Before the objection can be answered it must be aimed at the right target, because most attacks on natural law demolish a position the Church has never held. Natural law does not mean “whatever happens in nature is good” — nature red in tooth and claw is no moral textbook. Nor does it mean rules God arbitrarily posted, to be taken on faith. It means something more precise: moral truth is grounded in what human beings are, and human reason — not only revelation, and not only Christian reason — can in principle know it.

St. Thomas Aquinas gave the tradition its classic definition: “the natural law is nothing else than the rational creature’s participation of the eternal law.” God orders all creation toward its ends; one creature can understand that order and freely live by it. The light of natural reason, Aquinas adds, “whereby we discern what is good and what is evil… is nothing else than an imprint on us of the Divine light.” Conscience, on this account, is not a private oracle or a cultural download. It is a created intellect reading a real moral order — sometimes squinting, sometimes misreading, but reading something that is there.

From this follows the first precept: “good is to be done and pursued, and evil is to be avoided.” That sounds empty until you see what Aquinas builds on it. Human nature has intelligible inclinations — to preserve life, to bear and raise children, to know the truth, to live in society — and reason, reflecting on them, derives real norms: do not murder, do not steal, do not lie, honor your parents, keep your word. These are conclusions about what human flourishing requires — which is why the Church, defending life or marriage or justice, does not simply quote Scripture at the world. She argues from human nature, and expects the argument to be intelligible to those who reject her Scriptures. Whether that expectation is reasonable is what the objection denies. So state it at full strength.

Begin with the charge easiest to test: that natural law is religious preference disguised as reason. If so, the doctrine should appear only where the religion does. It does not. Its fullest ancient statement comes from a pagan — Cicero, in De Re Publica, around 51 B.C.: “There is indeed a true law, right reason, agreeing with nature, diffused among all, unchanging, everlasting, which calls to duty by commanding, deters from wrong by forbidding… Nor, truly, can we be released from this law, either by the senate or by the people… Nor will there be one law at Rome and another at Athens; one law at the present time, and another hereafter: but the same law, everlasting and unchangeable, will bind all nations at all times.”

Every load-bearing element of the Catholic doctrine is already there: a moral law grounded in nature, known by reason, binding all peoples, beyond any legislature’s reach. Cicero drew on the Stoics, who drew on Aristotle and Plato, who were arguing against the sophists — the original “morality is convention” theorists. This debate is not Church versus Enlightenment; it is as old as Greek philosophy itself.

C. S. Lewis made the point empirically in The Abolition of Man, assembling an appendix of moral maxims from Egyptian, Babylonian, Hindu, Chinese, Greek, Roman, Norse, and Jewish sources — the same core recurring everywhere: duties to parents and children, prohibitions of murder, theft, and false witness, obligations of good faith and mercy. He called it the Tao, deliberately choosing a non-Christian name. No culture has been found that admires cowardice, prizes betrayal of friends, or counts gratitude a vice. The Church did not invent this law. She found it already promulgated — and she can tell you by whom.

Here is the remarkable thing about the Bible’s treatment of universal morality: it does not argue for it. It presupposes it — and in one famous passage, explains it. St. Paul, on how Gentiles who never received the Law of Moses can still be judged: “For when the Gentiles, who have not the law, do by nature those things that are of the law; these having not the law are a law to themselves: Who shew the work of the law written in their hearts, their conscience bearing witness to them, and their thoughts between themselves accusing, or also defending one another” (Romans 2:14–15). Paul states an observed fact — pagans do, “by nature,” what the revealed law commands — and gives its cause: the law’s work is written in their hearts, and conscience reads it. Natural law doctrine, complete, in a single sentence of Scripture.

Paul had already made the parallel claim about knowledge of God Himself: “For the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity: so that they are inexcusable” (Romans 1:20). Reason, reading creation, can reach the Creator; reading human nature, it can reach His moral law. And the ground of that law’s universality is laid on the Bible’s first page: “And God created man to his own image: to the image of God he created him: male and female he created them” (Genesis 1:27). Dignity is not conferred by cultures or majorities, and therefore cannot be repealed by them.

Be precise about what is claimed here, because a careful critic will insist on it. Scripture’s say-so cannot refute a relativist who rejects Scripture — quoting Romans at Hume is circular. The point runs the other way: Christianity stakes itself on a testable claim, that the same moral core will be found among peoples who never heard of Moses — the prediction Section II showed confirmed. The philosophical case stands on reason’s own feet; Romans 2 explains why reason finds what it finds. But the case must still answer the three strands. Take them one at a time.

