Thomas Aquinas is one of the most influential philosophers in Western thought. He made substantial contributions to the natural law tradition in ethics through his synthesis of ancient Greco-Roman philosophy with his Christian worldview. In this article, Thomas D’Andrea sets forth the content and structure of Aquinas’s ethics.

For Aquinas, the universe is to be understood as God’s creation, directed by God’s eternal law toward perfection for God’s glory. The natural law is simply the eternal law as planted in the very design or nature of any particular creature, manifested through inclinations toward what is good. Non-rational creatures strive toward their perfection automatically. Creatures with intellect and will such as humans achieve their perfection through free, informed cooperation with God's plan for them; they therefore need knowledge of the natural law if they are to direct their actions well.

All humans with a properly functioning intellect and a minimum of experience can know the most fundamental precepts of the natural law without the aid of a special communication (“revelation”) from God. But because individuals cannot immediately grasp all of the historical details of God’s designs for them, human communities create supplemental laws to help guide their members to fulfillment. Such man-made laws are truly laws only if they agree with the precepts of the natural law.



Thomas Aquinas was born in 1225 at Rocca Secca, a hilltop castle between Rome and Naples. At the age of five, he entered the Benedictine monastery at Monte Cassino as an oblate. Some years later he began studies at the University of Naples, where he was introduced to the newly translated texts of Aristotle as well as to the newly founded Dominican Order of Preachers. Despite the protests of his noble family, Thomas became a Dominican and studied for a time under Albert the Great in Cologne. Thomas’s later career included time in the papal courts of Italy, in Paris, and in Naples. In 1274, as he was making his way to the Council of Lyon, Thomas fell ill and died at the Cistercian abbey of Fossa Nova.

Albert the Great once exclaimed of his student, “We call him the dumb ox, but in his teaching he will one day produce such a bellowing that it will be heard throughout the world.” Aquinas was a prolific, systematic writer. He contributed numerous commentaries as well as the great theological treatises Summa contra Gentiles and Summa Theologiae. Thomas Aquinas was canonized as a saint of the Roman Catholic Church in 1324, just fifty years after his death.  Because of his great philosophical and theological contributions, he was recognized as a Doctor of the Church in 1568.

To read more about Thomas Aquinas's life and works, see this encyclopedia article or visit




having to do with passions and desires, as opposed to reason and intellect.


possessing structural or architectural qualities.


a fourth-century-B.C. Greek philosopher who wrote vastly influential works on logic, metaphysics, physics, rhetoric, poetry, politics, and ethics. His doctrines exerted great sway over medieval thought and found their most famous expositor in Aquinas. For more information on Aristotle and his relationship to the natural law tradition, please see the section of this website on "Aristotle, Natural Law, and the Founders."


of, relating to, or characteristic of Aristotle or his philosophy. See also ARISTOTLE.


having to do with thought and reason.




the study or view of the universe as a whole, along with its origins, structure, and nature.

eternal law:

the dictates of reason by which God governs the universe.

first principles:

the basic truths from which one begins a rational argument. The first principle of natural law as worded by Aquinas is “good is to be done and pursued, and evil is to be avoided.” See also SELF-EVIDENT.

instantiate; -ed; -ing:

to serve as a concrete example or “instance” of a general idea.

man-made law:



the branch of philosophy that studies the fundamental nature or first principles of reality and what it means to “be.” See also FIRST PRINCIPLES.

natural law:

the imprint of the eternal law in the design of man’s inner makeup or “nature” (sometimes called man’s “share of” or “participation in” the eternal law). From this law man gets the inclinations to do and seek out the kinds of things that are good for human beings. The natural law’s dictates are self-evident, reasonable, and good. See also ETERNAL LAW and SELF-EVIDENT.


a fourth- and fifth-century-B.C. Greek philosopher. He is arguably the most influential philosopher in history and became most famous because of the many “Dialogues” that he wrote. He believed that beyond the visible world was an invisible world of the Forms: eternal patterns that give reality its structure and shape. For Plato, the Forms, and not God, are what make things exist. For more information on Plato and his relationship to the natural law tradition, please see the section of this website on "Platonic Philosophy and Natural Law."


of, relating to, or characteristic of Plato or his philosophy. See also PLATO.

positive law:

law that is created, enacted, and enforced by men, as opposed to eternal or natural law, which men discover. See also ETERNAL LAW and NATURAL LAW.


having to do with the soul and the body.


description of a truth that cannot be proven by referring to some other truth; such a truth is its own proof or evidence (hence “self-evident”). Anyone who understands correctly the meaning of a self-evidently true statement necessarily also understands that the statement is true.

