This dissertation takes as a starting point Thomas Aquinas’s treatment of the Natural Law, in which it is possible to identify the consolidation of a natural law theory. The natural law theory present in Thomas Aquinas has a theological basis. Despite not leaving aside the role of reason, God fulfills a fundamental role in his argument. With the analysis of the historical context of the modern period, which characterizes itself as a moment of intellectual crisis, we look for the conditions of the possibility of the break offered by the thought of Hugo Grotius. We intend to clarify, with special attention given to On the Right to War and Peace, how the Grotius’s natural law theory seeks another basis for the rights of men, as an answer, also, to the ancient sceptical arguments which were revitalized at the time. The result is a natural Law theory with a basis in rationality, casting aside theological arguments to the rights of men.