Grotius's successors responded to Hobbes and Spinoza by asserting that man's capacity for reason removes him from the anarchic condition of animals and permits him to choose freely to be bound by law. The State, pre-cisely because it is a sovereign, will seek agreements and uphold them generally because it is rational to do so as a way of life for the State, whether or not upholding a particular agreement is to its advantage at a particular moment in time.44 By means of this argument, Hobbes and Samuel von Pufendorf—the latter the most famous of Grotius's immediate followers—are brought into a measure of agreement: “Hobbes' opinion that the law of nature and the law of nations were the same was accepted by Pufendorf. He also accepted the assumption that separate states, having no common political superior, stood in the same relations to each other as men in a state of nature.”45
Pufendorf, moreover, believed that the law of nature provided the only basis for international law, because there was no other source for law such as exists in a society with a sovereign. Indeed he went so far as to deny the key Grotian insight that international law arose from the customs and practices of states, maintaining instead that only those rules that are derived from universal reason were lawful, this being the means by which natural law was apprehended. The law of gravity governed heavenly bodies; the law of states was to be derived by much the same rational means.
This is not the place to dwell on the shortcomings of Pufendorf; Leibniz has done this definitively.46 Nor is it necessary to complain about the habits of philosophers that addict them to imagining “states of nature” from which to extrapolate, heedless of the one natural state they know something about, namely the one they are in at the moment, which must be presumed to govern to some degree their speculations about other such environments. Indeed I think it equally likely that both Hobbes and Spinoza found their inspiration for the natural state of man in what they observed in the behavior of kingly states—Hobbes focusing on the domestic scene, Spinoza on the international—and reasoned back from this paradoxical interface of absolute domestic authority and apparent international anarchy to the nature of man, rather than the other way around.
However that may be, the political actors of the time confronted the problem of post-Westphalian law and order—namely, that in the absence of a universal sovereign every kingly state, which Westphalia had made the sole preserver of the liberty, authority, and even the life of the political society largely composed of such states, would attempt to aggrandize itself to the limit of its power. They solved this problem in a somewhat less reified, though no less abstract, way than the philosophers. Like the philosophers, the politicians and diplomats of this era were impressed by the power of reason to discover the principles of nature, and the power of rational systems to exploit those principles. This solution thus had much in common with the mathematics of the Age, but it was not achieved by or expressed in philosophical terms. Rather it was expressed in law, arrived at, as we shall see, by the legal means of a constitutional convention and the achievement of consensus at that congress.