Natural Law

St. Thomas Aquinas on the Natural Law.

After his Five Ways of Proving the Existence of God (ST Ia, 2, 3), St. Thomas Aquinas is probably most famous for articulating a concise but robust understanding of natural law. Just as he claims and demonstrates in his proofs for God’s existence that natural human reason can come to some understanding of the Author of nature, so in his exposition of natural law, Aquinas shows that human beings can discover objective moral norms by reasoning from the objective order in nature, specifically human nature. While Aquinas believes that this objective order of nature (and the operation of human reason which discovers it) are, in fact, ultimately grounded and established by God’s intelligent willing of the good of creation (i.e., His love), Aquinas’s understands that one need not know that God’s providence underpins the objective order of nature. Thus, whether or not one believes in or rationally proves there is a God, one can recognize and be bound by the natural law for, according to Aquinas, it applies to all people at all times, and in some sense is known by all rational humans (though, of course, they do not (but should) always act in accordance with it).

Eternal Law

In his monumental Summa Theologiae, St. Thomas Aquinas, devotes relatively little space to the natural law – merely a single question and passing mention in two others. There, he bases his doctrine of the natural law, as one would expect, on his understanding of God and His relation to His creation. He grounds his theory of natural law in the notion of an eternal law (in God). In asking whether there is an eternal law, he begins by stating a general definition of all law: Law is a dictate of reason from the ruler for the community he rules. This dictate of reason is first and foremost within the reason or intellect of the ruler. It is the idea of what should be done to ensure the well-ordered functioning of whatever community the ruler has care for. (It is a fundamental tenet of Aquinas’ political theory that rulers rule for the sake of the governed, i.e., for the good and well-being of those subject to the ruler.) Since he has elsewhere shown that God rules the world with his reason (since he is the cause of its being (cf. Summa Theologiae I 22, 1-2)), Aquinas concludes that God has in His intellect an idea by which He governs the world. This Idea, in God, for the governance of things is the eternal law. (ST I-II, 91, 1)

Defining the Natural Law

Next, Aquinas asks whether there is in us a natural law. First, he makes a distinction: A law is not only in the reason of a ruler, but may also be in the thing that is ruled. Just as the plan or rule for constructing a house resides primarily in the mind of an architect, so that plan or rule can be said also to be in the house so constructed, imprinted, as it were, into the very composition of the house and dictating how the house is properly to operate or function (cf. ST I-II, 93, 1). In the case of the eternal law, the things of creation that are ruled by that law have it imprinted on the them through their nature or essence. Since things act according to their nature, they derive their “respective inclinations to their proper acts and ends” (final cause) according to the law that is written into their nature. Everything in nature, insofar as they reflect the order by which God directs them through their nature for their own benefit, reflects the eternal law in their own natures (ST I-II, 91, 2)

The natural law is properly applied to the case of human beings, and acquires greater precision because of the fact that we have reason and free will. It is our nature as humans to act freely (i.e., to be provident for ourselves and others) by directing ourselves toward our proper acts and end. That is, we human beings must exercise our natural reason to discover what is best for us in order to achieve the end to which our nature inclines. Furthermore, we must do this through the exercise our freedom, by choosing what reason determines to be naturally suited to us, i.e., what is best for our nature.

Now among all others, the rational creature is subject to Divine providence in the most excellent way, in so far as it partakes of a share of providence, by being provident both for itself and for others. Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law .

(ST I-II, 91, 2)

The natural inclination of humans to achieve their proper end through reason and free will is the natural law. Formally defined, the natural law is humans’ participation in the eternal law, through reason and will. Humans actively participate in the eternal law of God (the governance of the world) by using reason in conformity with the natural law to discern what is good and evil. Just as the proper functioning of the body and its organs in order to achieve an optimal physical life defines health and the rules of medicine and healthy living, so the proper functioning of all human faculties defines the human good and the rules for living well (morally, spiritually, socially, etc.). The natural law encompasses the rules and precepts by which humans do good actions and live well, individually and collectively.

