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2: Sources of International Law

What systems allow international law to come into being?
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At the International Court of Justice. Photograph by the United Nations.

In this globalised era, pervasive and efficient modern technologies have opened up even the most remote village to the rest of the world. In spite of significant and persisting cultural, linguistic, religious, political and economic differences amongst countries and societies, it is now commonplace to speak of the existence of an ‘international community’. Few concepts better evoke this appeal to universal solidarity than the claim of ‘international law’.

In what follows, the meaning and scope of international law are laid out as its origins are examined. Attention is drawn to the important distinctions that exist between the language and ideals of international law on the one hand and of international politics and international relations on the other.


i) Contested meanings

Law is ubiquitous, and daily lives are enveloped in it. Although it radiates neutrality and certainty, its definition is far from straightforward[1]. The difficulty exists because explaining what constitutes law is not simply a matter of describing some “objective” reality – it invariably entails explanation and justification, an exercise that cannot be separated from one’s particular take on morality. The Western legal tradition, which has been predominant in the shaping of international law, and continues to dominate it, has been influenced by two primary philosophical schools of thought: natural law and legal positivism. Each provides a distinctive way of assessing the legitimacy of different developments in international law.

Despite the contestations that they present, a serviceable definition of law typically embodies the following:

First, law is a set of instructions or commands that are set in a form that can be communicated from one person to another. This means that, ordinarily, there is a specific written text that can be pointed to even when persons disagree about the meaning of the text.

Secondly, these instructions must be authoritatively issued (or, in the jargon of the profession ‘promulgated’) by a body, person or institution that is empowered to do so. This might be the head of a government, a council or cabinet of ministers, a legislature, court, administrative body or some other unit of leadership within the society. The relevant society can take any of several political structures: a village, municipality, state, country or even the international system at large[2]; this body is known as ‘the sovereign’[3].

Thirdly, the issuer of the instructions (i.e. the sovereign) must intend that the order bind those to whom it is addressed, and do so regardless of the desires or preferences of the addressee or subject.

In short, ‘law’ is a legitimate order that is issued by a politically competent body that commands compliance or obedience from those who are subject to the jurisdiction of the issuing body. Although there might be disagreements about the meaning or reach of a text, the authority or legitimacy of the sovereign, once such technical disputes are resolved, one should be able to say whether or not the particular set of instructions bind the addressee.

ii) Different conceptions of morality – positivism vs. natural law

For one school of legal thought – legal positivism – law is seen as the command of a duly constituted authority, which contends that the validity of a legal rule must be evaluated independently of the moral backing or content of the order. For natural law scholars, however, even a set of instructions whose text or meaning is pristinely clear, and which otherwise meets the requirements of promulgation and authoritativeness may nonetheless be viewed as not satisfying their definition of law.

Those who subscribe to this latter perspective contend that embedded in the idea of law must be the object of using its authority to promote core normative values such as justice, right behaviour and reason. Any set of instructions that do not satisfy these values cannot rightly be deemed to be law, regardless of how they were generated, or who issued them[4].

The ‘natural law’ position thus shifts the primary focus in the definition of law from the process of its creation to concerns over its content. Natural law thereby demands that a valid law must confront some fundamental questions that are otherwise avoided by legal positivists: namely, why do subjects comply with or obey laws? Is a set of instructions that is ignored or never enforced still law? In short, is the voluntary assent of the subject or his/her coerced compliance a necessary element of the definition of law? These are crucial components of the discourse of international law, and a crucial lens through which readers should assess international developments later on in the course.

These philosophical debates over the nature of law typically take place within the framework of the most common arena for the operation of legal rules; namely, the nation state. Although these ideas have equal relevance for the discourse of international law, one must nonetheless acknowledge the ways in which the making of international law may differ from that of domestic law. There are two notable distinctions that are worthy of special attention. These are 1) the absence in the international legal order of an institutionalised hierarchy for laying down the law; and 2) a similarly underdeveloped network of enforcement institutions.


The idea of international law as understood and practiced today owes its origins and foundational principles to two sets of intertwined transnational movements that radically reshaped European society during the late medieval period of European history, between the 15th and 17th centuries.

