In today’s world, international law (hereinafter: IL) plays an important role in governing the mutual relations between states. This article aims to critically analyze the current theories surrounding its sources. The traditional positivist theory is based on Article 38 of the ICJ Statute, we will take a closer look at these formal sources and the theory’s current evolution (I). An analysis of the current theory of informal sources of law, focusing on unilateral declarations of states and soft law will provide us with a general overview of the sources of IL (II).
I FORMAL SOURCES OF POSITIVE INTERNATIONAL LAW
In the 19th century naturalism ruled the theory of IL. This approach defines IL as a complete system with an inexhaustible source of law emerging from the will of God or human reason (ratio). However, from the 20th century it is the school of positivism that dominates the theorization of the sources of IL. It should be noted that in this view, IL faces practical incompleteness in particular fields of law. The positivistic approach to the traditional theory of sources is based on the will(s) of states to be legally bound by an international rule. Five formal sources can be identified, as enshrined in Article 38 of the ICJ Statute: Treaties, customary IL, general principles of law, judicial decisions and teachings.
- Treaties are generally seen as international agreements between states. Treaties are based on the principle of good faith and embody the idea of explicit consensualism, as States may only be bound to the extent expressly consented. Treaties have a contractual nature as illustrated in articles 26 (pacta sunt servanda) and 34 (pacta tertiis nec nocent nec prosunt) of the VCLT. Because of this contractual nature it is very easy to determine each state’s precise obligations. It is important to note that under national law contracts cannot be seen as law, yet in IL a treaty, as a contract, creates law.
- Rules of customary IL (hereinafter, CIL) are created and sustained by a general practice of states and their conviction that such practice is legally required (opinio juris). The major weakness of this source lies in the difficulty of the determination of the exact moment of its existence and precise content, causing a greater degree of legal uncertainty. As this source is based on the idea of tacit voluntarism, it is a contrario recognized that states are not bound by CIL if they persistently, expressly and publicly object to the rule during its formation. However, this creates an inconsistency in the law as traditional custom IL presupposes that all states act in a uniform way. General statepractice should be virtually uniform and extensive. Most weight will be accorded to the actions of the States who have a special interest in the subject-matter. This differentiation between States is also inconsistent with the basic theory of custom IL. Furthermore, regional CIL is not established, yet debatably bilateral CIL is recognized by the ICJ. As for the temporal element, even instant customary law may be accepted. Again, this approach seems highly inconsistent with the general understanding of custom IL requiring a long established practice. Concerning the subjective element of opinio juris, the acceptance of a state that a CIL reflects a legal norm may be deduced from the State’s attitude and may even tilt the scale in favor of the existence of CIL, even when the general practice is inconsistent, producing yet another inconsistency of the law.
It is useful to briefly explain the relation between treaty and CIL. In principle treaty rules only bind member states, however if they become a rule of CIL, the rule itself will also bind third states. Moreover, the practice of codification, and progressive development of CIL in/by (draft) treaties is encouraged to meet the criticism regarding the difficult determination of the exact scope of CIL.
- General principles of law are a subsidiary source of IL based on the nation's common principles of positive law. They are mainly used to fill lacunae in the law and to avoid the risk of non-liquet. The principle of good faith is seen as one of the most important examples.
- Judicial decisions and Teachings are also subsidiary means to identify the applicable rules in IL. As for the case law, there is no formal rule of stare decisis, yet its substantial status seems to evolve towards the application of the own case law of the court as precedents. This does not seem illogical as courts are encouraged to adopt a consistent logical reasoning.Teachings have become less significant but still influence IL, as they regularly serve as the basis for negotiations in diplomatic conferences and may be consulted in case of dissenting opinions.
The traditional theory of the sources of IL stipulates that there is no precedential order, nor any hierarchical order among the formal sources of IL. However, it is currently accepted that there are three exceptions to the no hierarchical order principle:
I. First of all, obligations under the UNC prevail over other IL obligations between member states.
II. Secondly, a hierarchy exists in the framework of international human rights. Non-derogable human rights prevail over other human rights. However, the idea seems debatable as it would entail a contrario that these other human rights might be derogated in the state of emergency, which would be highly problematic given that multiple nations currently find themselves in an emergency state due to the Covid-19 pandemic. Moreover, in an extreme case such as a (civil) war it is unlikely that a non-derogable right, like the right to life will be respected. Furthermore, in the latter case, the independent system of international humanitarian law becomes applicable revealing further inconsistencies in the law.
