enact and even successfully enforce immoral laws, such laws are legally invalid. The view is captured by the maxim: an unjust law is not a true law, where 'unjust' means 'contrary to the natural law.' Natural law theory has medieval origins in the philosophy of Thomas Aquinas, especially in his Treatise on law. In late 20th century, John Finnis revived interest in the theory and provided a modern reworking of it. Legal positivism, which is the view that law depends primarily on social facts. Legal positivism has traditionally been associated with three doctrines: the pedigree thesis, the separability thesis, and the discretion thesis. The pedigree thesis says that the right way to determine whether a directive is law is to look at the directive's source. The thesis claims that it is the fact that the directive was issued by the proper official within a legitimate government, for example, that determines the directive's legal validity—not the directive's moral or practical merits. The separability thesis states that law is conceptually distinct from morality. While law might contain morality, the separability thesis states that "it is in no sense a necessary truth that laws reproduce or satisfy certain demands of morality, though in fact they have often done so." Legal positivists disagree about the extent of the separability thesis. Exclusive legal positivists, notably Joseph Raz, go further than the standard thesis and deny that it is possible for morality to be a part of law at all. The discretion thesis states that judges create new law when they are given discretion to adjudicate cases where existing law underdetermines the result. The earliest proponent of legal positivism was John Austin who was influenced by the writings of Jeremy Bentham in the early 19th century. Austin held that the law is the command of the sovereign backed by the threat of punishment. Contemporary legal positivism has long abandoned this view. In the twentieth century, two positivists had a profound influence on the field: Hans Kelsen and H. L. A. Hart. Kelsen is most influential for his notion of 'grundnorm,' an ultimate and basic legal norm, which some scholars, especially in Europe, accept today. In the Anglophone world, Hart has been the most influential scholar. Hart rejected the earlier claim that sanctions are essential to law and instead argued that law is rule-based. According to Hart, law is a system of primary rules that guide the