Self-evident statements are not always “obvious to everyone” as the term is often understood today. If someone does not understand a self-evident truth to be true, the statement is not therefore not self-evident; rather, the person in question does not understand the meaning of some part of the statement.

The principle of non-contradiction is a classic example of a self-evident truth: “a thing cannot be said to be both true and false in the same way at the same time” (one could not say that “God is always good, but sometimes He is evil”).

Aquinas taught that the first principles of the natural law were self-evident.


able to exist independently; not reliant on anything else for existence; uncaused.


relating to Stoicism, a school of ancient Greco-Roman philosophy that advocated harmony with nature and the ordering of the passions to reason.



I. Introduction – Aquinas’s legacy and work

A. Aquinas’s work synthesized ancient Greco-Roman philosophy with Christian revelation.

B. Aquinas’s natural law doctrine must be understood in relation to his whole work.

C. The divisions of the Summa Theologiae discuss God’s nature and how God brings about creation’s perfection.

II. Aquinas on human nature

A. Man is a union of soul and body, and he is perfected by a harmony of his capacities and powers.

B. Intellect is the highest capacity of man.

1. In knowing things, the intellect unites itself with those things.

2. Because man’s highest end is to be united with God, man must know God.

III. God, law, and creation

A. Unlike Aristotle, Aquinas says that God is not only perfect in Himself but is also the creator of the universe who orders everything to perfection.

B. Created beings without intellect and will automatically tend to their perfection; creatures with intellect and will achieve perfection only through conscious, rational decisions and the free cooperation of the will.

C. Since rational beings (humans) must will their perfection but also have the ability to reject it, law becomes necessary as an extrinsic guide to perfection.

IV. The nature of law and the eternal law

A. Law is oriented toward not only personal human fulfillment, but also the good of society.

B. Law must be authoritative and public, binding and directing those subject to itself.

C. God is the supreme lawgiver, and He designs and directs the universe through the eternal law.

D. Through the eternal law, everything in creation comes to its perfection.

V. Eternal law and natural law

A. Humans, with intellect and will, must act in accord with the eternal law freely and with understanding.

B. Humans can discover the eternal law from within by reflecting on their nature.

C. The natural law is a rational creature’s share of or “participation in” the eternal law, which therefore can be discovered by reason.

D. Natural law can be known without special revelation from God.

VI. Natural law and self-evidence

A. The natural law is composed of basic, self-evident precepts.

1. The most fundamental precept is that good is to be pursued and evil is to be avoided.

2. Other principles specify what things are to be pursued or avoided.

B. Humans can grasp that certain acts are perfective of human nature. Some acts require further moral reasoning, though not every person is equally skilled in such reasoning.

VII. Conclusion: Natural law and positive law

A. Humans cannot fully know the eternal law; they must invent man-made or “positive” laws in communities to supplement (but not to contradict) the precepts of the natural law.

B. Humans must derive specific precepts from the general ones, or apply the principles of the natural law to real-life contexts.

C. Only those positive laws that do not contradict the natural law are truly laws, and must therefore be obeyed.

D. Positive laws against the natural law are irrational and are not laws, but they may still be obeyed for prudential reasons.



Part I. Basic Interpretation of Aquinas

If you are interested in the thought of Thomas Aquinas after reading Dr. D’Andrea’s essay, please go to the Primary Source Documents to read some of the essential passages of the Summa Theologiae that relate to the article. Please also refer to the biography. As you go back to the primary sources, keep in mind the following questions:

  1. In what sense is the natural law “natural”?
  2. What is the relation between natural law and positive law? Why do people need to create positive laws?
  3. Does Aquinas think that the knowledge of God is necessary for knowing the natural law?
  4. How does Aquinas define “law”? What is the difference in obedience to the law for humans and for non-rational creatures?
  5. What is distinctive about Aquinas’s view on human nature?