III. Thirdly, a peremptory norm of general IL or jus cogens is recognized as enshrined in article 53 VCLT. Legally, the entire treaty or unilateral act becomes void if it conflicts with a rule of jus cogens. Furthermore, these rules cannot be circumscribed by reservations nor by countermeasures. The idea of jus cogens might be seen as highly problematic in light of the decentralized, unorganized society that is IL. It might even become dangerous for the stability of treaties as member parties or even third parties may mobilize jus cogens to declare a treaty void, escaping the principle of pacta sunt servanda. It is my personal opinion that this legal consequence should at least be limited to the articles conflicting with the jus cogens norm instead of the whole treaty. However, from a practical point of view it is very unlikely that treaties or unilateral acts would include articles conflicting with existing jus cogens rules.
The idea of obligations erga omnes is another development in the theory of the relations among the sources of IL, that conflicts with the traditional view that states may only be legally bound by their will(s). The idea of an obligation of all states towards the international community as a whole first emerged in the Barcelona Traction case and is currently formalized in the 2001 ILC Draft articles on state responsibility for international wrongful acts and in the 2005 resolution of the ILL on obligations erga omnes. The existence of this obligation may create a right for all states to sue the infringing state regardless of the (lack of) harm suffered. The existence of this actio popularis in international law is not yet recognized by the ICJ as it could create confusion in the judiciary. A difficult balance has to be found between the traditional idea of no action without direct affection and the idea of protecting certain groups of people.
II INFORMAL SOURCES OF IL
At present new, informal sources of IL have emerged.
First of all, a Unilateral declaration of a state can create legally binding (new) rights and obligations based on the principle of good faith. A clear and explicit intention of the state to be bound must be generally known but no response of any subject of IL is required. Multiple forms are recognized: the legal act could be connected to treaties or CIL such as the establishment of a state’s territorial sea, the conferral of nationality, the acceptance of the compulsory jurisdiction of the ICJor they could be autonomous for example a claim of a right with no undisputable basis of positive law, a recognition or a promise etc. However, from a political viewpoint in international relations the foundations of this source may be seen as too naïve, as the ICJ bases its reasoning on the idea of “an age in which cooperation is becoming increasingly essential”. Yet, paradoxically states may be more hesitant to make such declarations, which normally could foster cooperation, if they risk being legally bound.
The second informal source of IL is soft law, generally seen as formally non-binding yet in practice it often de facto binds states. Over the years, States have de facto been complying with soft law instruments like resolutions and recommendations for various reasons: it stimulates progressive development, it may be seen as “codified” evidence of CIL, from a functional approach it may be seen as quasi-legislation or they can even be standards of legality. Current theories focus on the function of soft law in IL:
Firstly, soft law may function as pre-law of a treaty, an authoritative interpreter of a treaty, a provider of technical rules for member states of a treaty or as evidence of CIL deriving from treaties. Furthermore, soft law may be de facto binding when integrated in a treaty by a reference clause or when inserted in operational policies of IGO.
On top of that, soft law can be de facto binding when created by inter-governmental bodies and non-governmental bodies. The latter may take the form of collective international or national schemes and individual firm schemes. However, a cautious approach seems in order as soft law is expanding towards high influential issues, like national security, more precisely cyberattacks with the proposition of Microsoft for a Digital Geneva Convention.
At present, in addition to the traditional sources of IL, states may as well be (de facto) bound by the informal sources of IL: Unilateral declarations and soft law. Aside from this expansion of sources, the theories of the relations among the sources have undergone numerous changes. Especially the recognition of jus cogens and obligations erga omnes have strongly influenced the traditional view that states may only be legally bound by their will(s). On the other hand, these evolutions and their consequences are themselves not without debate as illustrated by this essay’s critical approach on the theories of the sources of IL.