The first was the overhaul of the place of religion in European political life. Although varying from one society to another in its speed and particulars, this movement saw justifications for power transform from appealing to the divine and sacred to the mundane and secular; that is, from belief in righteously anointed rulers to leadership based on functional abilities. These transformations were fostered and facilitated by splits and breaches within the institutions and power structures of religious institutions including the emergence of Protestantism and of reform movements within the Roman Catholic Church. This divorce of the legality of temporal power from religious sanctification was enshrined in the Treaty of Westphalia in 1648, generally taken as one of the preeminent constitutive documents of modern international law.

The second late-medieval movement that gave rise to modern international law was the fierce competition among European societies for maritime voyages of discovery and the commerce that accompanied such discoveries. During the so-called ‘age of discovery’, European kingdoms and principalities vied to equip entrepreneurial merchants, geographers, scientists, seafarers, and adventurers who sailed the high seas to discover, conquer and trade with ‘new lands’ in the Americas, Africa and Asia. How to regulate this competition became an integral element of international law-making.

Thus, while internal European religious fragmentation gave rise to and shaped international law doctrines as secularism, sovereignty and self-determination, the forces of externally driven competition contributed to other international law doctrines such as those relating to the freedom of navigation on the high seas, freedom of commerce, and the use of force. In turn, these generated exceptions, and counter-exceptions, which resulted in the body of evolving doctrines and principles that currently constitute international law.


The starting place and linchpin for comprehending international law is the centrality of the role of the nation state in identifying, making and applying rules of conduct and behaviour in the international system. Indeed, prior to the 19th century, international law was commonly referred to as ‘the law of nations’.

i) From nation to individual

International law was viewed simply as the product of relations among nation states, something made by and for the benefit of nation states. Its obligations, duties and liabilities were assumed by nation states, and correspondingly, its benefits, privileges and immunities redounded exclusively to the nation state.

However, as law increasingly tries to accommodate and reflect contemporary socio-political realities, vibrant debates have emerged that seek to transcend this rigid divide. Three lines of questioning underlie this transition:

- Does international law regulate only the behaviour of states, or does it reach beyond the state to embrace interests within the state such as individuals, corporations, non-governmental organisations, or indeed intergovernmental organisations as well?

- Similarly, who are the beneficiaries of international law? Do such beneficiaries have a right to assert their beneficial interests directly and independently of the state?

- Finally, who gets to make international law? Is such law-making solely the function of the institutions of the state, or do other interested persons such as individuals, civil and corporate non-governmental organisations have a role to play in the process?

There is no single categorical answer, and current answers are by no means permanent. Central to an informed understanding of the available range of answers is an appreciation of the sources of international law.

ii) Article 38 of the ICJ

Article 38 of the Statute of the International Court of Justice (ICJ) provides a widely agreed upon set of standards for evaluating whether a statement purporting to be a rule of international law is to be validly considered as such[5]. Although Article 38 is addressed exclusively to the ICJ, which is in itself entirely a creature of international law, it has become accepted that Article 38 provides the conceptual framework for ascertaining when a pronouncement should be considered a legally-binding obligation rather than a statement of a preferred value or norm.

According to Article 38, there are four sources of international law. These are 1) conventions or treaties to which a state is a party; 2) international custom or practice that international society has come to accept as law; 3) the general principles of law that are recognised by civilised states; and 4) the views of highly-qualified jurists writing on a point of law. International lawyers seek to pigeonhole particular pronouncements into one or more of these sources of legal legitimacy. Although at the margins these elements or sources of international law are not without difficulty of application, they nonetheless have reasonably well-settled meanings.

a) Conventions and treaties

The most uniformly accepted source of international law is the convention or treaty[6]. A treaty is an agreement between two or more countries. Treaties come in numerous forms, from bilateral understandings between two friendly states, to those that end world wars or create international arrangements like the United Nations Organisation. They cover the entire scope of human activities from politics, economics and the arts to the sciences, agriculture, youth exchanges and family relations. They vary in the level of formality and solemnity with which they are concluded.

