conduct of law's subjects, and secondary rules that regulate how the primary rules may be changed, identified, and adjudicated. Hart's theory, although widely admired, sparked vigorous debate among late twentieth century philosophers of law including Ronald Dworkin, John Rawls, Joseph Raz, and John Finnis. Legal realism, which asserts that law is the product of decisions made by courts, law enforcement, and attorneys, which are often decided on contradictory or arbitrary grounds. According to legal realism, law is not a rational system of rules and norms. Legal realism is critical of the idea that law has a nature that can be analyzed in the abstract. Instead, legal realists advocate an empirical approach to jurisprudence founded in social sciences and the actual practice of law in the world. For this reason, legal realism has often been associated with the sociology of law. In the United States, legal realism gained prominence in the late 19th century with Oliver Wendell Holmes and John Chipman Grey. Legal realism became influential in Scandinavia in the 20th century with Axel Hägerström. Legal interpretivism, which denies that law is source-based because law necessarily depends on human interpretation that is guided by the moral norms of communities. Given that judges have discretion to adjudicate cases in more than one way, legal interpretivism says that judges characteristically adjudicate in the way that best preserves the moral norms, institutional facts, and social practices of the societies in which they are a part. It is consistent with legal interpretivism that one cannot know whether a society has a legal system in force, or what any of its laws are, until one knows some moral truths about the justifications for the practices in that society. In contrast with legal positivism or legal realism, it is possible for the legal interpretivist to claim that no one in a society knows what its laws are (because no one may know the best justification of its practices.) Legal interpretivism originated with Ronald Dworkin in the late 20th century in his book Law's Empire. In recent years, debates about the nature of law have become increasingly fine-grained. One important debate exists within legal positivism about the separability of law and morality. Exclusive legal positivists claim that the legal validity of a norm never depends on its moral correctness. Inclusive legal positivists claim that moral considerations may determine the legal validity of a norm, but that it is