Often erroneously referred to as "the Father of Natural Law," Hugo Grotius may have done more than anyone else to throw the whole concept into confusion and disrepute. Part of the problem is that modern commentators ignorant of history and more than a little hazy about the principles of Aristotelian and Thomist philosophy assume that Grotius developed his thought out of nothing.
Very much the contrary. Grotius made the first formal presentation of a discussion of the respective roles of natural law and positive law in the new field of international law. That, however, had been discussed and argued for centuries. What was new in Grotius's thought was the distinction between national law and international law.
Grotius's work was in response to the growth of the concept of the absolutist ruler and the nation-state and the new political doctrine of divine right that developed during the Reformation. These were a concept and doctrine that (as Heinrich Rommen pointed out), "contradicted utterly" the individualistic principle of personal interpretation of scripture and religious liberty that constituted the raison d'etre of the Reformation. (Heinrich Rommen, The State in Catholic Thought, op. cit., 101.)
Grotius was therefore faced with a paradox. His primary goal was to support the absolutist divine right monarchy of James I/VI Stuart of England/Scotland. He had to do this in such a way, however, as to demonstrate the continuity of the new concepts and doctrines with medieval philosophy.
This is because the Reformation claimed to be a restoration of the purity of original Christianity. Consequently the reformers could only claim to be tossing out additions and accretions, not the substance of what constituted (in their opinion) true Christianity. Besides, King James's mother had been a Catholic. Piling paradox upon paradox, James's presumed divine right to rule came from or through the Catholic tradition, which could not be discredited without calling divine right itself into question . . . even as the Catholic Church's political philosophy rejected divine right. (See St. Robert Cardinal Bellarmine, De Laicis, the Treatise on Civil Government.)
As we might expect, Grotius was able to resolve the contradictions by the simple expedient of developing new and confusing definitions of terms. We see this, for example, in his definition of "sovereignty." As George Sabine explained,
"Grotius, more than Althusius, was hampered in his thinking about philosophical principles by his fidelity to the letter of positive law. After defining sovereignty as a power not subject to the legal control of another, he distinguished between a common and a special possessor, or subject, of the power. The common subject of sovereignty is the state itself, the special subject is one or more persons, according to the constitutional law of each state. The sovereign is therefore either the political body itself (Althusius's state) or the government, a use of terms which hardly made for clearness." (George H. Sabine, A History of Political Theory, Third Edition, op. cit., 420-421.)
Here in Sabine's analysis we see both the problem and Grotius's resolution of it: "his fidelity to the letter of positive law." As Rommen described the conflict, "Grotius did not profess the implied complete autonomy of human reason as the sole and not merely the proximate source of the natural law. He considered God to be the highest source of the natural law, and he likewise regarded Holy Scripture as a principle of knowledge on an equal footing with reason." (Rommen, The Natural Law, op. cit., 63.) As Grotius explained this,
"The law of nature is a dictate of right reason, which points out that an act, according as it is or is not in conformity with rational nature, has in it a quality of moral baseness or moral necessity; and that, in consequence, such an act is either forbidden or enjoined by the author of nature, God." (Hugo Grotius, Book I, Ch. I, § x, 1.)
The contradiction should be immediately evident. Grotius first acknowledged that, "the law of nature is a dictate of right reason." This is all well and good. He then stated that, "an act is either forbidden or enjoined by . . . God."
This is an insurmountable problem. That there is a God can be known by reason alone, as can the content of the natural law, as the Catholic Church teaches. (Pius XII, Humani Generis, §§ 1-2.) Thus it can be established by the process of inductive reasoning that a God exists, that He (and it is legitimate at this point in the argument to add "She or It") is a perfect Being, and that this perfect nature is self-realized in His Intellect, thereby being discernible by reason alone. This is a matter of true and certain knowledge, not opinion. But there's more.