In this essay, Walter Nicgorski describes the formative influence of Cicero on natural law theory. Cicero, an ancient Roman philosopher and statesman, set out one of the earliest expositions of natural law, thus providing intellectual inspiration for many medieval and modern thinkers, including the American Founders. Drawing on both Greek philosophy and the Roman juridical tradition, Cicero uses probing arguments to demonstrate the existence of a natural law that fulfills both human nature and the divine order of the universe. Specifically, he grounds both the knowledge and implementation of natural law in the practice of virtue. This gives him the ability to confront challenges to the very idea of natural law, and to show how it can be useful in making practical decisions, such that familiarity with his thought is essential for any contemporary student of the subject.



Marcus Tullius Cicero was born on January 3rd, 106 BC in a town near Rome, the eldest son of a wealthy landowner. At an early age, Cicero displayed great interest in, and capacity for, philosophical inquiry and public speaking. Cicero studied in Rome and Rhodes, served as a soldier and, in 75 BC, was elected to the office of quaestor, which brought him membership in the highest council in the Roman Empire: the Senate. Meeting with early success in political life, Cicero enjoyed widespread popularity. However, the years of the triumvirate of Julius Caesar, Pompey, and Crassus were exceedingly turbulent for Cicero. He was exiled for more than a year before he was able to return to Rome, serving as a lawyer for eight years. It was during this time that Cicero wrote some of the most influential texts on natural law, such as On the Republic and On the Laws.

In 49 BC Caesar crossed the Rubicon River and overthrew the Roman government under Pompey. Caesar became dictator for life and Cicero was forced to stay out of politics. Most of the rest of his life was devoted to studying and writing about philosophy. After Caesar's assassination, Octavian, Antony, and Lepidus established themselves as the dictators of Rome, outlawing many of their enemies, including Cicero. Attempting to flee belatedly, Cicero was murdered at the hands of Antony's men on the 7th of December, 43 BC.

Although his own experience of politics was quite turbulent, Cicero contributed some of the most important works on republican government, providing the philosophical inspiration for post-Renaissance thinkers such as Locke, Hume and Montesquieu. Indeed, Cicero's thought and very phrases reached even to America's founding generation.

To read more about Cicero's life and works, please click here.  



academic skepticism:

a school of thought originating in Greece in the 3rd century BC. It considered itself to be following the tradition of Plato even while it emphasized suspension of belief or withholding absolute assent though approving the probable. The latter was the form of skepticism of the New Academy to which Cicero professed allegiance. It was prepared to use the standard of the probable with respect to all claims to knowledge including those concerning a right course of action.


Fourth- century-BC Greek philosopher. He was Plato’s most famous student, and, though he wrote extensively on all topics, is particularly admired for his virtue-based system of ethics. 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."  

cardinal virtues:

practical wisdom, temperance, courage, and justice. These four virtues were believed to encompass the basic requirements of character for living a good life, and all other virtues were either derived from them or were subsets of them.  

civil law:

Here, civil law encompasses all the explicit laws enacted by the state, as opposed to the natural law. It does not refer to the division within the written law between civil and criminal law. 


a popular form of philosophical writing in the classical period in which arguments are advanced through stylized discussion among several characters.


a school of Greek philosophy noted for its materialism; disbelief both in gods involved in any way with human life and in the immortality of the soul; the pursuit of pleasure as the highest good; and insistence on following a way of life detached from civic affairs. All of these beliefs were strongly opposed by Cicero.  


a natural tendency to behave in a certain way, which, when ingrained in a habit and confirmed in the intellect and will, has the potential to become either a virtue or a vice. 


the idea that everything in the universe, including what we would call the “mind” or “soul,” is composed of matter. 


of the school of philosophy founded by Aristotle (see also ARISTOTLE).


Fourth-century-BC Greek philosopher. His work, the Republic, an extended dialogue about the nature of justice, whether it is truly preferable to injustice, and the best form of government for man, deeply influenced Cicero and provided the model for his work of the same name. 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."