All that matters is that the parties entering into the treaty see themselves as making commitments that other parties have a legal right on which to rely. Not surprisingly then, although the existence of a treaty will rarely be beyond dispute, occasionally disputes arise over whether an agreement or understanding has been reached between two states, and if so, whether the parties intended to make binding legal commitments to each other.

These and the several other issues that are presented by the pervasive part played by treaties in international relations have been addressed in a treaty: the 1969 Vienna Convention on the Law of Treaties, otherwise known as ‘the treaty on treaties’. Like any treaty, the terms of the Vienna Convention bind only those who are parties to it, and some important members of the international society, such as the United States, are not. Yet, it is generally accepted that many of the provisions of the treaty constitute customary international law (discussed below), which binds non-parties.

Among the core provisions of the Vienna Convention are the following: treaties are made only by and among states as defined under international law; by signing a treaty, a state undertakes to comply with its provisions in good faith and not to undercut its purpose; once ratified (a process that occurs under the domestic laws of the ratifying country) and notified to the other parties, a treaty imposes legal obligations according to its terms on the ratifying party, and creates internationally-recognised interests for all parties to the treaty; a treaty should be interpreted by reference to its text which must be construed in light of the treaty’s purpose or object; the terms of a treaty should not be in conflict with certain peremptory norms of international behaviour, so-called jus cogens norms; just as parties should not be forced by duress or fraud to enter into treaties, they are also free to withdraw or renounce treaties, subject to the conditions for withdrawal or renunciation contained in the treaty.

Central to the idea of treaty law creating legal obligations and rights is the view of the state as a sovereign entity that is free to consent or withhold consent as it deems fit. Ostensibly, treaties are voluntary undertakings which, once accepted, signify a commitment to be bound, and create a reliance interest for other parties.

b) Custom as a source of law

A second uniformly-accepted source of international law is customary international law. There are three conditions under which the general behaviour of states becomes a rule of customary international law: a) if the behaviour is widespread, b) practices are followed over a not insignificant period of time, and c) it s viewed by it is practitioners as mandated by law.

Questions, however, remain: at what point does a practice become sufficiently widespread among states and of sufficient duration in time so that it should be deemed a general practice? And how does one know whether the states are engaging a practice because they view it as law, or for some other reason, such as convenience?

Nonetheless, some of the most venerated rules of international law either originated as customary practices among states that were subsequently codified as treaties, or continue to be derived from such custom. Increasingly, however, the trend is less to transform customary laws into treaty law than the reverse. Given the proliferation of treaties and the diversification of international society from its West European cultural roots, those who want a uniform standard of behaviour among states increasingly look to treaties to extrapolate customary law, often insisting it should be binding on all states regardless of participation in a specific treaty regime. The result is some sort of a symbiosis between treaty law and customary international law.

For evident reasons, ascertaining and applying customary international law in specific situations is often problematic. However, procedural problems with its crystallisation process are not its gravest. The doctrine seemingly contradicts the two established principles that undergird the treaty regime: sovereignty and consent.

The application of customary international law therefore often requires attempts to reconcile the universality of its obligations and rights with national rights such as sovereignty and consent. The result is a mishmash of doctrines. For example, on the one hand, a state that persistently objects to a particular rule of customary international law is thereby freed from compliance (the persistent objector rule); and on the other hand there are certain so-called jus cogens,peremptory rules of international law that no state can derogate from them. These problems of interpretation are compounded by the third possible source of international law.

c) & d) General principles of law and the views of highly qualified publicists

The third broadly accepted source of international law is the so-called general principles of law which is, in the phrasing of the Statute of the International Court of Justice, “recognised by civilised nations”. The primary difficulty lies in deciphering what this vague formulation actually means in specific cases.

If it is intended to incorporate no more than broad principles of law present in all reasonably developed modern legal systems – e.g. wrongs should have remedies, claimants and defendants should be given fair opportunities to present and defend their positions, decisions should be based on reasoned analysis of evidence, and the decision-maker should be impartial – it might be asked whether the rule serves any useful purpose.

