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. Grotius' conception of the nature of natural law is set forth in his De Jure Belli ac Pacis (On the Law of War and Peace), published in 1625. In it Grotius contended that natural law prescribes rules of conduct for nations as well as for private individuals. He derived much of the specific content of international law from the Bible and from classical history. Although he did not condemn war as an instrument of national policy, he maintained that it was criminal to wage war except for certain c