BIBLIOGRAPHY See the 1927 SS Lotus judgment p. 18: “emanate from their own free will”, “Restrictions upon the independence of states cannot therefore be presumed”. The basic assumptions of the law of treaties are state sovereignty, voluntarism and consensualism. Successful multilateralism often requires the possibility for states to make reservations. If a reservation is expressed, the state shall not be bound by this obligation. However, a reservation may become without effect if it conflicts with a rule of jus cogens (cf. infra); principle of good faith as expressed in the ICJ Nuclear test case §46.  The two element approach as expressed in the 2018 ILC draft conclusions and the 1969 North Sea Continental Shelf cases §77.  Conclusion 15 of the 2018 ILC draft conclusions; Anglo-Norwegian Fisheries case.  Conclusions 4-8 of the 2018 ILC draft conclusions; North Sea continental shelf cases.  For regional CIL see the Asylum Case (Colombia v. Peru) p. 277. However, it is important to note that the idea of regional CIL is inserted in Conclusion 16 of the 2018 ILC Draft conclusions; For bilateral CIL see the Right of Passage Case (Portugal v. India) §39. Note: It might have been better to handle the right of passage case as a historic title instead of CIL.  As established in the Asylum Case (Colombia v. Peru) p. 275 and 276.  As established by the ICJ in the North Sea Continental Shelf case §74. Although it will depend on the degree of uniformity of the practice etc. The prohibition of the use of force is now recognized as a rule of CIL but enters into conflict with the CIL rule of the right to self-defense. As established in the Nicaragua case p. 188.  Art. 13(a) UNC; Art. 15 of the ILC Statute; Conclusion 11 of 2018 ILC Draft conclusions.  Art. 2(2) UNC; Art. 1 ILC Guiding principles on unilateral acts; Nuclear test case §46. Mostly ICJ case law but also other international and regional courts like the ICTY, ICC or even semi-judicial organs like the UN Human Rights Committees. See C. RYNGAERT, G. DE BAERE, J. WOUTERS and T. RUYSE, International law: a European perspective, Hart publishing, Bloomsbury, 2019, 159. The absence of a formal rule of stare decisis follows from Conclusion 13 of the 2018 ILC Draft conclusions and Art. 59 of the ICJ Statute. However, we find the substantial status of stare decisis for certain courts, like the ICC art. 21 of the Rome Statute of the ICC; For the ICJ see the Land and Maritime boundary between Cameroon and Nigeria case §28, Nuclear test case §45.  Conclusion 13 and 14 of the 2018 ILC Draft conclusions.  As enshrined in art. 103 UNC.  Reoccuring non-derogable rights are the right to life, the prohibition of torture, the prohibition of slavery, the prohibition of nulla poena sine lege and the principle of non-discrimination. See Art. 4 of the 1966 International covenant on civil and political rights which contains a derogation clause that selects Arts. 6-8 and 11, 15-16 and 18 as non-derogable rights. The same exception to the idea of hierarchy is found in the derogation clause of Art. 15 ECHR and Art. 27 of the 1960 American convention of Human Rights.  For the debate on the existence of jus cogens see arguments pro A. Verdross and arguments contra G. Schwarzenberger. For the most recent draft on jus cogens see the 2019 ILC Draft conclusions on peremptory norms of general international law. The ILC has incorporated the idea of jus cogens and some deriving obligations see Art. 64, 66(a) and 71§2 VCLT. For the recognition of jus cogens by the ICJ see the advisory opinion of 1996 on the legality of the threat or use of nuclear weapons §83; For the legal consequence on a unilateral act in conflict with jus cogens (cf. infra) see Principle 8 of the 2006 Guiding principles applicable to unilateral declarations of States capable of creating legal obligations and Conclusion 15 of the 2019 ILC Draft conclusions on peremptory norms of general international law.  For reservations to treaties see point 4.4.3. of the 2011 Guide to practice on the reservations to treaties; For obligations not affected by countermeasures see art. 50§1(a) of the 2001 ILC Responsibility of states for Internationally wrongful acts.  Seen in light of the non-exhaustive list of jus cogens which includes the prohibition of genocide, torture, slavery etc.; See Conclusion 23 of the 2019 ILC Draft conclusions on peremptory norm of general international law.  