Jonathan McIntosh (Ph.D., University of Dallas) is a Fellow of Humanities at New Saint Andrews College in Moscow, Idaho, where he teaches on the history of western political and economic thought, philosophical theology, natural law ethics, Aquinas, Anselm, and J.R.R. Tolkien. He is the author of The Flame Imperishable: Tolkien, St. Thomas, and the Metaphysics of Faërie, and blogs at The Natural Law Libertarian and The Flame Imperishable.
Having laid its ultimate metaphysical foundation in God’s eternal law governing all created reality, and having then laid its ultimate moral and anthropological foundation in the natural law governing all human action, Aquinas at last turns to the question of man-made law, or what he calls human law. He raises four questions in particular about human law: (1) What is its use? (2) From whence do political communities derive human law? (3) What are some of human law’s leading “qualities” or aspects? And (4) what are some of the principal divisions of human law? Both the questions themselves, but especially how Aquinas answers them, will prove of significant interest to the libertarian.
Aquinas begins his discussion of the usefulness of human law (ST I-II.95.1) by quoting a highly suggestive passage from the early medieval encyclopedist Isidore of Seville (AD 560-636): human laws are made in order that through fear “human audacity might be held in check, that innocence might be safeguarded in the midst of wickedness, and that the dread of punishment might prevent the wicked from doing harm.” From here Aquinas argues that while the private admonition of parental training is the normal and best way of inculcating virtue in people, human law is nevertheless necessary for those “depraved” individuals who have either failed to receive such training in virtue or, because of their being “prone to vice” or otherwise “not easily amenable to words,” are only able to be “restrained from evil by force and fear.” In this way, such men might be kept from doing evil and “leave others in peace,” and hence over time might even be habituated to do that good voluntarily and virtuously what they previously did only under compulsion.
Aquinas’s account of the usefulness of human law is remarkable in that it involves a much more libertarian perspective on law than Aquinas has given thus far. Previously in his “Treatise on Law,” Aquinas had identified the end of law as nothing less than the “universal happiness” of the community as a whole (ST I-II.90.2), as being more effective than private forms of admonition because of its use of coercion (ST I-II.90.3, reply 2), and as able to make men to be good, not in a relative but in an absolute sense, making men to be good as men (ST I-II.92.1). Here, however, Aquinas would seem to set law’s sites much, much lower: coercive human law doesn’t positively bring about communal happiness so much as it merely helps ward off a particular kind of vicious behavior; its use of coercion makes it in important ways actually less effective in leading to virtue than the verbal or persuasive means of private admonition; and thus law makes men good not absolutely speaking, but in the relative sense of forcing only some them to be less bad than they otherwise might be. However non-libertarian Aquinas’s general account of human law may be, his account of specifically its usefulness is as libertarian as one could ask for.
After its usefulness, Aquinas next inquires into human law’s origins: from where do we get human law (ST I-II.95.2)? The answer is one that we have seen before: if human law is to be legitimate and just, it must be derived from the natural law. Aquinas here distinguishes two different ways in which this may happen. First, and most straightforwardly, a human law may be derived from the natural law by way of strict logical deduction, the way a conclusion is derived from its premises in a syllogism. The example he gives is how, from the general natural law principle that “one should do harm to no man,” we deduce the human-law conclusion that “one must not kill.” The second way a human law may be derived from natural law is less clear cut, as it involves a more indefinite, discretionary, and even creative act that Aquinas calls a “determination of certain generalities,” in which a general principle could be reasonably applied in any number of ways, leaving the lawmaker to make a prudential decision as to how it might best be applied in the context of a particular community. Here the example is how, although the requirement that an act be punished may be deducible from the natural law, just what that punishment should be is not so derived, but is something that must be prudentially resolved, specified, and in that sense “determined” from the natural law.
The significance of Aquinas’s discussion of the twofold origin of human law in the natural law perhaps become most clear in the context of the last question he raises about human law, in which he inquires into some of the principal divisions of human law (ST I-II.95.4). The most important division Aquinas incorporates into his account is the classic Roman law division of human law into the “law of nations” (ius gentium) on the one hand, which are those laws that all or nearly all political societies hold in common, and “civil law” (ius civile) on the other, which are those laws that are peculiar to a particular society. Ingeniously, what Aquinas does is identify this historic, Roman law distinction between the law of nations and civil law with his above, epistemological distinction between the two ways in which human law may be derived from natural law. Thus, on Aquinas’s interpretation, the law of nations that all societies hold in common corresponds with those human laws that are logically deduced from the natural law, and which he illustrates now with the law of nations stipulation that there should be “just buyings and sellings,” but which he says also includes those laws “without which men cannot live together.” The Roman category of civil law, by contrast, and in which one society differs from another, Aquinas identifies with second way of deriving human law from natural law, namely “by way of particular determination.”
While Aquinas gives here a somewhat more idealized account of the law of nations than was really historically the case, his discussion is nevertheless quite suggestive from a libertarian point of view. Aquinas’s two examples here of a human law “deduction” from the natural law are the prohibition of murder and the requirement of justice in exchange, but to which list he would also add such laws concerning the prohibition of treason, rape, fraud, theft, and the general provision for the punishment of crime, all of which involve either acts of aggression (or the threat thereof) against others or else the community’s coercive, punitive response. Thus, although Aquinas never directly develops, as the libertarian attempts to do, an ethic of human law coercion from the natural law, he implies both the possibility of deducing such an ethic, and that the result of such a deduction is, for all intents and purposes, a libertarian ethic of non-aggression. This would suggest, moreover, that all of the other, higher functions that Aquinas assigns to human law, principally its directing community to the common good of universal happiness and of leading men to virtue, fall under the category of the civil law “determinations” of the natural law that vary from community to community.
Understood in these terms, accordingly, the debate between the libertarian and Aquinas may be summarized thus: whereas the libertarian believes that human law ought to be limited to those uses of coercion expressly warranted and required by the natural law (and hence recognized by virtually all nations), Aquinas believes that each political society is also permitted, in the interest of promoting virtue and the common good, to use coercion in enforcing its own unique set of “civil laws” that go above and beyond the libertarian law of nations. To this, however, the libertarian could meaningfully reply that, however important such ends might be, to use coercion to achieve such ends is itself a violation of the non-aggression ethic logically required by the natural law, and therefore not to be attempted.
 The law of nations, for example, traditionally also included such miscellaneous rules as regulations over the occupation of fortifications, the taking of captives in war, slavery, building codes, peace treaties, marriages with foreigners, respect for parents, and proper religious observances, all topics that would not seem to fit Aquinas’s notion of a logical “deduction” from the natural law. Jose Manuel De Aguilar, “The Law of Nations and the Salamanca School of Theology,” The Thomist, vol. 9, no. 2 (April 1946): 190-200.
 John Finnis, Aquinas (Oxford: Oxford University Press, 1998), 266.