Take Hume first, because his gap is real — against a certain picture of nature. If nature is a value-free field of particles in motion, no description of it yields an ought, and Hume wins. But natural law never tried to squeeze values out of value-free facts; it questions Hume’s picture of nature itself. Living natures are teleological — ordered toward ends — and once ends are in the picture, facts and values were never separate. From “this is a heart” it follows what a good heart does, and no one thinks that step smuggles in mysticism. The claim is that “human being” works the same way: a rational, social animal has a specifiable flourishing, and conclusions about what frustrates it — deceit, betrayal, murder — are discoveries, not decorations. Here is where the dispute actually now stands: the modern naturalist does not concede that biological “ends” are real, normatively loaded features of the world at all — he takes them to be merely functional or etiological descriptions, dressed in the language of purpose but reducible to selection history. That is a live, unsettled fight in contemporary philosophy of biology, not a premise natural law is entitled to assume as already won. Hume’s guillotine severs ought from a nature already stripped of ends; it never touches a nature that has them — but whether nature has them is precisely the question the argument must still win, not simply post. And note the objector’s predicament: the moment he says we ought not believe what evidence cannot support, he has vaulted his own gap.

Mackie’s “queerness” argument is less an argument than a metaphysical bill of costs: objective values would be strange furniture in a materialist universe, therefore deny them. Granted — but the same reasoning disqualifies logical necessity, mathematics, and the reliability of reason itself, equally “queer” residents of a world of mere matter. Mackie saw the price and paid it: ordinary moral thought, he conceded, really does claim objectivity — hence error theory: every moral judgment ever made, including “the Holocaust was wrong,” is false. A theory whose output is that nothing is actually wrong has not explained morality. It has abolished it.

The anthropological strand deserves the most respect, because the data are real: societies have honored practices their neighbors abominated. But Aquinas anticipated the point seven centuries before the field workers shipped out. The natural law’s general principles are the same for all; its remote conclusions can fail where reason is corrupted by passion or custom — “theft, although it is expressly contrary to the natural law, was not considered wrong among the Germans, as Julius Caesar relates.” Yet even then, “the natural law, in the abstract, can nowise be blotted out from men’s hearts.” Look closely at the famous cases: cultures differ far less over principles than over application — above all, over who counts. The society that exposes infants does not believe murder is fine; it has drawn the circle of “neighbor” cruelly small. That is a factual error about persons, not a rival morality — and moral progress has consisted precisely in widening the circle, which would be meaningless without a truth to widen it toward. Variation in moral codes no more disproves moral truth than variation in ancient astronomies disproved the sky.

Euthyphro, finally, is a genuine dilemma — for the gods of Athens. It has no grip on the God of classical theism, because it assumes God and the good are two things needing to be related by His will from one side or the other. The Catholic tradition rejects the assumption, not a horn: God does not have goodness, nor obey a standard outside Himself — He is goodness itself, subsistent Being in whom the standard and the One who wills are identical. His commands are neither arbitrary (they express His unchanging nature, which cannot will cruelty as good) nor redundant (the standard does not float above Him; it is Him). And note what the dilemma needs in order to bite: an objective goodness by which even a divine command could be measured. Euthyphro is not a weapon relativism can borrow — it presupposes the moral realism the relativist denies.

Strip away the natural law and something must fill the space where moral argument used to happen — and what fills it is force. If there is no truth about justice for reasoning to reach, disputes about justice cannot be settled, only won. Law becomes the will of whoever commands the legislature; rights become grants from the state, revocable by the state; and “that is unjust,” addressed to the powerful, means only “I lose, and I dislike losing.” This is a conceptual entailment, not a slippery-slope prophecy — and the twentieth century ran the experiment at scale. Lewis’s deeper argument in The Abolition of Man was precisely this: the men who step outside the moral law to remake values do not become freer than the law — with reason’s access to objective good denied, nothing remains to guide the remaking except appetite and power.

And his simpler observation in Mere Christianity still cuts: listen to any quarrel — “you promised,” “that’s not fair.” Quarreling means appealing to a standard both parties are presumed to know; a man who truly believed morality was private taste would no more argue about it than about whether cilantro tastes good. The universal practice of moral argument — including every argument ever mounted against the Church — is a daily demonstration that everyone believes in the law written on the heart. The Church’s insistence on natural law is not an imposition on public reason but its defense — the claim that when the strong wrong the weak, the weak have something better than a grievance: they have a case.