If the rule is intended to go beyond broad procedural niceties to impose additional substantive obligations on states (other than those that may already exist either under treaties or customary international law), then it is fair to ask what those additional obligations might be. Perhaps here, one ought to read the ‘general principles of law’ source along with the fourth source cited in the ICJ statute: “the teachings of the most highly qualified publicists of international law”.

The function of ascertaining, sanctifying and legitimising so-called general principles of law may depend on the existence of consensus among highly-qualified international law scholars and jurists as to whether a particular behaviour or rule should be considered as mandated by international law, independently of what states may have accepted or done. What is clear is that the concept of ‘general principles of law’ under international law is at best underdeveloped. Whatever the intellectual underpinnings of international law may be, it is difficult to imagine that this unrepresentative process for fashioning ‘general principles of law’ by appeal to the teachings of publicists does not undercut its legitimacy as a tool of governance.

iii) Institutional sources of international law

The sources considered above are the explicitly stated formal sources of international law. However, as a cultural institution, international law, like any legal order, is dynamic and adaptable. It is fashioned by human beings to serve their needs which vary with time, place and environment. A final source of international law, then, must account for the human beings, institutions and the contingencies of politics and economics that shape, interpret and deploy law to serve particular ends[7]. Such laws are less the product of specific rules, doctrines or procedures than they are of the policy preferences of interested subgroups within international society.

Sometimes applicable laws are ascertained less by a formalistic resort to written texts and shared practices than they are by the needs of a particular group – power rather than reasoned analysis is here the coinage of rule-making.

Although Article 38 of the ICJ Statute may be silent about the place of power as a source of law, no-one doubts that an agreed-upon policy of the United Nations Security Council – and indeed of each the five permanent members of the Council – may well constitute international law, even though such policy does not receive the benediction of any one of the formal criteria.

The beliefs and views of diplomats, international civil servants, economists, non-governmental organisations and of course statesmen in certain circumstances now prove to be just as influential in determining international law as those of legal specialists.

International law (and indeed any legal order) cannot be seen simply in terms of static rules that can be ascertained from rule books. Law may not be politics, but no legal order is intelligible without reference to the political environment in which it operates.


Within a domestic legal order, hierarchy is established over time through the exercise of political power. This hierarchy might emanate from foundational constitutional arrangements, or it may be the result of extra-constitutional power grabs; but in either event, the structure of authority within a domestic legal order is both clear and hierarchical. This is important because of the certainty that is derived from infallibility. That finality ultimately is backed up by the coercive power of the state exerted through enforcement mechanisms.

There is no equivalent route to finality in the international system. International rules are interpreted by a myriad of institutional actors acting more or less independently of each other. There is no one institution whose pronouncements override all others, and there is no enforcement system that guarantees compliance. But this does not result in international rules being indeterminate or rarely obeyed.

It does say, however, that the level of compliance that is brought about by the existence of predictable and final rules is, at best, underwhelming. The emerging role of national, regional and specialised international courts or tribunals as interpreters and enforcers of international law may reduce some of the gaps between the aspirations of international law and its practical applications, but the chasm remains.

Put another way, the effectiveness of a legal order depends on the coherent exercise of political power within a political community. Against constant appeals to an ‘international community’, reality shows a society of national political communities. It is neither a substitute for those communities, nor does it operate independently of them. International law offers humanity at best a hope for coordinating these varied legal orders.

For more information about the course please read our introductory blog post.

Think Africa Press welcomes inquiries regarding the republication of its articles. If you would like to republish this article, provide feedback, ask or answer questions or request new content please contact or get in touch via Twitter at @romromromTAP.


Anthony Anghie, Imperialism, Sovereignty and the Making of International Law (2005)

David J. Bederman, International Law Frameworks (2d ed. 2006)

Thomas Buergenthal & Sean D. Murphy, Public International Law in a Nutshell (4th ed. 2007)

Ian Brownlie, Principles of Public International Law (7th ed. 2008)

Antonio Cassese, International Law (2d ed. 2005)

T. O. Elias, The Modern Law of Treaties (1974)

Mark Janis, An Introduction to International Law (5th ed. 2008)

Sean D. Murphy, Principles of International Law (2d ed. 2012)


1. What problems are posed by the formal equality of nation states? How does this differ from domestic legal systems?

2. What values do natural law and legal positivism embrace? Which one do you find more useful for looking at international law?