Fifth- century-BC Greek philosopher known for his vigorous style of debate in which all beliefs are systematically questioned, and for his unprecedented emphasis on morality as the proper subject for philosophical reflection.


a school of Greek philosophy which Cicero admired and helped to introduce to the Latin-speaking world. It taught that there was a unifying cosmic order in nature, and that men could live in accordance with nature through the practice of reason and virtue. 


usefulness, or effectiveness in achieving a pre-determined goal. In ethics, a contrast is often set up between performing the action that is good for its own sake, and doing the action that fulfills some definition of utility.


an ingrained habit of character that is conducive toward living a good life. 



I. Cicero’s ideas and influence

A. Early proponent of the idea that what is right has a basis in nature
B. Was admired by medieval and early modern thinkers, and the American Founders

II. Cicero’s intellectual forebears

A. Drew on various Greek philosophers, while retaining a critical stance towards them

1. Plato, Aristotle
2. Stoicism

B. Socrates and Academic Skepticism

1. Most closely aligned with this school.
2. Focus on ethical and political questions.
3. Takes questioning stance and uses argumentative style to arrive finally at the truth.

III. Cicero on natural law

A. Republic

1. Reflections on the nature of justice and the ideal state.
2. Consideration of the order in the universe leads to belief in the “law of nature.”
3. Authority of the law of nature precedes that of individual states, is a source of rights and duties.
4. Variation in civil law is no argument against the natural law.

B. On the Laws

1. Natural law stems from human nature and man’s role in the universe.
2. Natural law is accessible to human reason.
3. A reasonable person is the best source for knowledge of natural law.

C. On Duties

1. Developing inclinations into virtues fulfills human nature and the natural law.
2. The right and the useful do not actually conflict.
3. Application of the idea of right to specific moral and political problems.

IV. Conclusion

A. Cicero one of the first thinkers to use explicit natural law language.

B. Natural law stems from divine order, but is knowable through reason.

C. Natural law then becomes the standard of justice and right.



Part I. Basic Interpretation of Cicero

For further information about Cicero after reading this article, please refer to his biography. You can also read the full passages regarding the natural law in On the Republic, On Duties, and On the Laws here. The following questions are meant to focus your thinking as you read further. 

  1. What is the relationship between the divine law, human nature, and natural law?
  2. How did Cicero embody the principles of Academic Skepticism?
  3. How do Cicero's formulations of natural law differ in On the Republic, On Duties, and On the Laws
  4. Does Philus present a convincing criticism of natural law? Why or why not?
  5. What is the relationship between law and virtue in Cicero's ethics?
  6. How does Cicero use the idea of the common good in relation to the natural law?

Part II. Connections to Other Thinkers

Cicero, in trying to adapt Greek philosophy for the Roman world, was naturally influenced by many of the Greek thinkers who preceeded him, and he in turn influenced many other medieval and early modern philosophers. As you look deeper into Cicero's writings, think about these questions in order to see how his work fits into the broader history of ideas.

  1. Cicero considered himself greatly indebted to Plato and Aristotle. Are there any natural law concepts to be found in these thinkers?
  2. Compare Cicero with Aristotle in the Nicomachean Ethics, where it says that we should look to a virtuous man as our moral exemplar. 
  3. One of Cicero's broad philosophical projects was to translate the work of Greek thinkers for a Roman audience. What in his writings specifically reflects the Roman legal and political system, as opposed to that of the Greeks?
  4. Nicgorski writes: "In reaching into nature and learning from her, the wise person shares in a divine force or the very mind of god." Compare this to the idea of Aquinas that the natural law is "man's participation in the eternal law of God."
  5. "Seeds of this true law remain in all, never to be entirely obliterated." Is Cicero correct? If so, why must it be the case that one can never entirely lose sight of the natural law? Is there something contradictory, especially in a Judaeo-Christian framework, in the idea of a person being completely evil in every respect? Compare with the Aristotelian/Thomistic idea that in acting, one always seeks some good.
  6. What was Cicero's influence on the early modern political philosophers, such as Locke, Hume, and Montesquieu?
  7. Thomas Jefferson specifically cited Cicero as a source for ideas in the Declaration of Independence. Are there any passages in the Declaration that seem particularly Ciceronian?

