In this essay, Samuel Gregg describes the role of ius gentium, or “law of nations,” within the natural law tradition. For the Romans, the ius gentium originated in the natural law and was the class of written law that applied regardless of citizenship. The idea that the ius gentium was essentially positive law derived from the natural law persisted through the time of Aquinas. However, in the early-modern period, groups of Jesuits and Dominicans defined the ius gentium as law constituted “by the common consent of the people.” Ius gentium also came to refer to the laws which govern interactions between states, or simply to the domestic laws which were in place nearly everywhere. For Francisco Suárez, this understanding maintained the tie to the natural law because a failure to follow the ius gentium was an indicator of a corrupt society. In the modern period, Hugo Grotius and Samuel von Pufendorf treated ius gentium more as a law between states (thus weakening its connection to the natural law), though both viewed it to be higher than mere convention. By the 18th century, the concept of ius gentium as the common law of humanity was largely marginalized. Only faint echoes of the early-modern natural law tradition remained as the conception of the law of nations became increasingly focused on the application of the laws in Hobbes’s “state of nature” to state relations, and in particular to international commercial relations.



Francisco Suárez was born in Granada, Spain in 1548. He entered the Society of Jesus at the age of 18 in 1564, and for eight years he taught philosophy and theology across Spain and Europe. His scholastic education grounded him firmly in the philosophical tradition of Thomas Aquinas, and his efforts to continue that tradition emerge in his large body of work.

Though Suárez taught extensively in the early years of his academic career, he did not publish his first work, De Deo Incarnato, until 1590. Subsequently he published twelve other volumes, including the two for which he is perhaps most famous, Disputationes Metaphysicae (1597) and De Legibus (1612). Suárez develops the natural law tradition most directly in De Legibus, in which he posits the inability of human reason to discern acts as right and wrong without God’s provident command. Among other issues he also addresses the necessity of human government and the origins of political power. Suárez laid early foundations for international law and is arguably one of its founders. His scholarship had a deep influence on later philosophers, including Hugo Grotius, René Descartes, and Gottfried Leibniz.

To learn more about Francisco Suárez’s life and writings, please click here.


Hugo Grotius was born in Delft, Holland in 1583. Raised in an economically stable and well educated family, Grotius showed signs of brilliance as a child. At the age of eight he could write Latin elegies, and at fifteen he accompanied Dutch politician Johan von Oldenbarnevelt on an embassy to France. Upon completing his law degree from the University of Orléans as a young man, he began a lengthy legal, political, and diplomatic career.

Impressed by Grotius’s highly successful law practice, Prince Maurice of Nassau appointed him Attorney General of Holland, Zeeland, and West Friesland in 1607. Though in the later years of his life he pursued more closely the problem of Christian unity, his early political support for religious freedom cost him his prestigious position. Although he was promoted from Attorney General to Pensionary of Rotterdam in 1613, he was later imprisoned by Maurice and a group of Calvinists after they overthrew supporters of religious freedom in the States General. In 1621, Grotius escaped prison in Amsterdam and fled to Paris, where he continued a period of prolific writing.

Grotius’s broad background in the legal field and foreign diplomacy shaped his scholarship on international and natural law. He was the author of approximately five-dozen books, including his two most famous, De iure praedae commentarius (Commentary on the law of prize and booty) and De iure belli ac pacis (On the laws of war and peace). Grotius devoted himself to questions about the origin of nations and their government by positive law, the legitimacy of law, and why humans ought to obey it. After a harrowing shipwreck and another harsh sea voyage, Grotius died in 1645 in Germany.

To read more about the life and writings of Hugo Grotius, please click here


Samuel von Pufendorf was born in Dorfchemnitz, a small town in Saxony, in 1632 and grew up a firsthand witness to the turmoil following the Thirty Years' War. His academic and diplomatic career would later be shaped by his childhood experience of political strife. Homeschooled until the age of thirteen, Pufendorf then entered a subsidized school where he developed a special passion for and proficiency in the classics. His competency in Latin and Greek was formative in his scholarship on natural law.

During his time as a student at the University of Leipzig and in his year of study at Jena, Pufendorf expanded his educational background quite rigorously. He studied philosophy, jurisprudence, and mathematics and was exposed to the works of Galileo, Grotius, Descartes, and Hobbes, all of which deeply influenced his own thought and works.

