Some laws are unjust. Not unpopular, not inconvenient — unjust.

Saying that commits you to something, because it measures a law against a standard the law itself does not supply. Natural law is the name for the claim that such a standard exists, that it is grounded in what human beings are, and that reason can get at it without revelation.

The four laws

Aquinas's treatment in the Summa Theologiae is the reference version, and its architecture is the clearest part of the doctrine.

Eternal law. God's ordering of everything toward its end. Identical with divine wisdom, and not directly available to us.

Natural law. A rational creature's share in that ordering. Because you can reason, you can work out some of it from what human beings are — and the first principle, that good is to be done and evil avoided, is self-evident in the way that a definition is.

Human law. What particular societies actually enact. It takes its authority from natural law but is not deduced from it: natural law says contracts should be enforceable and says nothing about filing deadlines. Enormous latitude, and a limit at the edge.

Divine law. What revelation adds, covering what reason cannot reach.

The consequential claim is the one about the third. A human law that contradicts natural law is defective as law — Aquinas says it is a corruption of law rather than a law — which is where the whole subsequent argument lives.

It rests on teleology, and that matters

The doctrine is doing something specific that is easy to miss: it is crossing the is-ought gap by denying there is one.

The move requires that human nature has ends built into it. If flourishing is what a human being is for, then facts about what serves that flourishing are already normative, and no extra premise has to be smuggled in — which is exactly the response to Hume that Anscombe revived in 1958.

So natural law stands or falls with teleology. The early modern collapse of teleological explanation in physics is the deepest reason the doctrine lost ground, and it is a much better reason than the one usually given, which is that people stopped believing in God.

It does not need God

That is the other thing usually got wrong, and the tradition settled it early.

Hugo Grotius, writing in the 1620s, made the argument that became famous in a phrase: natural law would hold etiamsi daremus — even if we were to grant that there is no God. The obligations follow from what human beings are and what social life requires, and the theological frame can be removed without the structure falling over.

That secularization is what let the doctrine carry into modern politics. Locke turned the objective duties of the older tradition into subjective rights held by individuals; the American founding documents are written in that vocabulary; and the Universal Declaration of Human Rights in 1948 is its direct descendant. Contemporary natural lawyers, John Finnis most prominently, work without theological premises entirely.

The case that tested it

The doctrine's sharpest modern test was a legal system that was formally impeccable and monstrous.

The Nazi statutes were properly enacted by the recognized procedures of a recognized state. After the war, German courts had to decide whether people who had informed on their neighbors under those statutes had acted lawfully.

H. L. A. Hart, for legal positivism, argued that they had — the laws were valid law, and the right response is to say so and then condemn them as wicked and punish under retroactive legislation, openly. Lon Fuller argued that a system so at odds with the requirements of law as such had forfeited the claim to be law, so there was nothing valid to obey.

Their exchange in 1958 is the best short statement of what is at stake. Hart's position keeps the concepts clean and asks people to break the law when the law is evil. Fuller's gives them grounds to refuse at the time and risks letting anybody decide which laws are real. Neither is comfortable, and the discomfort is the subject.