The Human Good

In applying this universal notion of natural law to the human person, one first must decide what it is that (God has ordained) human nature is inclined toward. Since each thing has a nature (given it by God), and each thing has a natural end, so there is a fulfillment to human activity of living. Even apart from knowing about this dependence on God, one can discover by reason what the purpose of living is, and so he or she will discover what his or her natural end is. Building on the insight of Aristotle that “happiness is what all desire,” a person can conclude that they will be happy when he or she achieves this natural end, specifically “a life of virtuous activity in accordance with reason” (Nicomachean Ethics, I, 7; see Aquinas, Commentary, Bk. I, Ch. 10, nos. 127-8). The commands or precepts, then, that lead to human happiness or flourishing is what Aquinas means by the natural law.

Aquinas distinguishes different levels of precepts or commands that constitute or comprise the natural law. The most universal is the command “Good is to be done and pursued and evil avoided” (ST I-II, 94, 2). This applies to everything and everyone, so much so that some consider it to be more of a description or definition of what we mean by “good.” For these philosophers, a thing is “good” just in case it is pursued or done by someone. Aquinas would agree with this to a certain extent; but he would say that that is a definition of an apparent good. Thus, this position of Aquinas has a certain phenomenological appeal: a person does anything and everything he or she does only because that thing at least “appears” to be good. Even when I choose something that I know is bad for myself, I nevertheless chooses it under some aspect of good, i.e., as some kind of good. I know the cake is fattening, for example, and I don’t choose to eat it as fattening. I do, however, choose to eat it as tasty (which is an apparent, though not a true, good). A true good is an object of desire which reason determines to be appropriate or fitting to a given person, in certain circumstances, in light of their universal human nature (ST I-II, 94, 3, esp. ad 3). Sometimes this will include eating cake, but not too much of it.

Precepts of the Natural Law

The precepts of the natural law are commands derived from the inclinations or desires natural to human beings; for Aquinas there is no problem in deriving “ought” from “is.” Since the object of every desire has the character or formality of “good,” there are a variety of goods we naturally seek. They are all subsumed under the First Principle of Practical Reasoning: The good is to be done and pursued, and evil avoided. This first principle is operative in all the precepts that comprise the natural law. Subsequent precepts of the natural law derive first from various sorts of inclinations as humans share these with other sorts of natural things, and second, as one discovers these goods in greater specificity (ST I-II, 94, 2).

InclinationCommon to substances: existCommon to animals: procreateProper to humans: act rationally
Primary preceptsPreserve human life.Marry.
Have children.
Educate the young.
Know truth about God.
Preserve social harmony.
SecondaryDo not kill.
Protect the weak.
Honor your parents.
Do not commit adultery.
Do not steal.
Shun ignorance.
Avoid giving offense.
Tell the truth.
Render to each his/her due.
intermediate levels.
Particular actionSave my friend, David.Leave the bedroom.Do not lie to soldiers.
Do not reveal David’s location.

On the level that we share with all substances, the natural law commands that we preserve ourselves in being. Therefore, one of the most basic precepts of the natural law is to not commit suicide. (Nevertheless, suicide can, sadly, be chosen as an apparent good, e.g., as the cessation of pain; often it is not chosen at all, but the result of mental illness.) On the level we share with all living things, the natural law commands that we take care of our life, and transmit that life to the next generation. Thus, almost as basic as the preservation of our lives, the natural law commands us to rear and care for offspring. On the level that is most specific to humans, the fulfillment of the natural law consists in the exercise those activities that are unique and proper to humans, i.e., knowledge and love, and in a state that is also natural to human persons, i.e., society. The natural law, thus, commands us to develop our rational and moral capacities by growing in the virtues of intellect (prudence, art, and science) and will (justice, courage, temperance) (ST I-II, 94, 3). Natural law also commands those things that make for the harmonious functioning of society (“Thou shalt not kill,” “Thou shalt not steal”). Human nature also shows that each of us have a destiny beyond this world, too. Man’s infinite capacity to know and love shows that he is destined to know and love an infinite being, God, and so, natural law commands the practice of religion.

