By the fifteenth and sixteenth centuries, when the modern international law started getting its form and content, the natural law had made heavy inroads and assumed strong ideological moorings. This can be clearly seen in the writings of Francisco de Vittoria (1480-1546), Francisco Suarez (1548-1617) and Alberico Gentilis (1552-1608). International law was binding on the States because it was part of the law of nature. However, it was Hugo Grotius (1583-1645) who secularized the concept of natural law while making a distinction between jus naturale and jus gentium. He based the law of nations on customs and treaties in addition to the law of nature. The law of nature, according to him, was the dictate of right and reason, not of divine prescriptions.