important of these was Hugo Grotius, whose treatise De Jure Belli Ac Pacis Libri Tres is considered the starting point for modern international law. Before Hugo Grotius, most European thinkers treated law as something independent of mankind, with its own existence. Some laws were invented by men, but ultimately they reflected the essential natural law. Grotius was no different, except in one important respect: Unlike the earlier thinkers, who believed that the natural law was imposed by a deity, Grotius believed that the natural law came from an essential universal reason, common to all men. This rationalist perspective enabled Grotius to posit several rational principles underlying law. Law was not imposed from above, but rather derived from principles. Foundation principles included the axioms that promises must be kept, and that harming another requires restitution. These two principles have served as the basis for much of subsequent international law. Apart from natural-law principles, Grotius also dealt with international custom, or voluntary law. Grotius emphasized the importance of actual practices, customs and treaties—what "is" done—as opposed to normative rules of what "ought to be" done. This positivist approach to international law strengthened over time. As nations became the predominant form of state in Europe, and their man-made laws became more important than religious doctrines and philosophies, the law of what "is" similarly became more important than the law of what "ought to be." According to the Jewish jurist and diplomat Shabtai Rosenne, the 17-century major figures of the law of nations were used to extensively refer to Jewish sources like the Codes of Law, Maimonides, Moses of Coucy, Ibn Ezra, Leon of Modena and Menasseh Ben Israel. The jurists referring to them were Grotius, Selden, Ayala, Gentili, Zouch, Samuel Rachel and Pufendorf. Peace of Westphalia The Westphalian treaties of 1648 were a turning point in establishing the principle of state sovereignty as a cornerstone of the international order. However the first attempts at formulating autonomous theories of international law occurred before this, in Spain, in the 16th century. Most prominent among the early theorizers were the Roman Catholic theologians Francisco de Vitoria and Francisco Suárez. Suárez is especially notable in this regard in that he distinguished between ius inter gentes and ius intra gentes which he derived from ius gentium (the rights of peoples). Ius inter gentes corresponds to modern international law. In 1625, Hugo Grotius followed with the first systematic treatise