All of these levels of precepts so far outlined are only the most basic. “The good is to be done and pursued and evil is to be avoided” is not very helpful for making actual choices. Therefore, Aquinas believes that one needs one’s reason to be perfected by the virtues, especially prudence, in order to discover precepts of the natural law that are more proximate to the choices that one has to make on a day-to-day basis. As is indicated in the table above, particular precepts, as they derive from more general ones in issuing forth in action, can conflict with each other. The command to save David might conflict with the apparent injunction that one ought not to lie to soldiers bent on unjustly executing him, but might cohere with the injunction not to give them information they have no just right to. Applying the natural law to cases, then, is more open to error, even though the general principles are true and apparent, and there is, in fact, a true and most rational application to certain particular circumstances (ST I-II, 94, 3).

Application of the Natural Law as an Absolute / Objective Standard

Given that the natural law depends on the inclinations inherent in human nature (as ordained by the intelligent, loving providence of God, i.e., the Eternal Law) it applies to all people, at all times. Yet Aquinas readily acknowledges that the laws and morals of people can vary wildly. Rather than succumbing to moral relativism (where what is morally good and right is merely what each society or person thinks is such), Aquinas seeks both to uphold the objective and universal basis of morality in the natural law, and to explain the variety of moral and legal injunctions. Despite his belief that the natural law applies universally, Aquinas explains how this variety arises, first from the perspective of the generality of the precepts, and then from the perspective of the knowledge an individual has of the precepts.

From the perspective of the generality of the precepts, Aquinas reasons that the more general a precept is, the less is it open to exceptions. The general principles of both speculative reasoning (e.g., mathematics and the sciences) and practical reasoning (arts, ethics and politics) are necessary, and so the primary precepts of the natural law apply in all cases. The good is always to be done; life is always to be preserved. Yet, as one seeks to apply the general principles to particular cases, the particular circumstances necessitate greater variety in the kinds of actions required by the principle. Preserving life in the community may require the execution of murderers.

Although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects. … In matters of action, truth or practical rectitude is not the same for all, as to matters of detail, but only as to the general principles: and where there is the same rectitude in matters of detail, it is not equally known to all .

(ST I-II, 94, 4)

From the perspective of the knowledge an of individual, the variety of ways of applying the natural law also leads to variability in knowing how to so apply the principles. The more general a precept is, the more likely it is to be known by a greater number of people. The more particular a precept of the natural law (or the application of a general precept to a particular case) is, the more likely it is that a particular individual will get it wrong.

Aquinas thus concludes that the greater the detail, the more likely it will be that people disagree about what the natural law requires:

Consequently, we must say that the natural law, as to general principles, is the same for all, both as to rectitude and as to knowledge. But as to certain matters of detail, which are conclusions, as it were, of those general principles, it is the same for all in the majority of cases, both as to rectitude and as to knowledge; and yet in some few cases it may fail, both as to rectitude, by reason of certain obstacles (just as natures subject to generation and corruption fail in some few cases on account of some obstacle), and as to knowledge, since in some the reason is perverted by passion, or evil habit, or an evil disposition of nature.

(ST I-II, 94, 4)

Even though it is true that as one makes more particular applications of the general precepts of the natural law, the form that application takes is likely to be displayed in a greater variety of actions, nevertheless, the same natural law is being applied in each case, and the same natural law commands a variety of actions as given situations demand. What is the right thing to do might vary according to a variety of circumstances, yet in each case, the right thing to do is objective and necessary, a rational deduction of the certain general principles of moral action.