Throughout his life Pufendorf often moved between different academic positions that were invariably intertwined with politics and diplomacy. In 1658, Pufendorf assumed his first post as tutor to the family of Paul Julius Coyet, Sweden’s envoy to Copenhagen. When the members of the envoy were imprisoned during Charles X Gustav’s war against Denmark, Pufendorf wrote his first major work, Two Books on the Elements of Universal Jurisprudence, which draws extensively on the theory of Grotius and Hobbes. After he was freed from prison, Pufendorf spent the next ten years of his life as a professor at the Palatinate’s university at Heidelberg and then at the university at Lund in Sweden, where he was a professor of the law of nature and of nations, and of ethics and politics.

In 1672 and 1673, Pufendorf wrote two other major works of his political theory canon, On the Law of Nature and of Nations and On the Duty of Man and Citizen According to Natural Law. In 1677, Pufendorf entered the final phase of his academic and diplomatic career when he took up the responsibilities of royal Swedish historian and later of privy councilor and private secretary to the dowager queen Hedwig Eleonora. In one of his last works, Of the Nature and Qualification of Religion in Reference to Civil Society, Pufendorf argued for the separation of church and state as a path to achieving religious toleration. He died of complications from a stroke in 1694 in Berlin.

To read more about Pufendorf’s life and works, please click here.


Aquinas, Thomas:

(1225-1274) a Dominican priest whose most famous work, the Summa Theologica, provides a philosophical and theological explanation for most of the Church’s teachings, including those on the law and the natural law. Aquinas distinguishes the ius gentium from the ius naturale on the grounds that the former is promulgated through positive laws, and from the ius civile on the grounds that it [ius gentium] is necessary for justice and social order in any community, easily deducible from the natural law, and applicable across jurisdictional boundaries. For more information on Thomas Aquinas and his relationship to the natural law tradition, please see the section of this website on “The Natural Law Theory of Thomas Aquinas.”


(2nd century A.D.) a Roman jurist and author of the Institutes. For Gaius, the ius civile was a particular law of the state, while the ius gentium was the law “observed by all mankind.” In Rome, the ius gentium was formally distinct from the ius civile because it applied to citizens and non-citizens alike, whereas each person was subject to the ius civile of his own state, even when in another.

Grotius, Hugo:

(1583 – 1645) a Dutch jurist and philosopher who helped develop international law based on the natural law. Though he considered the ius gentium to be the law between states, distinct from the civil law and grounded in the law of nature, he noted that because ius gentium is positive law, it is a product of human will.

inter nationes:

between nations.

Isidore of Seville:

(560 – 636) a Spanish archbishop who, in his Etymologiae, listed institutions like peace treaties and the treatment of prisoners in wartime as part of the law of nations because they applied to almost everyone.

ius civile:

the law particular to a state; the law based on custom or legislation that applied exclusively to Roman citizens. (Compare with IUS GENTIUM and IUS NATURALE.)

ius gentium:  

the law of nations. Originally a term used within ancient Roman jurisprudence to codify the treatment of foreigners, it later came to refer to law common among nations. Over time, the concept was more or less associated with the first principles of natural law but became increasingly concerned simply with the application of the early-modern understanding of the laws of nature in the so-called “state of nature” to state or international relations. By the 18th century, the term was even more narrowly focused on international commercial relations.

ius naturale:

the natural law, discernible through human reason.

Pufendorf, Samuel von:

(1632 – 1694) a German philosopher and jurist, Pufendorf considered the ius gentium to be more than mere convention but accepted Grotius’s argument that the law of nations referred to the law between states as opposed to the law shared by all humanity.

Suárez, Francisco:

(1548 – 1617) a Jesuit priest and scholastic philosopher who situated the ius gentium between the natural and positive law, though closer to the natural. This included the civil laws governing property and domestic commerce and, as he thought more fitting, international laws.

Treaty of Westphalia:

a series of treaties signed in 1648 that ended the Eighty Years’ War between Spain and the Dutch Republic and the Thirty Years’ War in the Holy Roman Empire.

Vattel, Emer de:

(1714 – 1767) a Swiss philosopher, diplomat, and legal theorist who played an important role in the development of natural law. In his most famous work, Droits des Gens (1758), he treats the law of nations as simply the law of nature of individuals in the state of nature applied to states. Unlike Hobbes and Pufendorf, Vattel believed that since nations and individuals were different, they also had different rights and obligations.

Vitoria, Francisco de:

(1483/1486 – 1546) a Dominican jurist and theologian who played a significant role in the development of international law and just war doctrine. He shifted the emphasis of ius gentium from inter omnes homines to inter omnes gentes, that is, from the law among all individual men to the law among all nations.


