avril 1, 2023

Margaux Schmit photo (souriante) (2)
Opinio Juris par Margaux Schmit

Margaux Schmit photo (souriante) (2)

Opinio Juris

Margaux Schmit est de retour !

C’est avec un très grand plaisir et une immense fierté, cher lecteur (rice) que je propose à votre lecture attentive un nouvel essai brillantissime de Margaux Schmidt. Margaux Schmit a dans le cadre du programme Erasmus et de son Master en droit, passé quatre mois au collège renommé de Queen’s Mary à Londres.
La sélection y est impitoyable. Ce collège a formé de très grands professeurs et des hommes politiques du monde entier.

Pour rappel Margaux vient juste de fêter ses 23 printemps. Vous avez été nombreux, très nombreux à apprécier la qualité de son travail. J’avais recueilli, il y a quelques mois, ses impressions de retour de voyage à Cuba. Elle revient sur Blogazoi pour ma plus grande satisfaction et le plus grand profit de tous.
Son essai est époustouflant quant au fond, quant à ses idées et quant à son esprit de synthèse. Quant à la forme son anglais est absolument impeccable. Nombre d’hommes politiques ou de hauts fonctionnaires aimeraient manier la langue de Shakespeare avec une telle précision voire une telle élégance.
Lire Margaux est donc un ravissement pour l’esprit. Je n’ai pu résister, au risque de froisser sa modestie, à publier le commentaire de son professeur. Il est vrai que rarement éloge aura été aussi mérité. La note qu’elle a obtenue est l’équivalent de la mention summa cum laude. Blogazoi est particulièrement fier d’avoir découvert, il y a déjà un an, ce jeune talent.
Son arc essai est fort dense et mérite réflexion fort donc patience. Mais celle-ci sera récompensée.
Albion n’ayant jamais autant mérité le qualificatif de perfide, il n’y aura jamais de ministre franco-britannique. Mais Margaux étant titulaire d’une licence de droit allemand, elle sera peut-être la première secrétaire d’Etat franco-allemande.

À toutes et à tous bonne lecture
Leo Keller
Neuilly le 08/07/2017


‘Not only must the acts concerned be a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule requiring it. … The States concerned must feel that they are conforming to what amounts to a legal obligation.’ (North Sea Continental Shelf cases, ICJ Reps, 1969, p. 3 at 44) Discuss

Dear Margaux,
Apologies for the delay. I gave you a mark of 66%, which by our standards is indeed a very good mark. My comments on your essay were “A very good and sophisticated analysis especially of opinio juris. However, more on state practice, in particular on how opinio juris is evaluated. Is it right that the practice of specially affected states should be given more weight? What is the status of inconsistent practice in light of Nicaragua? Can omissions count as practice?’
Otherwise, a well written essay, carefully structured and meticulously researched. I liked the fact that you were able to draw on both English and French sources.

Happy Holidays and Happy New Year.


Phoebe Okowa
Professor of Public International Law
Queen Mary, University of London
Mile End Road
London E1 4NS
Tel. +44(0) 20 7882 3961

International Public Law
First topic

Über dem Nirgendssein spannt sich das Überall
Over the nowhere arches the everywhere
Rainer Maria Rilke

The mystery of custom, that which has the power to change the fact into a norm, still continues to challenge the authors. D’Amato said in substance: “The question of how custom comes into being and how it can be changed or modified are wrapped in mystery and illogic”. In the Lotus case, the International Court of Justice (“ICJ”, “the Court”) spoke in psychological terms, holding that States must be “conscious of having a duty” in order for a rule of customary law to be present. Similarly, in the North Sea Continental Shelf Cases, the Court referred to opinion juris as “a subjective element” in the composition of customary law:

“Not only must the acts concerned be a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule requiring it. … The States concerned must feel that they are conforming to what amounts to a legal obligation.”

This traditional definition of opinion juris is obviously unsatisfactory because it leads to the paradox that a rule cannot begin to become a law unless states believe that it is already a law. Goldsmith and Posner wrote of opinion juris: “Opinio juris is described as the psychological component of customary international law because it refers to an attitude that States have toward a behavioural regularity. The idea of opinio juris is mysterious because the legal obligation is created by a State’s belief in the existence of the legal obligation. Opinio juris is really a conclusion about a practice’s status as international law; it does not explain how a widespread and uniform practice becomes law.” One question remains: when it comes to the formation of a customary rule, is the opinio juris part of the practice or does it have an independent and necessary role to play?

I. The formation of the customary law: from the praxis to the opinio juris

A. The own existence of the opinio juris, although it is deeply related to the practice

The existence of a customary norm is attested by the cumulative and convergent reunion of two categories of criteria: a general practice, accepted as the law (opinio juris). This is the very wording of Article 38 (1) (b) of the Statute of the ICJ, which is thus exactly applied. Custom is brought to light by a relevant practice, corresponding to the exercise of rights and obligations perceived as such by the persons concerned.

