Hugo Grrotius (1583-1645) was one of the pioneering natural rights theorists of the late 16th and early 17th centuries, Grotius defined natural law as a perceptive judgement in which things are good or bad by their own nature. This was a break from Calvinist ideals, in that God was no longer the only source of ethical qualities. These things that are by themselves good are associated with the nature of man. The Dutch Republic had been founded on principles of religious toleration but had become a Calvinist theocracy. Grotius, a humanist and Dutch patriot, struggled with Calvinism all of his life. In this struggle, he dealt with the international laws of war and issues of peace and justice. Although most famous for his theories of natural law, Grotius was also considered to be a great theologian. While occasionally writing about Christianity and religion, his intention for law was to write of it as independant of religious opinions.

Grotius' conception of the nature of natural law is set forth in his works De Jure Praedae (Commentary on the Law of Prize and Booty) and De Jure Belli ac Pacis (On the Law of War and Peace). On the Law of War and Peace, which was published in 1625, is a seemingly expanded version of On the Law of Prize and Booty, which was written in the late months of 1604 and the early months of 1605. On the Law of Prize and Booty was not published until 1868 when it was discovered at a book sale by several professors from the University of Leyden. Although this manuscript was not found until the late 19th century, Chapter Twelve of the book was published separately in 1609 as Mare Liberum (The Freedom of the Seas). Shortly after his arguments for the liberty of the sea, Grotius became involved in disputes with the Calvinists. Grotius sided against predestination and Calvinism and took up the Arminian cause of free will. He publicly claimed that Calvinist beliefs could have political and religious dangers to Protestantism. Grotius tried to devise a formula for peace that did not go against Calvinism. His attempts failed and ultimately led to his imprisonment. Grotius was famous for his daring escape from the castle of Loevestein in March of 1621. Grotius writes about similar topics and ideals in both the Commentary on the Law of Prize and Booty and in On the Law of War and Peace. According to Grotius, all law, should be divided into what is divine and what is human. He distinguishes between the primary laws of nature and secondary laws of nature. Primary laws of nature are laws that completely express the will of God. Secondary laws of nature, on the other hand, are rules and laws that lie within reason. Grotius discusses war as being a mode of protecting rights and punishing wrongs. It is a mode of judicial procedures. Although war was considered a "necessary evil," it needed to be regulated. The "just war," in the eyes of Grotius, is a war to obtain a right. Grotius discusses three methods of for settling a dispute peacefully. The first is conference and negotiation amongst two rivals or contestants. The second method is called compromise, which is a settlement in which each side gives up some demands or makes concessions. The third is that of single combat or choosing by lot. Grotius believed that it is sometimes better to renounce rights than to try and enforce them. When it comes to bargaining and mediation he holds that for any of the three methods listed above, it is of extreme importance to select a judge with character and decency. Grotius discusses these methods of achieving peace to ultimately obtain some form of justice. He says, "For justice brings peace of conscience, while injustice causes torment and anguish... Justice is approved, and injustice condemned, by the common agreement of good men." (Prolegomena)

Grotius intended moral laws to apply to both the individual and the state equally. Although Grotius was somewhat conservative in his views, his ideas on war, conquest, and the law of nature continued to be revered and expanded by more liberal philosophers like John Locke in his Two Treatises on Civil Government (1689). Locke agrees with Grotius in using the analytical device of a state of nature that exists before civil government and in the general claim that might does not make right as well as the claim that just wars aim to preserve rights. A child prodigy and remarkable international law theorist, Grotius helped form a concept of international society. International society is a community that is joined together by the notion that states and rulers have rules that apply to them all. All men and all nations are subject to this international law and the international community is held together by written agreement in states of instituted customs. The applications of international relations and political implications of the international society (possibly called "world" or "global" community in more contemporary times) can presently be seen in governments like that of the United States and much of Europe. King Henry IV in 1598 remarked that Grotius (who was only 15 at the time) truly was "the miracle of Holland."