3. What historical changes brought about international law?

4. In which article and statute can one find the sources of law? What are the four sources of international law?

5. How do treaty law and customary law interact and in what ways do they oppose each other?

6. What problems are there with ‘general principles of international law’ and the ‘teachings of the most highly qualified publicists’?

[1] The branch of law that worries about this sort of issues is known as “jurisprudence.” Among those who have contributed substantially to this branch of learning in the Western “canon” are Plato, Aristotle, Cicero, Thomas Aquinas, Thomas Hobbes, John Locke, Immanuel Kant, Jeremy Bentham, John Austen, Oliver Wendell Holmes Jr., Lon Fuller, H. L. A. Hart, John Rawls, and Ronald Dworkin, to name but a few of the most prominent scholars.

[2] Whether one ought to include in this definition rules made by nonpolitical bodies such as religious organizations, sporting federations or cultural associations is purposefully left out of the discussion that follows.

[3] In “democracies,” the “sovereign” is idealised and said to reside in “the people.”

[4] This school of thought is often associated with philosophers of law who are either religiously inclined, and/or who see “morality” as an integral element of law. Recent struggles for civil and human rights, social justice, and against abortion and the death penalty have provided much emotional support for the intellectual arguments of natural law scholars.

[5] National legal orders have national equivalents, even if they are not quite as explicitly stated as Article 38. In the United States of America, for example, it is accepted that an authoritative official statement can be law if and only if it does not contravene the written Constitution of the United States. In the United Kingdom, on the other hand, there being no written Constitution, it is generally said that any bill that is passed by parliament thereby becomes the law of the land. In many post-colonial African states with their very fluid histories of both constitution-making and unmaking through military coup d’états and internal insurgencies, ascertaining the appropriate institutional framework for resolving this difficulty can be at times no mean task.

[6] A convention or a treaty may be referred to by a variety of other appellations such as “covenant,” “pact,” “agreement” or even “understanding.” Under international law, the actual term used is of no consequence as long as the parties to the agreement intend that the instrument in question creates legally binding undertakings between or among the parties who have subscribed to it. See The Vienna Convention on the Law of Treaties, Article 2.

[7] International lawyers have referred to these participants in the making of international law as the “invisible college”; but the self-deprecation should not lead to an off-handed dismissal of the importance of these participants as law-givers.

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"what problems are posed by the formal equality of nation states?" er... how about start with the fact they aren't equal?! and international law can't do its job properly unless they've got a big country with lots of guns willing to back it. so the big country with the big army is always going to get away with more stuff. if america tells togo what to do its gonna do it.

What I always wonder is how international law is going to change in the next century considering the rise of non-Western countries like China and India. If the system we have now has largely been developed since Westphalia in 1648, then it is a product of quite a unique three-and-a-half centuries of massive European dominance. Now, at the beginning of the 21st century, Asian powers are being able to properly flex their political muscles on the world stage. Surely, then, in the future the path taken by international law will begin to reflect traditions from the great Asian powers? More Confucius, less Thomas Aquinas perhaps?

@ jules_rules I don't know what you mean about more powerful nations. I reckon Tuvalu could give China a run for its money in a nuclear arms race any day.

@jules_rules Of course all nations aren't equal! That's why it says "formal equality"

What is clear is that the concept of ‘general principles of law’ under international law is at best underdeveloped. Whatever the intellectual underpinnings of international law may be, it is difficult to imagine that this unrepresentative process for fashioning ‘general principles of law’ by appeal to the teachings of publicists does not undercut its legitimacy as a tool of governance.I most certainly concur.  It is also my understanding that the debate over the very origin of the said 'general principles of law' as enumerated by Art. 38 of the ICJ Statute, has been gaining prominence in recent years, particularly as it pertains to the inclusion of non-Western, non-Eurocentric legal concepts.  Would Western states be willing to abide by such principles should the latter ever be labeled as 'general'?  Very informative analysis!