Natural Law without Nature? Aquinas to the Rescue

Natural law has had a hard time in the modern world. Jeremy Bentham (1748-1832) castigated the natural law theorists of his day for promoting “personal opinions and sentiments” as standards of right and wrong. He offered utilitarianism as a replacement, versions of which have taken the lead in modern university courses on ethical theory. Immanuel Kant (1724-1804), dissatisfied with utilitarianism and committed to a “Copernican revolution” in ethics and metaphysics, developed a subjective replacement for natural law: “Act only on that maxim whereby thou canst at the same time will that it should become a universal law.” Kant surmised that if we simply sat down and considered whether we would want everyone to do what we were planning to do, without self-contradiction, we would have sure-fire guidelines for right and wrong.

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This is called the “universalization hypothesis.” Kant’s successors were unconvinced. G. W. F. Hegel (1770-1831), for example, argued that with the Kantian method one could justify either communism or a system of private property without any self-contradiction. More recently, Henry Veatch shows how a hypothetical Nazi, trying to “universalize” his maxim, “I should eliminate all Jews,” could justify it by carefully rephrasing the moral “test.”

Opposition to natural law theory continues in our own time. The magazine First Things, dedicated to defending “first principles, for the right ordering of public life,” published Paul Griffiths’s “The Nature of Desire,” which attacked natural law on the basis of the infinite number of unrealizable desires, as evidenced in the plays of Shakespeare and other literature. J. Budziszewski wrote “Natural Law, Revisited,” in the same magazine and, aside from a brief reference to a natural law governing reproduction, was silent about any particular version or theory. Even Hadley Arkes, whose book First Things initially inspired the creation of the magazine by that name, devotes only a single chapter in his most recent book, Constitutional Illusions and Anchoring Truths: the Touchstone of the Natural Law, to natural law, in which he disclaims any interest in any “theory” of natural law and gravitates toward Kant’s “universalizability” approach.

Arkes puts forward a brilliant analysis of the application of legal doctrines like ex post facto and “prior restraint” and brings out blatant infractions of the law of non-contradiction in specific cases like Lochner, which is considered a famous landmark by legal scholars like himself, and the “Pentagon Papers,” published during the Nixon administration. But such important constitutional insights are not traditional natural law as espoused by Cicero, Aquinas, the late-medieval Catholic scholastics, and even later Protestant spokesmen like Grotius, Cumberland, and Pufendorf.


Much of the contemporary disinterest in traditional natural law can be traced to David Hume (1711-1776) who, in his ethical work, warned against “deriving an ‘ought’ from an ‘is’” — in other words, basing moral rules on factual states of affairs. My 2004 book, Natural Law: An Introduction and Reexamination, argues that Hume has been widely misunderstood. And Hume himself, in the moral theory he ended up with, derives the fundamental “oughts” of human nature from inclinations and sentiments, which certainly qualify as an “is.” Rev. Frederick Copleston, S.J., famous for his nine-volume History of Philosophy, suggests that Hume himself should be categorized as a “natural law” philosopher.

But contemporary ethicists, fearing ridicule for even the appearance of deriving theories from something existing (and gradually emerging by biological evolution) like “human nature,” tend to look to “rationality” itself, in various guises, as the source of morality. Thus “new natural law” theorists, like John Finnis and Germain Grisez, noted for their stalwart defense of Pope Paul VI during the firestorm of rejection that followed his encyclical on contraception, propose a theory “without metaphysics” depending on certain “self-evident” basic human values, including knowledge, life, play, aesthetic experience, sociability (friendship), practical reasonableness, religion, and the pursuit of ultimate questions.

But I began to suspect the alleged “self-evidence” of these values when I read the long and intricate proof in Finnis’s Natural Law and Natural Rights that the value of knowledge is self-evident. The self-evidence he has in mind is certainly not the intuitive kind. I and other philosophers have concluded that this “new natural law” theory is interesting and perhaps valid, but it is not a natural law theory based on the givens of human nature.

Thomas Aquinas, who lived before the Humean restrictions about “is” and “ought,” dared to go beyond the abstract principle, “Do good and avoid evil,” to point out three specific “oughts” derived from the tendencies and appetites of human nature itself. (This triad of precepts is not original with Aquinas, but appears in Cicero, the Roman jurists, Aquinas’s scholastic predecessors and successors, and even as late as the 18th century in Montesquieu.)

The most important precept is indeed, as contemporary ethicists often maintain, to act rationally. But for Aquinas, rationality is not just about avoiding self-contradiction; he points out two specific and necessary manifestations of rationality: 1) in the theoretical realm, always pursue the truth; and 2) in the practical realm, contribute to communal and societal harmony. A second precept, experienced by humans as well as animals, is to procreate and care for offspring; humans, of course, require an extraordinarily long period of nurturing. And the third precept is what is called in common parlance the “law of self-preservation.”

These natural laws are also rights. The duty of seeking the truth implies the right to the truth, and certainly involves a right to necessary information and education; the duty of contributing to communal harmony implies the right to work towards that end — a right with political implications; the duty of caring for one’s offspring (Aquinas would consider this the main argument against abortion, not the general “right to life”) implies the right to reproduce (not the bizarre modern inversion of “reproductive rights”); and the duty of self-preservation implies the right of self-preservation (implying the right to the wherewithal necessary for maintaining physical life).


Although Protestant as well as Catholic philosophers have argued for natural law, some Protestants, for example Karl Barth, have found it inimical to the primacy of the Christian gospel. And the reaction against traditional natural-law theory continues among Protestants. A forthcoming Lutheran anthology on natural law boasts that it is not concerned with the wrong (Thomistic) version of natural law: “No contemporary thinker is interested in a wooden repristination of the natural law that is tied necessarily to the particular metaphysical foundations in the Thomistic-Aristotelian synthesis.”

The Thomistic theory of natural law has three important advantages over its avatars: 1) it is based legitimately on the realities of human nature, on the basic impulses that all humans share; 2) it is intuitively connected with natural rights, as “the other side of the coin”; and 3) it offers specific guidelines about moral behavior, whereas general guidelines about “universalization” or conforming to “basic values” do not.

By default, the Thomistic “metaphysical” natural law has been connected historically with Catholic tradition. In this tradition, natural law has been considered a bridge toward ethical and legal unanimity with non-Catholics and even non-Christians, on the supposition that the theory defends the basic moral presuppositions for all humans.

Optimally, however, Christianity will not be satisfied with mere adherence to natural law, but will impel believers to go even beyond the basic tenets — going “the extra mile” (Mt 5:41), offering your coat to someone who takes your tunic (Lk 6:29), lending to would-be borrowers and forgiving debts (Mt 5:42, 6:12), possibly even “turning the other cheek” when assailed by adversaries (Mt 5:29; Lk 6:29). Such responses, of course, leave natural law far behind.


  • Howard Kainz

    Howard Kainz is professor emeritus at Marquette University. He is the author of several books, including Natural Law: an Introduction and Reexamination (2004), The Philosophy of Human Nature (2008), and The Existence of God and the Faith-Instinct (2010).

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