The Dutch humanist Hugo Grotius (1583–1645) presented in his magnum opus De Iure Belli ac Pacis (1625) a comprehensive and unified theory of natural law on a secular rational basis. He declared that natural law is a separate discipline from divine law and other forms of volitional norms, and may in principle be exclusively identified by reason. The article explains Grotius' three formulations of natural law, and concludes that the concept of natural law understood as a personal moral quality, in the form of expletive jus – or the perfect rights to one's own – was given a pivotal role in his system of natural law. This concept of law, which Grotius distinguishes from other rationally derived ethical norms and virtues, had a major impact on the development of political and legal ideas in the Enlightenment. The article also contains sections on Grotius’ method, as well as short comparisons with natural law conceptions of the Roman jurists, as well as those of Thomas Aquinas and Thomas Hobbes.