Thomas Aquinas and Natural Law Theory


Natural law is a theory in ethics and philosophy that says that human beings possess intrinsic values that govern their reasoning and behavior.  Natural law maintains that these rules of right and wrong are inherent in people and are not created by society or judges.  This is in contrast to what is called “positive law” or “human law,” which is defined by statute and common law and may or may not reflect the natural law.[1]  Unlike those laws enacted by governments to address specific needs or behaviors, natural law is universal, applying to everyone, everywhere, in the same way.[1]

The prohibition of murder is a clear example of natural law. This law, unlike some civil or criminal laws, is universally accepted and understood without the need for formal legislation. It is inherently understood, for example, that taking another human life unjustly is morally wrong.

Historical Review

Aristotle (384–322 BCE) held that what was “just by nature” was not always the same as what was “just by law,” that there was a natural justice valid everywhere with the same force and “not existing by people’s thinking …”  More than 300 years later, the Apostle Paul wrote in Romans 2:14-15 that “Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. They show that the requirements of the law are written on their hearts …[2]

Even early economists of the medieval period, including Thomas Aquinas and the Scholastic monks of the School of Salamanca, heavily emphasized natural law as an aspect of economics in their theories of the just price of an economic good.[3]

Natural Law Theory can be applied to human conduct by both theists and atheists.  The atheist uses reason to discover the laws governing natural events and applies them to thinking about human action.  Actions in accord with such natural law are morally correct.  Those that go against such natural laws are morally wrong.  For the theists there is a deity that created all of nature and created the laws as well and so obedience to those laws and the supplement to those laws provided by the deity is the morally correct thing to do. [4]

The Roman Catholic Church understands natural law to be inherent in nature; this understanding is in large part due to the influence of Thomas Aquinas (1225-1274 A.D.).

Aquinas was the greatest figure of thirteenth-century Europe in the two preeminent sciences of the era: philosophy and theology.  He took his inspiration from antiquity, especially Aristotle, and builds something entirely new. Viewed from a theological perspective, Aquinas has often been seen as the summit of the Christian tradition that runs back to Augustine and the early Church. Viewed as a philosopher, he is a foundational figure of modern thought.[5]

Aquinas argues that because human beings have reason, and because reason is a spark of the divine, all human lives are sacred and of infinite value compared to any other created object, meaning all humans are fundamentally equal and bestowed with an intrinsic basic set of rights that no human can remove.[6]

For Aquinas what role, if any at all, does God have when it comes to morality?  For him, God’s commands are there to help us to come to see what, as a matter of fact, is right and wrong rather than determine what is right and wrong. But then this raises the obvious question: if it is not God’s commands that make something right and wrong, then what does?  Does not God just fall out of the picture? This, according to Aquinas, is where Natural Law Theory comes in.[7]

The likely answer from a religious person as to why we should not steal, or commit adultery is “because God forbids us.” Or if we ask why we should love our neighbor or give money to charity then the answer is likely to be “because God commands it.”  Drawing this link between what is right and wrong and what God commands and forbids is what is called the Divine Command Theory (DCT).

This view of morality in which what is right is what God commands, and what is wrong is what God forbids, is one that ties together morality and religion in a way that is very comfortable for most people, because it provides a solution to pesky arguments like moral relativism and the objectivity of ethics.[8]

However, the DCT seems to fall apart in what is known as the Euthyphro Dilemma.

The Euthyphro Argument comes from Plato’s dialogue in which Socrates asks: Is an action wrong because God forbids it or does God forbid it because it is wrong?

If God forbids an action because it is wrong, we admit that there is some standard of right and wrong that is independent of God’s will.  We concede that the wrong action was already wrong prior to God’s forbidding them.

If an action is wrong because God forbids it, then we reduce ethics to religion.  God’s Commands are arbitrary.  If things aren’t right or wrong or good or bad independent of God’s commanding or forbidding them, then it seems God has no basis on which to choose what to command and what to forbid.  He has no good reasons for forbidding the things he forbids.

Either God commands something is right because it is, or it is right because God commands it. If God commands something because it is right, then God’s commands do not make it right, His commands only tell us what is right. This means God simply drops out of the picture in terms of explaining why something is right.[9]

Thomas Aquinas never explicitly addresses the Euthyphro dilemma, but he rejected the Divine Command Theory because of these very logical dilemmas it presented.[10]

Aquinas scholars often put him on this side of the issue: Aquinas draws a distinction between what is good or evil in itself and what is good or evil because of God’s commands, with unchangeable moral standards forming the bulk of natural law.

Aquinas distinguishes four kinds of law: (1) eternal law; (2) natural law; (3) human law; and (4) divine law.

