Natural Law and Its Precepts

Aquinas's whole account of natural law rests on a single foundational claim. In Question 94, Article 2, he states what he calls the first precept of natural law:

Read quickly, this sounds like a platitude. It is actually the opposite: it is a claim about the basic structure of practical reasoning. Aquinas is saying that whenever we deliberate about what to do, we are presupposing that the good is to be pursued. That presupposition is built into the very act of choosing. We never sit down and ask "should I pursue the good?" because pursuing what we take to be good is just what choosing is.

What makes this a substantive principle is that it puts a structural constraint on ethics. Whatever else turns out to be true about morality, Aquinas is saying, any specific moral precept will take the form of an application of this first one. "Do not murder" is a specification of "do good and avoid evil" once we have worked out that murder is evil. "Honor your parents" is a specification once we have worked out that honoring parents is good. The first precept does not by itself tell us what is good and what is evil, but it tells us the form that all further moral reasoning has to take.

Once the first precept is in place, Aquinas needs to fill in what counts as good. His method, which has been influential ever since, is to start from the basic natural inclinations of human beings. We are drawn, by our nature, toward certain things. Those inclinations reveal what is good for us, and the natural law directs us to pursue those goods.

Aquinas identifies three layers of natural inclination, corresponding to three layers of our nature.

First, we share with all substances the inclination toward self-preservation. Every being naturally resists its own destruction. From this Aquinas derives the natural law precept that we should preserve our own life and protect ourselves from what would destroy us.

Second, we share with other animals the inclination toward species preservation and the raising of young. Animals reproduce and care for their offspring. From this Aquinas derives precepts about marriage, sexual conduct, and the obligations of parents to children.

Third, and distinctively, we have as rational creatures an inclination toward the good of reason itself: to know the truth, including the truth about God, and to live in community with other rational beings. From this Aquinas derives precepts about honest inquiry, about avoiding ignorance where we can help it, and about the requirements of social life: not harming those we live among, not lying to them, honoring legitimate authority.

This three-layer structure is worth noticing. Aquinas does not treat humans as disembodied reasoning machines. He treats us as the kind of animal we actually are: physical, social, reproductive, rational. The natural law has things to say about all of these dimensions, because our good spans all of them.

Notice also that the precepts become more specific as we move down the layers. "Preserve your life" is almost as general as the first precept itself. The social precepts derived from our rational nature (do not bear false witness, honor contracts, respect property) are specific enough to do real moral work. Aquinas believes all of these precepts are accessible to human reason without revelation, and all are binding on all human beings.

Aquinas's universality claim is sharp. Natural law, he says, is the same for all human beings, because human nature is the same for all human beings. If natural law follows from what we are, and we are all humans, then what counts as good for us should be the same.

The obvious objection is that different cultures have held very different moral beliefs. Some societies have practiced things that other societies found monstrous. If natural law were really universal, shouldn't we find more agreement?

Aquinas's response, in Article 4 of Question 94, is nuanced. He distinguishes between first principles and proximate conclusions, on one hand, and secondary precepts and remote conclusions, on the other. The first principles of natural law (the basic inclinations and the most general precepts derived from them) are the same everywhere and known to everyone. You will not find a culture that sincerely believes gratuitous cruelty is good or that breaking promises for no reason is admirable.

Secondary precepts, the more specific applications, can fail to be known. Aquinas offers two explanations for this. First, people can be corrupted by what he calls "evil persuasions" or bad customs: living in a society where some evil is normalized can genuinely warp people's moral perception. Second, reasoning from general principles to specific conclusions is hard, and people can make honest mistakes. A community that has never thought carefully about slavery, for instance, might fail to see that it violates the precepts they already accept.

This is a philosophically significant move. Aquinas is admitting that moral disagreement exists while denying that it undermines the universality of natural law. The disagreement, on his view, is evidence of human fallibility, not of the absence of an underlying moral order.

Does natural law change over time? Aquinas says no, in the sense that the fundamental precepts remain what they are. But he allows that natural law can be added to (human communities can develop more detailed codes that do not contradict natural law) and that people can come to understand natural law more fully over time. What cannot happen is for a true natural law precept to become false or vice versa. Murder was always wrong, even in the era when people did not yet see that some forms of killing they practiced counted as murder.

This position is philosophically demanding. It requires us to hold both that moral knowledge is genuinely universal and that human beings are capable of massive, sustained, collective moral failure. The conjunction is not impossible, but it is a lot to defend. Whether Aquinas can defend it is one of the live questions in natural law theory today.

In Question 95, Article 2, Aquinas asks whether every human law is derived from natural law, and whether a human law that contradicts natural law really counts as a law at all. His answer is bold. Following Augustine, he writes:

The famous Latin slogan summarizing this position is lex iniusta non est lex: "an unjust law is no law." The claim is not merely that unjust laws are bad. The claim is stronger: an unjust law, on Aquinas's view, lacks the very thing that makes something a law in the first place.

Remember the four-part definition from Question 90: a law is an ordinance of reason, for the common good, made by proper authority, and promulgated. A statute that contradicts natural law fails to be an ordinance of reason, because reason is what natural law is. Such a statute may have been enacted by a lawful authority and published in the appropriate form, but it still lacks the essential feature of law. It is an act of violence dressed up in legal clothes.

This is a theoretically powerful claim and a practically consequential one. If it is true, citizens living under an unjust human law are not bound in conscience to obey it, and may even have a duty to resist it. Aquinas is careful: he allows for prudential reasons to comply with unjust laws where resistance would cause greater evil. But the philosophical position is clear. A genuinely unjust law has no moral authority, because moral authority flows only from its proper source, the natural law.

This thesis has had a long afterlife. Martin Luther King Jr., in his 1963 Letter from Birmingham Jail, drew explicitly on Aquinas to explain why he was willing to violate the segregation laws of Alabama. An unjust law, King wrote, following Aquinas, is no law at all. The segregation statutes may have been enacted through proper legal procedure, but they failed the test of conformity to natural law, and therefore did not bind the conscience of the person asked to obey them.

You do not have to be a Thomist to find this thesis attractive. You also do not have to accept it uncritically. The obvious worry is: who decides what counts as an unjust law? If everyone is authorized to disobey laws they personally find unjust, the result is chaos. Aquinas's response is that the standard is not personal preference but natural law, which is in principle objective and accessible to reason. Whether that response is adequate is one of the questions the reflection prompt will ask you to take up.

Watch this lecture after you have worked through the material above. It will reinforce the structure of natural law theory and connect it to ongoing debates.