I.               Classical origins of ius gentium, or “law of nations”

A.     The ius gentium is first articulated by Greek and Roman classical philosophers and jurists.

B.     In Gaius’s Institutes,  ius gentium is associated with ius naturale.

1.     Ius civile is the special law of the state.

2.     Ius gentium is the “law observed by all mankind.”

3.     Ius naturale differs from ius gentium in that ius naturale originates in human reason, whereas ius gentium is its practical application.

C.     A second sense of ius gentium within Roman law emerges to address the treatment of foreigners rather than just the application of law according to citizenship.

D.     Isidore of Seville (560 – 636) preserves and codifies Gaius’s concept of the ius gentium in his Etymologiae and lists some customary laws that appear to have force amongst almost all peoples.

II.             Medieval treatment of ius gentium

A.     Thomas Aquinas (1225 – 1274) agreed with Gaius’s distinction between ius civile and ius gentium and argued that ius gentium is the aspect of the positive law deduced from the natural law and application across jurisdictional boundaries.

1.     Aquinas uses the examples of contracts and property, which are necessary for justice and social order in any community, to show that  ius gentium is deduced from natural law (and hence not merely ius civile).

2.     Since ius gentium is promulgated through the positive law, it is distinct from natural law proper.

III.           Early modern period: breakdown of distinctions

A.     Some Jesuit and Dominican scholastics thought all laws were either natural or positive, but since these categories rendered ius gentium redundant, they introduced instead the new definitional feature that ius gentium is constituted by “the common consent of the peoples.”

B.     Francisco Suárez situated ius gentium between the natural and positive law, identifying two groups:

1.     Laws that are part of the domestic law of most commonwealths, e.g., laws governing property and domestic commerce (civil law).

2.     Laws that coordinate relationships between peoples and commonwealths (laws inter nationes), e.g., laws governing war, international commerce, and the treatment of diplomats.

C.     Francisco de Vitoria made a similar point in shifting the emphasis of ius gentium from inter omnes homines (among all men) to inter omnes gentes (among all peoples).

D.     Suárez believed that the ius gentium had a universal character. It extended to everyone, not because all people consented to its content, but because all people independently recognized its content.

1. Failure to recognize the ius gentium was considered evidence of a society’s corruption.

IV.            Rise of the modern nation state: further modification

A.     The Treaty of Westphalia (1648) brought about a reconsideration of the role of ius gentium in international law.

B.     Hugo Grotius (1583 – 1645) took up the notion of “a body of law that is maintained between the states” that was grounded in “the law of nature and nations,” but he slightly diminished the connection of ius gentium to the natural law.

C.     Samuel von Pufendorf (1632 – 1694) further weakened the connection by accepting Grotius’s idea that the law of nations is the law between states rather than the universal human law, and he also adopted Hobbes’s idea of the law of nature and its division into the laws of man and those of states.

1. Though he rejected Hobbes’s belief that the state of nature was one of war, he nonetheless held that laws were necessary to maintain a rather feeble peace.

V.              18th-century changes

A.     The concept of ius gentium as the common law of humanity became largely marginalized.

B.     Emer de Vattel in Droits des Gens (1758) referred to it as the law of nature of individuals in the state of nature applied to states.

1. Unlike Hobbes (and Pufendorf), he distinguished nations from individuals, especially with regard to obligations, rights, and commercial relations.


Part I. Basic Interpretation

If you are interested in learning more about the ius gentium after reading Samuel Gregg’s essay, please go to the Primary Source Documents to read some of the critical passages that were referenced in the essay. As you return to the primary sources, keep these questions in mind:

1.         What is the ius gentium?

2.         How did it change over time?

3.         What is the relationship between the natural law and the ius gentium?

4.         To whom does the ius gentium apply? In what sorts of situations?

5.         How much authority does the ius gentium have? What is the source of this authority?

6.         What role does consent play in the ius gentium for the different thinkers mentioned?

Part II. Connections to Other Thinkers

In order to properly understand the ius gentium, it is important to understand the different ways it was used in various historical contexts.

1.  Compare Aquinas’s view of the ius gentium to Pufendorf’s. What implications did this have for the way in which it was employed? Did this reflect a changing view of the natural law? Compare their views to that of Hobbes.

2.  Isidore of Seville was unfamiliar with the sorts of cannibalistic societies that informed Montaigne’s thinking. Would the introduction of societies that disobey the natural law undermine Isidore’s system? Can Montaigne or Isidore identify cannibalism as wrong?

3.  How does Gaius’s description of the source of law’s authority compare to Locke’s view about popular sovereignty?

4.  Gaius’s definition of ius gentium seems very similar to Isidore’s definition of the natural law. What distinctive role does ius gentium play in Isidore's thought? Does this represent a difference in his legal philosophy?