Practice must be state, coherent, general and relevant, and these different characteristics, cumulative, mix and mutually reinforce each other.

The requirement of a relevant practice relates more directly to opinio juris, since relevance is largely a function of the feeling of being legally obligated. Thus a State practice largely followed and coherent for lack of relevance. In the Continental Shelf case (Libyan Arab Jamahiriya v Malta), for example, the Court stated, after noting the extensive nature of a given practice, that it “…has no doubt about the State practice. Yet that practice, however interpreted, falls short of proving the existence of a rule prescribing…” . Relevant practice may, however, sometimes be objectively defined, as in the case concerning the delimitation of the continental shelf in the North Sea , where the practice taken into consideration must include “State whose interests were specially affected”.

In this sense, a well-known dictum of the Court in the 1969 case: “The essential point … is that … these acts … would not suffice themselves to constitute the opinio juris; for, in order to achieve this result, two conditions must be fulfilled. Not only must the acts concerned be a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule requiring it… The frequency, or even habitual character of the acts is not in itself enough.”

“Must also be such” and “carried out in such a way” refer, as it were, to a motivated practice, and this motivation implies material signs which may be very diverse. Thus, in the Case concerning military and paramilitary activities in and against Nicaragua (Nicaragua v USA) : “This opinio juris may, though with all due caution, be deduced from, inter alia, the attitude of the Parties and the attitude of States towards certain General Assembly resolutions”. This opinio juris is then completely independent from the practice and me be proven as a distinct feature of a customary norm.

B. The role of the opinio juris: to achieve the recognition of the customary law

The function of this second component is to reveal the nature of the behaviour of States by distinguishing the one, which is capable of generating a new rule from the one that is not. The opinio juris is the adjuvant, which makes it possible to read the practice and to legally translate it. Hugh Thirlway defines his role as « the philosopher’s stone which transmutes the inert mass of accumulated usage into the gold of binding legal rule » .

In the absence the feeling of being legally obligated, the behaviour, be it constant and uniform, is only « courtesy », « opportunity » or « tradition »; it is the opinio juris, which makes de facto a legally relevant practice of it. Moreover, as stated in the Nicaragua case, it is the opinio juris that permits the analysis of a behaviour either as a violation of an existing rule or as the seed of the creation of a new rule.

The opinio juris, or the acceptance of the practice « as being the law » indeed confers to the practice a “label of authenticity”, or of juridicity, which completes the recognition of a custom. It is an element of its finding; It is also closely linked to its formulation, since it makes it possible to determine the rule implicitly contained in practice. It incorporates the substance of the custom, of which the opinio juris is at the same time a condensation and an expression. Nevertheless, we cannot oppose practice as a finding, and opinio juris as a formulation, since this opinio juris must also be found.

Furthermore, any practice does not generate custom, and it is the opinio juris which, playing the role of a reducer, limits the generating power of custom to the only practice, which is the object of a feeling of a legal obligation. Neither the practice nor the opinio juris are sufficient isolated one from another. It is from their combination that the customary norm emerges.

II. The opinio juris, a disputed concept due to its subjectivity

A. The legal enforceability of a psychological element

Legal voluntarism is built on the basis of a fundamental affirmation: the rules of law are produced by the human will, they exist for and through this will. If law is imposed on all the members of the community, it is because it emanates from a will, which is superior (the State) and occupies the supreme position in the society. The reference to sovereignty places voluntarism in front of a dilemma when it comes to the field of international law. In the absence of superior authority in the international order, how can we explain that a sovereign State can be obliged solely by its own will and even more to oblige a third State?

A particular version of the denial of opinio juris, however, can be found in a recent jurisprudence. In the Gulf of Maine case , the Court held: “A body of detailed rules is not to be looked for in customary international law which in fact comprises a limited set of norms for ensuring the co-existence and vital cooperation of the members of the international community, together with a set of customary rules whose presence in the opinion juris of States can be tested bi induction based on the analysis of a sufficiently extensive and convincing practice, and not by deduction from preconceived a-ideas”. On the basis of this argumentation, one may have the impression that the Court opposes two categories of customs: those which would be necessary, dictated by the requirements of coexistence between States, and constitute an irreducible core of customary law, independent of any opinio juris, and a fortiori of any consent, as a natural customary law of the inter-State society ; and those, then, which would be purely contingent, abandoned to the free decision of States, and hence to practice and opinio juris.