Eternal law is comprised of those laws that govern the nature of an eternal universe.  One can “think of eternal law as comprising all those scientific (physical, chemical, biological, psychological, etc.) ‘laws’ by which the universe is ordered.”[11]

Divine law is concerned with those standards that must be satisfied by a human being to achieve eternal salvation. One cannot discover divine law by natural reason alone.  The precepts of divine law are disclosed only through divine revelation.

Natural law is comprised of those precepts of the eternal law that govern the behavior of beings possessing reason and free will. The first precept of the natural law, according to Aquinas, is the imperative to do good and avoid evil. Here it is worth noting that Aquinas holds a natural law theory of morality: what is good and evil, according to Aquinas, is derived from the rational nature of human beings. Good and evil are thus both objective and universal.

Human Law that which is promulgated by human beings. But Aquinas is also a natural law legal theorist. In his view, a human law is valid only insofar as its content conforms to the content of the natural law; as Aquinas puts the point: “Every human law has just so much of the nature of law as 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,”[12] To paraphrase Augustine’s famous remark, an unjust law is really no law at all.

The Principle of Double Effect

Imagine a case where a soldier sees a grenade thrown into her barracks. Knowing that he does not have time to defuse it or throw it away, he throws himself on the grenade. It blows up, killing him but saving other soldiers in his barracks. Is this wrong or right? Aquinas says this is morally acceptable given the Principle of Double Effect. If we judge this act both internally and externally, we’ll see why.

Aquinas’s Doctrine of Double Effect which states that if an act fulfils four conditions, then it is morally acceptable. If not, then it is not.

The first principle is that the act must be a good one.  The second principle is that the act must come about before the consequences.  The third is that the intention must be good.  The fourth, it must be for serious reasons.

The intention – the internal act – was not to kill himself even though he could foresee that this was certainly what was going to happen. The act itself is good, to save his fellow soldiers (1). The order is right, he is not doing evil so good will happen (2). The intention is good, it is to save her fellow soldiers (3). The reason is serious, it concerns people’s lives (4).


In summary, for Aquinas everything has a function (a telos) and the good things to do are those acts that fulfil that function. Some things such as seeds, or the wings of a bird, just do that naturally. However, humans are free and hence need guidance to find the right path. That right path is found through reasoning and generates the “internal” Natural Law. By following the Natural Law, we participate in God’s purpose for us in the Eternal Law.[13]


[1] This is discussed in some detail under theories of Behavioral Economics:  The Investopedia Team. “Natural Law.”, 19 Dec 2023, “Natural Law in Ethics.” Investopedia, Rohrs%20 Schmitt. Accessed 3 Mar. 2024.‌

[1] ““Natural Law: Definition and Application.” ThoughtCo,‌. Accessed 3 Mar. 2024.‌

[2] Britannica, The Editors of Encyclopaedia. “Natural “Law”. Encyclopedia Britannica, 28 Feb. 2024,

[3] Izbicki, Thomas, and Matthias Kaufmann. “School of Salamanca.” Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta and Uri Nodelman, Fall 2023, Metaphysics Research Lab, Stanford University, 2023, Accessed 5 Mar. 2024.

[4] Ethics – An Online Textbook Copyright Stephen O Sullivan and Philip A. Pecorino 2002. All Rights reserved. Chapter 7. Deontological Theories: Natural Law Section 4. Natural Law Theory

[5] McInerny, Ralph, and John O’Callaghan. “Saint Thomas Aquinas (Stanford Encyclopedia of Philosophy).”, 7 Dec. 2022,

[6] Maritain, Jacques (9 October 2018), Human Rights and Natural Law, UNESCO.

[7] Dimmock, Mark, and Andrew Fisher. “Chapter 4. Aquinas’s Natural Law Theory.” OpenEdition Books, Open Book Publishers, 19 Apr. 2018, books.‌

[8] “Divine Command Theory and the Euthyphro Argument.” Matt Mullenweg, 19 Feb. 2003,‌

[9] “Handout 1 – the Euthyphro Problem.”, Accessed 6 Mar. 2024.‌

[10] “Divine Command Theory and the Euthyphro Argument.” Matt Mullenweg, 19 Feb. 2003,‌

[11] Himma, Kenneth Einar . “Natural Law.” Internet Encyclopedia of Philosophy, 2022,

[12] (ST I-II, Q.95, A.II).

[13] Dimmock, Mark, and Andrew Fisher. “Chapter 4. Aquinas’s Natural Law Theory.” OpenEdition Books, Open Book Publishers, 19 Apr. 2018,