5.  Does Pufendorf's reference to a "state of nature" (see Hobbes) represent a fundamental break with the preexisting ius gentium tradition? Many Enlightenment-era critics [link] of the natural law rely on "state of nature" reasoning. Do Pufendorf's arguments contain explicit or implicit criticisms of the older understanding of the ius gentium or natural law, or is his reference to the "state of nature" simply a different way of framing the issue and a reordering of terminology? 

Part III. Critical Interpretation

Now that you have a basic understanding of the ius gentium, let us turn a more critical eye towards the works in which the concept is used and debated. Is it a useful concept? Is it applicable to other issues that are not directly addressed by these authors? Use the questions below as your guide:

1.         Isidore of Seville writes that the natural law is “common to all nations,” whereas the law of nations is so called “because almost all nations observe this law.” If the distinction of the natural law is that every nation recognizes it, does a nation's non-recognition of a law mean that it can't be a natural law? Is there room in Isidore's thought for a nation’s failure to adhere to the natural law? If not, is the natural law fundamentally distinct from other forms of the law, including the law of nations?

2.         Aquinas addresses this question in Article 4 of Question 95 of the Summa Theologiae. Is Aquinas’s defense of Isidore a compelling one? To understand Objection 1, look to Article 2 “I answer that..." Be careful to note Aquinas's discussion of animals. (Emer de Vattel also addresses this point when he discusses the distinction between natural law and the ius gentium.)

3.         Consider Gaius's discussion (§3-6) on the source of laws' authority. Is it implicit that their basic source of authority lies in the ius gentium? If so, why do they vary from place to place? (Aquinas might prove helpful in answering this question.) Or, if they are ultimately nothing but positive law, how can they be authoritative?

4.         Can civil laws conflict with the ius gentium? If so, does this mean the civil laws lose their authority? Or are they simply not laws? Do Gaius’s and Isidore of Seville’s accounts sufficiently address this question?

5.          In the essay, Gregg argues that Pufendorf weakened the ius gentium; indeed, in "The Law of Nature and Nations" Pufendorf denies that “there is any positive law of nations proceeding from a superior.” Evaluate his argument here. Does it entail a denial of natural law together? How does he distinguish nations from individuals? In what way does it weaken the ius gentium?

Part IV. Contemporary Connections

Let us now turn to some contemporary debates and see how the ius gentium tradition may be helpful in addressing them:

1.         Many modern theories of international relations—most notably, realism—assume anarchy among nations. In Hobbes’s view, such a state of nature (among states) inevitably leads to war; for Pufendorf, war was not a necessary consequence. For Emer de Vattel, the law of nature provided these states with different maxims from those of individuals because states are a different type of entity. How should states determine appropriate actions on the international scale? Do different theories of ius gentium prove helpful in answering this question?

2.         Consider the question of humanitarian intervention. Because the United States is committed to republican principles, it’s easy for us to identify the wrongness of human rights violations and political oppression. Because of these same republican principles, however, we believe that sovereignty rests on the consent of those governed, and thus, perhaps, are hesitant to intervene against popularly supported injustice. Does the source of authority for the ius gentium shed any light onto how we might address these questions? How might the different thinkers answer this question?

3.         In 1998, former Chilean dictator Augusto Pinochet was indicted internationally for human rights violations. The European judges appealed to the principles of universal jurisdiction, but Pinochet’s lawyers argued that he could not be tried because he had been a head of state. Can your studies of the ius gentium help answer the question of when individuals, specifically state leaders, can be tried internationally? If this is allowed, what can be the basis for the international law under which they are tried?

4.         In Article 1, Reply to Objection 2 of Question 95 of the Summa Theologiae, Aquinas justifies the written law by quoting Aristotle: “it is easier to find a few wise men competent to frame right laws, than to find the many who would be necessary to judge aright of each single case.” The U.S. Constitution was written for such a function, but how should critics of the founders approach it? For instance, many of the Framers held Lockean beliefs, and someone who is unimpressed with Locke (or who believes that the American Revolution was not a just war) is likely to question the wisdom of the framers. Would this undermine the validity of the law? Or does convention mandate that we conform to the constitutional tradition regardless of its founders’ wisdom?

5.         Can the ius gentium be discovered in a society where individuals don't share many values or in a world where nations don't share many values? Or, if ius gentium is merely a description of the values that are shared, could it ever be politically or rhetorically useful? Is it possible to argue about the content of the ius gentium? What would be required for a nation, or a quorum of nations, to prevent certain laws from being included in the ius gentium?