Part III. Critical Analysis

Now that you are more familiar with Cicero's arguments as expressed in the readings, let us subject them to further scrutiny. Use the following questions to help understand the underlying basis of his thought, further consequences of his arguments, and whether his conclusions are convincing.

  1. Virtue-based and law-based systems of ethics are often contrasted with each other. Yet Cicero seems to want to have it both ways. He says that virtue is necessary to know the natural law, but also writes: "It is proper for the law to be the corrector of vices and the recommender of virtues." What is the proper relationship between law and virtue?
  2. Cicero writes: "And one god will be in common as though he were a teacher and general of all the people. He will be the author, umpire, and provider of this law" (De Republica). To what extent does Cicero's concept of natural law depend upon a divine lawgiver? Is it possible to have any sort of understanding of natural law without a belief in the divine?
  3. In On the Republic, Philus argues that there cannot possibly be a universal natural law, since actual laws and customs vary so much. Cicero has a rebuttal to this argument, but what if Philus were to make the weaker claim that variation in civil law implies that the natural law must at least be difficult to discern correctly? What if he were to note not only the disagreements in civil laws, but precisely all of the disagreements about what is contained in the natural law? Does natural law cease to be a useful tool in public discourse when so much uncertainty as to what it constists of remains?
  4. Does disagreement about the natural law point to a lack of virtue in those trying to discover it, since virtue is supposedly necessary to know the natural law? Cicero says that if there is uncertainty about the natural law, one should look to "the mind and reason of the prudent man." But are people who disagree about the natural law going to agree on what prudent man to use as an example? Did Cicero, living in a society that had more moral consensus than our own, believe that all reasonable people would agree about the natural law? To what extent is it possible for reasonable people to disagree?
  5. Cicero writes that human nature is "the source [in which]...laws and right can be found." What is the relationship between the natural law and human nature? Does our nature somehow require us to follow the natural law, however imperfectly? Or is it just that people will only be happy/flourishing/at peace with one another if they follow the natural law? If the natural law truly leads to human flourishing, does this mean that empirical data can never be used to discover the natural law, by showing what sort of behavior, on average, leads to success in life?
  6. Speaking through Laelius, Cicero writes that "not every law is indeed a law," and thus the variation in positive law does nothing to impugn the natural law. He seems to be saying that laws that go contrary to the natural law are not actually laws. What does this mean? For they can have all the relevant appearances of law. Is he saying that unjust laws need not be obeyed? Would he advocate civil disobedience? Does one always have a right to resist an unjust law? Is one required to resist it? How does this answer change when one considers whether the law merely hurts you, or whether it hurts others?
  7. Stoicism, with its emphasis on a deep underlying order in nature, heavily influenced Cicero. Moreover, in the "Dream of Scipio," reflection on the order of the cosmos gives rise to a belief in natural law. What does he mean by there being an "order in nature"? If it only means that the universe is governed by the laws of physics, this seems uncontroversial. And yet there are many who believe in the laws of physics, but not in a natural law for man. To what extent does order in the physical world give us information about order in the moral sphere?
  8. Cicero argues that "natural justice means that one must never do harm and must always serve the common good" [Nicgorski's words]. Such a statement, however true, seems rather unhelpful for giving guidance on concrete issues. For instance, consider the contentious issue of whether to ban smoking in restaurants and public places. Does the ban serve the common good by reducing smoking and protecting people from the harms of secondhand smoke? Or does it hurt the common good by reducing freedom of choice and harming the business of restaurant owners? Obviously, there are many moral situations in which someone will be harmed no matter what option is chosen. "Never do harm," therefore, instead of providing a simple litmus test on whether an action is acceptable, is a guideline that one must know how to apply correctly. Does this mean, therefore, that prudence is required not only to discover the natural law, but also to carry it out? Is it the case in general that the natural law, even once determined, is not specific enough to be carried out without the virtue of practical reasonableness?
  9. "The absence of a written law at Rome concerning defilements during Lucius Tarquinius's reign did not mean that Sextus did not violate Lucretia." This seems to imply that natural law retains its force regardless of whether it is confirmed by positive law. Does this mean that positive law does not add any moral force to something already confirmed by the natural law? Is it just in place so that the state can legally punish certain offenses?