Two different categories of obligations are distinguished: Obligations of a state vis à vis another state or obligations erga omnes see Barcelona Traction case §33-35; Examples of obligations erga omnes provided by the ICJ concern the prohibition of aggression, prohibition of genocide, Prohibition of slavery and prohibition of racial discrimination. Later the right of self-determination was added see the 1995 East Timor case §29. See especially Art. 48 on the idea of obligations erga omnes and articles 33§1, 42(b) and 48(b) on the idea of an international community as a whole.  See preamble, article 1-3 and 5 in which similar wording from the ICJ 2004 Legal consequences of the construction of a wall by Israel in the occupied Palestine territory case can be found. As the ILL mostly follows the judgements of the ICJ this resolution can be seen as a codification of IL. However, it also contains some progressive development factors like the idea of actio popularis enshrined in article 3.  1966 South West Afrika case §88. However, an on ongoing case on the crime of genocide (The Gambia v. Myanmar) might lead to a different approach.  Preamble and Principle 1 of the 2006 ILC Guiding principles on unilateral declarations of States capable of creating legal obligations; Nuclear test case p. 253 and §43,46 and 49.  See C. RYNGAERT, G. DE BAERE, J. WOUTERS and T. RUYSE, International law: a European perspective, Hart publishing, Bloomsbury, 2019, 161.  See for example declarations on Art. 3 UN Convention in the law of the sea.  Nottebohm case §2.  See for example declarations on Art. 36(2) of the ICJ Statute.  See for example the Truman proclamation of 28 September 1945.  See for example the Ihlen declaration on the Legal status of Eastern Greenland case p.3§-37  Nuclear Test Case (Australia and New Zealand v. France) §42-43 and §45-46.  Nuclear Test Case (Australia and New Zealand v. France) §46.  L. VAUGHAN, International law, Oxford university press, Oxford, 2007, 95-96.  Ideas invoked respectively by I. Brownlie, M. Akehurst, R.A. FALK, The idea of soft law as a standard of legality follows from Softlaw (Soft law sensu stricto).  For example, a GA resolution like the 1948 Universal declaration on human rights which functions as pre-law of the later 1966 International Covenants of human rights.  UNGAR 2625, a declaration which has served as an authoritative interpreter of Art.2§4 UNC (treaty).  The SBSTA produces recommendations on technical rules but each member state of the 1992 United nations framework convention on climate change transposes them in their national legal system, and in this way become de facto bound.  In the 1986 Nicaragua case the ICJ relied on the 1987 Declaration on principles of international law concerning friendly relations and co-operation among states in accordance with the UNC as evidence of the prohibition of the use of force as contained in Art. 2§4 UNC.  For example, Art. 21 and 211 of UNCLOS recognize the IMO as a competent IO to adopt resolutions. As state-parties incorporate these resolutions in national law they are de facto bound by them. Same applies for the Codex Alimentarius commission and the international office of epizootics in the framework of the SPS agreement of the WTO.  For example, the operational policies guidelines of the World Bank include environmental treaty standards. If States want to receive finances of the World Bank their finance applications have to respect these obligations, therefore they are de facto bound.  SDGs are currently incorporated in most state’s national legislations and even companies follow them as a part of company strategies. Yet, they were originally introduced in a simple UNGA resolution of 2015 Transforming our world: the 2030 Agenda for sustainable development.  For example, the standards of the international organization for standardization (ISO) which is a non-profit organization under Swiss civil law and composed of national standardization organizations. The organisation produces non-binding documents of standardization yet the national standardization organization’s States legislate them under national law, de facto binding them.  For example, the strictly French national scheme Label Rouge produces i.a. food standards. As France is an influential importer and exporter of food, these standards may push other states to amend their legislation in order to ensure exportation. As a result, they would be de facto bind by these standards.  For example, TESCO natures choice is a norm produced by a private organization yet it may de facto affect national legislation of its produces and supplies because of its importance.