Part II. Connections to Other Thinkers

In order to understand Aquinas, it is important to place him in his proper context. While Aquinas inherited the Aristotelian-Platonic tradition, he was also one of the pioneering figures of the natural law tradition and has been one of the most influential natural law thinkers over the past seven hundred years. By comparing Aquinas to thinkers before and after him, his own unique philosophical position will be more greatly illumined.

  1. For Aquinas, the proper effect of law is to lead its subjects to virtue. In contrast, contemporary liberal thinkers such as John Rawls or Ronald Dworkin argue that the state and its laws should not take any position on moral matters but rather should try to let each citizen pursue his own conception of virtue and the good. To justify this position, these thinkers invoke the “harm principle,” according to which the state should make illegal only those action which would harm other people. Compare this liberal notion with Aquinas’s ethics. Are these positions directly at odds, or is there some common ground? Do Rawls and Dworkin adopt Aquinas’s idea that law should lead men to virtue, but just the virtue of tolerance? Or does Aquinas adopt a version of the harm principle, but just a more comprehensive version of it?
  2. Locke and Hobbes argue that people give up rights by contract when they enter into society. Consider Locke and Hobbes in relation to Aquinas. For Aquinas, do people give up their rights upon entering society? Does the notion of a “right” even make sense according to Aquinas’s moral framework?
  3. Hobbes explicitly rejects the "old moral philosophers" who use such teleological notions as Finis Ultimis (utmost aim) and Summum Bonum (greatest good) to describe human nature individually and in society. However, the ostensible end or aim of Hobbes’s system of natural rights is to attain “civil peace” for society. How does Hobbes’s “civil peace” differ from Aquinas’s notion of “common good”? And how do their respective understandings of law contribute to those formulations of what is good for society?
  4. Aquinas is one of the greatest philosophers to write in the Aristotelian tradition, appropriating and expounding Aristotle’s views on physics, metaphysics, politics, and ethics. What are some of the ways in which Aquinas’s theory of natural law crucially differs from Aristotle’s?
  5. As D’Andrea writes, Aquinas held that “[a]ny human law . . . that directly contravenes a dictate of the natural law ipso facto fails as a law and has the status of an irrational command instead.” So it seems that, for Aquinas, what is immoral is also irrational. Compare this to the position of the New Natural Law Theory that immoral acts are nevertheless rational acts.
  6. According to Aquinas, man is a “psychosomatic unity” composed of body and soul. This view stands in contrast to Hobbes’s mechanistic view that man is only a machine, seeking pleasure and avoiding pain. What differences do these divergent premises of Aquinas and Hobbes lead to in their respective philosophies?
  7. Thinkers like Niccolo Machiavelli and Francesco Guicciardini have argued that a ruler must assume that men are evil and capricious. Thus, for Machiavelli and Guicciardini, in order to preserve the order of the state, certain brutal measures have to be enforced because people cannot be made virtuous and will not obey without coercion. Moral laws must be suspended to preserve the state. Compare this view to Aquinas’s position that moral laws must always be obeyed in every act, and that the aim of law is to make men virtuous and good. Is politics that follows morality (like Aquinas’s) practical? Is “practical” even the right criterion for judging a political system?

Part III. Critical Interpretations of Aquinas

With a basic understanding of Aquinas’s thought, let us examine his work more critically. Are his arguments persuasive? Can we expand on Aquinas’s thought and determine what he would say about issues that he did not directly address? Use the questions below as your guide:

  1. As D’Andrea says, law for Aquinas is “an extrinsic source or principle of human perfection.” At the same time, however, the natural law is based on human nature. Indeed, Aquinas says elsewhere that if man’s nature were different, so also would be his duties. It would seem, then, that natural law is intrinsic rather than extrinsic. How would Aquinas respond to this apparent contradiction?
  2. Aquinas argues that the human mind can grasp precepts of natural law as self-evidently good. Being self-evident, these precepts cannot be proven from any other principles; they are fundamental. So how does Aquinas justify holding to these principles when they cannot be proven? Moreover, how could they really be self-evident when many people doubt them?
  3. For Aquinas, it is “normative or rational” to obey human laws if they are not in conflict with the natural law. Moreover, should laws against the natural law “be observed for prudential reasons”? Isn’t obeying an immoral law itself immoral?
  4. According to Locke, a government is legitimate only by consent of the governed. Does Aquinas hold a similar view? Consider a scenario in which a man, by immoral means, takes control of a country and declares himself dictator. Suppose further that, having seized power, the dictator implements laws that are perfectly in accord with natural law. Since those laws are reasonable and ethical, it seems that, according to Aquinas’s philosophy, the citizens of that country would be obligated to obey those laws, and therefore would not be justified in leading a revolution to re-establish the old system of government. Thus it would appear that Aquinas’s ethics could justify a tyrannical or dictatorial system. Do you think this conclusion follows from Aquinas’s position? If not, what would Aquinas say in response?
  5. D’Andrea writes that “law does not recommend or suggest, it binds and commands.” In Aquinas’s view, this applies not only to positive law, but also to natural law. Is Aquinas convincing in arguing that natural law is binding? Indeed, in what sense is natural law binding? Does this view only make sense with God as the divine lawgiver?   

Part IV. Contemporary Connections

Aquinas has had a tremendous influence on moral and political thinking in Western society. Therefore, let us now turn to some contemporary issues and see how Aquinas’s thought may be applied to them.

  1. In modern society, one alternative moral system to natural law is utilitarianism, which holds that we should seek to maximize happiness or pleasure and minimize their opposites. A clear example of utilitarian reasoning concerns the issue of torture. Supporters of torture argue that the pain inflicted is ultimately used for a greater good, namely, gaining important information in order to save more innocent lives. While utilitarianism is usually placed in contrast to Aquinas’s natural law ethics, it still seems to follow Aquinas’s most basic precept of natural law: “anything good is to be pursued, and the opposite of this good, evil, is to be avoided in all human acts.” Indeed, it would seem that the torture is performed for the sake of pursuing some greater good and avoiding some greater harm. This raises the question of whether natural law can be used to justify performing evil for the sake of good. While one might be tempted to say no, Aquinas does hold that immoral laws should (only) be “observed for prudential reasons, such as to avoid some greater harm that might arise in the social order.” Is Aquinas a utilitarian? If not, what does Aquinas say that would indicate otherwise? Are some acts intrinsically evil, and therefore not to be done?
  2. Is a natural law approach viable in contemporary society, when there is great disagreement over both what the precepts of natural law are, and whether the natural law approach is even the best form of ethics? Or is natural law viable only for the medieval Europe of Aquinas’s day, which was united under a shared set of moral standards?
  3. Much political debate has focused on whether the good of the individual or the good of the state should be given priority. On the one hand, proponents of individualism often contend that individuals are persons and therefore have absolute worth, while the state is merely an abstract entity with no inherent worth. On the other hand, proponents of statism argue that the state, because it is concerned with everyone in society, aims at the greater good and should therefore be given priority. What would Aquinas say about this debate? Are the goods of the state and of the individual necessarily in conflict?
  4. With the advances of science and technology, many people would argue that the world can be understood in purely physical terms. Is such a materialistic view of the universe compatible with Aquinas’s natural law philosophy? Is Aquinas’s natural law dependent upon his metaphysical and theological views, or can it still be defended apart from them?
  5. As the world becomes more globalized, the issue of international relations becomes increasingly important. Organizations such as the United Nations and international codes such as the Geneva Convention have attempted to regulate actions in the global community. This brings up crucial issues related to natural law. For instance, what role should natural law theory play at the international level? More interestingly, the question that the case of international law raises is (a) under what conditions a law becomes binding, and (b) whom it binds. According to Aquinas’s philosophy, is one obligated to abide by the U.N.’s proclamations simply because one lives in the world? Or can one choose not to obey them because one does not recognize the U.N. as an authority? In brief, when does a law legitimately apply to oneself, and when can one rightly reject it?