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.
In the previous post in this series, we considered, from a libertarian perspective, St. Thomas Aquinas’s teaching on the eternal law, that law regulating the entirety of God’s creation, by which he sovereignly and providentially directs his creation to its appointed end. The next category of law to receive Aquinas’s closer inspection is that of the natural law, that subset of God’s eternal law dealing with human action in particular. As we saw in his introductory account earlier, natural law consists of those commands of human reason that are drawn from our natural “inclinations” towards action, inclinations that have been formalized into normative rules for human behavior.
This brings us to the question of what those rational rules of action are, precisely, that the natural law commands us to keep. Just what kinds of action does reason charge us to do? In short, what is the content of the natural law? Aquinas begins his answer to this question by making the interesting claim that the very first rules or “precepts” of the natural law function in much the same way in our practical reasoning (that is, our reasoning about human action) that first principles or axioms function in more theoretical sciences such as geometry. In both cases—in ethics as in mathematics—reason proceeds deductively or logically from self-evident first principles or premises to more derivative, though still necessary, theorems or conclusions. Because the first thing known or grasped by practical reason is the notion of a thing as good, i.e., as something desirable and hence to be sought after in action, the most basic truth, axiom, and hence rule or precept of all practical reason, and hence of the natural law, is the self-evident principle that “good is to be sought after and evil is to be avoided.” This, according to Aquinas, is the very “first principle” of the natural law from which all other precepts of the natural law are derived. The natural law, in sum, consists of all those rules for human action by which a human being is rationally directed towards a real good or away from a real evil. Whenever human beings seek that which is good and avoid that which is evil, they are following the natural law.
This is not to say, however, that the natural law commands every good or prohibits every evil equally or in the same measure. Rather, just as there is a real, objective order or hierarchy to the goods that a human being needs and desires, as well as to the evils that he is to avoid, so there is an order or hierarchy to the natural law precepts directing human action towards or away from those respective goods and evils (ST I-II.94.2). Moving from the more general to the more specific, in the first place is the natural inclination that human beings share in common with all other existing things, namely the desire for self-preservation. For this reason, the natural law commands that (all other things being equal) a person act so as to preserve his own life and avoid those things tending towards his destruction. A second order of natural goods and natural inclinations—and hence a second order of natural law precepts—concern those matters which human beings share with all other animals, including sexual intercourse and the raising of offspring. A third and final order of natural goods, inclinations, and corresponding natural law precepts concern those goods which are proper or specific to human nature itself, including especially the knowledge of God, living in society with other human beings, and the corresponding avoidance of ignorance and giving undue offense to one’s neighbors. Self-preservation, procreation, and the pursuit of God and a well-ordered society: these comprise Aquinas’s well-known “three precepts” of the natural law.
These three precepts, however, are still very general, whereas the natural law in fact directs human beings to every good to which their nature inclines them, a consequence of which is that the natural law is understood by Aquinas to command every act of every virtue (ST I-II.94.3). In a word, the natural law encompasses all of human morality. And although the natural law is one and the same in all men, Aquinas admits that the more we descend into particular matters of detail concerning the natural law, the more difficult it is to know precisely what the natural law teaches (this is one of the reasons Aquinas thinks that Scripture, with its supernaturally revealed divine law, is so important), the result being that men often differ from each other over their understanding of what the natural law actually teaches or requires (ST I-II.94.4). Nevertheless, Aquinas insists that the natural law does not change in its fundamental precepts (ST I-II.94.5), nor can its primary precepts be completely eradicated from the hearts of men, though, again, in some of its more derivative or down-stream precepts he allows that the natural law can be practically erased from the consciences of men through bad customs or habits (ST I-II.94.6).
What might the libertarian make of Aquinas’s above teaching on the natural law? In and of itself, there is little if anything in Aquinas’s traditional account of the natural law that is inconsistent with libertarianism per se, and, indeed, much to commend it. The natural law, as the moral foundation of all human action, and hence of all human law, is objective, universal, obligatory, and knowable, and obedience to or compliance with the natural law is necessary if human beings are to flourish as their God-given nature dictates. As Aquinas will develop later, and in a principle that is arguably the central political insight and application of his doctrine of the natural law, this means that for any human law to be legitimate, authoritative, and hence obligatory, it must legislate in a manner consistent with and in some sense derivable from the always more foundational moral authority of the natural law. Viewed from this perspective, the central thesis of libertarianism might be instructively viewed as merely an attempt to apply this principle a step further: not only must the actions regulated by law be consistent with the natural law, but the regulators’ own act of coercion must likewise be consistent with the natural law, using force to restrain only those violations of the natural law that the natural law itself would require.
Although Aquinas (unfortunately) never develops a consistent natural law ethic of coercion, one natural law principle of coercion we might draw from his above account concerns the variability with which he says the natural law is grasped or known in its more derivative or secondary principles. If it is the case, after all, that the more we descend into matters of detail, the more difficult it is to know the natural law and, hence, disagreements over the natural law arise more frequently, shouldn’t this fact alone have some moral bearing on which parts of the natural law it is in fact natural for human law to try to enforce? According to Aquinas’s own earlier definition of law, a law is only a law if it is clearly promulgated (ST I-II.90.4), but as Aquinas himself will give us to understand later, the most universally promulgated form of human law, because the most directly and clearly derived from the natural law, is the “law of nations,” that body of law affirmed by virtually every political society, and yet which is primarily concerned with such aggressive acts as violence, theft, and fraud. If so, then already in his account of the natural law, it is possible to see the seeds being sewn for a properly libertarian ethic of non-aggression.
 I add the qualification “all other things being equal” to the principle of self-preservation, for as Aquinas recognizes, of course, there are certain goods which are greater than, and so are to be valued higher than, our own life, in which case it is rational, natural, and moral for us to risk and even sacrifice our own lives for the sake of these higher goods. This doesn’t alter the case, however, that the inclination towards self-preservation is natural and hence part of the natural, moral law.