In accordance with this judgment, Article 38 of the Vienna Convention, entitled « Rules in a treaty becoming binding on third States through international custom”, says: “Nothing in articles 34 to 37 precludes a rule set forth in a treaty from becoming binding upon a third State as a customary rule of international law, recognized as such”. In the North Sea Continental Shelf case, the judicial debate was limited to the possibility for a customary rule developed on the basis of the treaty to bind non-parties to it. Third States were then obliged to respect the customary rule even though their opinio juris would have been different. It was on this principle that part of the argument of Denmark and the Netherlands was based in the North Sea Continental Shelf cases. They argued that even if Article 6 of the 1958 Convention did not embody or crystallize any customary rule, such a rule had arisen since the Convention, partly because of the influence exercised by it and also because of the subsequent practice of States.

But to question the basis of the binding force of custom is, as Julio Barberis said, « a question that makes no sense. To ask what is the source of the validity of custom is to ask whether the meter deposited at Sèvres really measures a meter » . Therefore, one could argue the same logic can be applied to the opinio juris, as the heart of the custom.

B. The limits of this concept

The most firmly negative position was expressed by Kelsen, followed by Guggenheim . The two authors believe that the opinio juris is only a pseudo-element, which is assessed by the judge in a totally discretionary manner, without any real constraint or support other than the practice. The « subjective » element would thus only reflect the subjectivity of the judge – or of any other interpreter – enabling him to characterize, among practices, those, which remain mere uses, and those which are recognized as the dignity of customary rules.

But it is on a theoretical basis that opinio juris is above all rejected. The argument seeks to show the impossibility, or the logical contradiction, of the subjective element. But how can one see in this feeling a factor of formation of custom, since precisely it is not yet obligatory? The opinio juris is supposed to consecrate the birth of the rule, which does not exist without it. This amounts to say that a custom is in some way an error, as States consider themselves engaged by a practice, which is not – not yet – obligatory.

Is it necessary to reply to this argument, since it was subsequently abandoned by its authors? The opinio juris attests the recognition of the binding nature of the rule, but it is not in itself constitutive. Thus, for the formation of the law of the continental shelf: the first state claims, and in particular the Truman Proclamation of 28 September 1945, provoked unilateral and collective reactions and developments, leading to the establishment of a general customary rule.

A more empirical argument, focusing on jurisdictional practice, and based on the difficulty of proving this intangible element, can also be presented. It was in particular developed by Mr. Sörensen . Very broadly, the thesis amounts to considering that actual and general practice constitutes a presumption of custom, which can only be reversed by the evidence of a lack of opinio juris. As P. Haggenmacher writes, « the two pretended elements have no individuality; They are inextricably intertwined in a unitary « practice ». This practice forms, so to speak, a single complex « element » made of « material » and « psychological » aspects” .

Thus, does practice itself not prove the opinio juris? Does not the way in which States behave make it possible to presume what they think is right? In the Gulf of Maine case, the opinio juris « can be tested by induction based on the analysis of a sufficiently extensive and convincing pratice »; And in the Nicaragua case « indications of a practice illustrative of belief » as a rule of law. On the other hand, when the practice is manifested by declarations, votes, pronouncements, the distinction between practice and opinion juris vanishes: behaviour is the expression of the opinio juris, and the opinio juris is the basis of behaviour. Contrary to that in the Continental Shelf case, Lachs J. wrote « to postulate that all States, even those which initiate a given practice, believe themselves to be acting under a legal obligation is to resort to a fiction”.

In conclusion, the opinio juris is an independent concept, certainly strongly related to the practice since both together lead to the formation of a custom. However, it is highly contested for being hard to determine as any psychological element forcing third States to adopt a new rule they would have never adopted otherwise.

Margaux Schmit

References and bibliography

Akehurst, Custom as a Source of International Law, 1974-75, 47 BYIL 53

A D’Amato, The concept of custom in international law, Cornell University Press, 1971.

Julio Barberis, Réflexions sur la coutume internationale, AFDI, 1990.

Goldsmith, Jack L. and Posner, Eric A., The Limits of International Law, 2005.

H. Kelsen, Principles of International law, edition reviewed by R.W. Tucker, 1966.
H. Kelsen, « Théorie du droit international coutumier » in Ecrits français de droit international, PUF, 2001.

P. Guggenheim, Les deux éléments de la coutume en droit international, Mélanges Scelle, 1950.
Paul Guggenheim, Traité de Droit international public, 1967, t. 1.

Krystyna Marek, Le problème des sources du droit international dans l’arrêt sur le plateau continental de la mer du Nord, Genève, RDBI, 1970.

Stephen C. Neff, Opinio Juris : Three Concepts Chasing a law, Duke Law School ed.

Shaw, Chapter 3

Sörensen, Les sources du droit international, 1946.

Hugh Thirlway, International Customary Law and Codification, 1972.

« La coutume est-elle une source du droit international ? », Le droit international au service de la paix, de la justice et du développement. Mélanges Michel Virally, Paris, Pedone, 1991.

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