Just and Unjust Laws

Aquinas thus argues that the natural law cannot be changed, except by way of addition. Such additions, he says, are “things for the benefit of human life [which] have been added over and above the natural law, both by Divine law and by human laws” (ST I-II, 94, 5). Nothing can be subtracted from the natural law with regard to the primary precepts, and thus, no human law which commands something contrary to the natural law can be just. Interestingly, he notes, that certain features of society, while not being provided to humans by nature, accord with the natural law under the general principle of being “devised by human reason for the benefit of human life” (ad 3). He includes among such non-natural features as consonant with natural law: clothing, private property and slavery. Yet by introducing the condition that just additions to the natural law must be for the benefit of human life, he allows, as we’ll see below, that should they fail this condition, they would thereby be subtractions from the natural law and so, unjust.

Given the universality and objective character of the natural law, Aquinas unsurprisingly asserts that it cannot be forgotten or “abolished from the human heart” (ST I-II, 94, 6). Nevertheless, he recognizes that many people act as though they do not recognize this universal and objective standard of morality since they are inhibited by the influence of concupiscence or other passions, by an error of reasoning, or “by vicious customs and corrupt habits.” Indeed, as the third objection notes, there are whole societies which operate according to laws at variance with the natural law, declaring their departures as “just.” Aquinas responds that such laws abolish only the “secondary precepts of the natural law, against which some legislators have framed certain enactments which are unjust.” (ST 94, 6 ad 3). This nuanced understanding of the natural law, then, provides a standard for judging just and unjust laws. He makes this criterion of just laws explicit when he turns to the origin of human law.

Consequently, every human law has just so much of the nature of law, as it is derived from the law of nature. But if in any point it deflects from the law of nature, it is no longer a law but a perversion of law.”

(ST I-II, 95, 2)

Aquinas, thus, seems to grant that whatever judges a system of human laws must stand outside and above that system. The laws of Nazi Germany which prescribed the execution of Jews and dissidents and forbade their protection constituted “crimes against humanity,” not because these laws were in violation of other laws of Germany, or the laws of France or the United States. The reason that the attempted destruction of the Jews was wrong was not even because the whole rest of the world thought it was wrong. The crimes of Nazi Germany could be judged as crimes because they were violations of a law that stands apart from and above the laws of every nation; they were violations of the natural law. Natural law, then, serves as the standard against which we determine whether human positive laws are just or not.

One can see these principles at work in “Letter from Birmingham City Jail” by Dr. Martin Luther King, Jr. As King says:

I would be the first to advocate obeying just laws. One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws. I would agree with St. Augustine that “an unjust law is no law at all.” Now, what is the difference between the two? How does one determine whether a law is just or unjust? A just law is a man made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law. Any law that uplifts human personality is just. Any law that degrades human personality is unjust. All segregation statutes are unjust because segregation distorts the soul and damages the personality. It gives the segregator a false sense of superiority and the segregated a false sense of inferiority.

The justification to which King appeals in order to show that segregation laws are unjust is not other laws of Alabama or of the United States, but the natural law as it is founded in human nature. Human nature demands a true sense of equality and dignity, and because segregation laws violate that equality and dignity, they are unjust. Segregation laws clearly diminished the dignity, and thus damage the personality of blacks in Alabama. Interestingly, King asserts that segregation laws were harmful to the white majority, distorting their proper dignity and damaging their personality as well. In both cases, segregation laws are an affront to human dignity, founded as it is in our common human nature.


The Thomistic notion of natural law has its roots, then, in a quite basic understanding of the universe as caused and cared for by God, and the basic notion of what a law is. It is a fairly sophisticated notion by which to ground the legitimacy of human law in something more universal than the mere agreement and decree of legislators. Yet, it allows that what the natural law commands or allows is not perfectly obvious when one gets to the proximate level of commanding or forbidding specific acts. It grounds the notion that there are some things that are wrong, always and everywhere, i.e., “crimes against humanity,” while avoiding the obvious difficulties of claiming that this is determined by any sort of human consensus. Nevertheless, it still sees the interplay of people in social and rational discourse as necessary to determine what in particular the natural law requires.

Revised and expanded August 26, 2021

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