his legal philosophy as a theory of legal knowledge based on a sequence of practical postulates. Kant is interested above all in how morality reveals the world of human relations to have legal significance. We then turn to consider the relationship between Kant’s legal philosophy and two contemporary legal theorists who have been deeply influenced by Kant: Robert Alexy and Jürgen Habermas. We argue that Alexy offers an elegant synthesis of contemporary misreadings which still overlooks Kant’s core claim about the conditions for the possibility of legal knowledge. Habermas, by contrast, understands the role of practical postulates in our knowledge of law, but re-presents these in a social constructivist mode. Neither ultimately succeeds in defending Kant’s core insight that facts about law can only be known to be law if they are treated as a reflection of an underlying moral idea.