The theory of consent80 requires that every state needs to agree to being bound by a norm of customary international law. It is said that this theory can easily describe intentional customary law-making, the processes of ‘initiation, imitation and acquiescence’,81 as may have happened with the law on the continental shelf through the Truman Proclamations 1945. Another advantage is that the ‘opinio iuris paradox’, one of the problems plaguing the antithetical approach, is avoided by taking the element of ‘belief in a law’ away and supplanting it with ‘consent that something be law’. As Raphael Walden argues: ‘The tacit consent theory, in all its forms, has the great merit of recognising the constitutive nature of custom.’82
Its greatest problem is inferred consent.83 It is unlikely that the majority of states actively or intentionally participate in the making of any one norm of customary international law. Most states will neither consent nor protest most developments. The question is – and it is one that has puzzled many commenta- tors over the years – how to connect this ‘inert mass’ of non-participating states to the creation of customary law. The staple solution has been to infer consent by way of a kind of ‘qualified silence’ called acquiescence.84 Only affected states which knew, or might have been expected to know, of a practice can be said to have acquiesced to it.85 This raises the valid question and, indeed, is the crux of consent
79 The distinction is taken from Mendelson (1996) supra note 13, but the dichotomy is older and will not be treated in this way here.
80 Danilenko (1988) supra note 11; Olufemi Elias, The nature of the subjective element in customary international law, 44 International and Comparative Law Quarterly (1995) 501–520; Wolfke (1993) supra note 26.
81 Mendelson (1996) supra note 13 at 185.
82 Raphael M. Walden, The subjective element in the formation of customary international law, 12 Israel Law Review (1977) 344–364 at 355 (emphasis added).
83 Hans Kelsen, Principles of international law (1952) 311.
84 MacGibbon (1958) supra note 67.
85 Aptly distilled by Mendelson (1996) supra note 13 at 186, from the ICJ’s reasoning in Fisheries (1951) supra note 66 at 138–139.
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theories, of whether implied or inferred views can really be evidence of state will.86 At best acquiescence is a legal fiction, but one may ask whether silence does in fact equal consent.87 Its proponents are open to the charge of being inconsistent,88 since individual consent means a positive emanation of will by every single state that will be bound, not only by some.
3.3.2 Opinio iuris properly so called
The second appraoch is the theory of the opinio iuris sive necessitatis,89 under- stood as the requirement of a genuine, true belief in customary law’s existence (or necessity). This view has traditionally90 been summed up by quoting from the Court’s judgment in North Sea Continental Shelf:
Not only must the acts concerned amount to 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 of law requiring it. The need for such a belief, i.e.
the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The states concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequence or even habitual character of the acts is not in itself enough. There are many international acts e.g., in the field of ceremonial or protocol, which are performed almost invariably, but which are motivated only by consideration of courtesy, convenience or tradition, and not by any sense of legal duty.91
While this may be how the theory turned out to be, it has other roots. The origins of the concept lie in theories for which customary law is merely a manifestation of pre-existing law.92 This does not present a particular problem, because the belief in a law that already exists is not constitutive, only declaratory. In fact, were the belief to be seen as constitutive, one might ask what the point of the customary process is if we have and are able to prove pre-existing law? With Franỗois Gộny’s Methode d’interpretation et sources en droit privé positif (1899), international legal
86 Byers (1999) supra note 15 at 144.
87 Qui tacet consentire videtur? Mendelson (1996) supra note 13 at 186; Torsten Gihl, The legal character and sources of international law, 1 Scandinavian Studies in Law (1957) 51–92 at 79: ‘On the whole it is difficult to draw any conclusion from the fact that a state has taken up a passive attitude.’
88 Kirchner (1989) supra note 3 at 17.
89 Also called the ‘Zwei-Elementelehre’, ‘Theorie von der Rechtsüberzeugung’ or ‘belief theory’.
90 Akehurst (1977a) supra note 15 at 31.
91 North Sea Continental Shelf (1969) supra note 57 at 45 (para 77) (emphasis added).
92 For example in the Volksgeist (‘spirit of the people’) of the German historische Rechtsschule: Georg Friedrich Puchta, Vorlesungen ỹber das heutige rửmische Recht (4th ed. 1854) 25–33 (para 11);
Friedrich Carl von Savigny, System des heutigen Rửmischen Rechts, Erster Band (1840) 34–38 (para 12). Described by D’Amato (1971) supra note 15 at 47–48; Kelsen (1952) supra note 83 at 309–311; Hans Mokre, Theorie des Gewohnheitsrechts. Problementwicklung und System (1932) 7–31; Walden supra note 82 at 357–359.
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scholarship has identified a turning point in the development of this view of the subjective element. He is said to have coined the expression opinio iuris sive neces- sitatis and he gave up the requirement of a necessarily pre-existing law.93
The opinio iuris paradox has become a persistent problem in international legal scholarship. If the belief that something is already law is what counts for law-making, then it can only be used to identify existent customary international law.94 With respect to a new norm, e.g. a norm governing conduct that had either hitherto not been covered by a norm, the belief that something is law which is only just becoming law cannot be true. Therefore, the belief is necessarily mistaken.95
Gény concludes that at the source of the formation of custom, there must be an errone- ous belief on the part of those who are the creators of custom that they are already legally bound by the very rule which they are in the process of creating: ‘an error seems at least at the beginning of a usage a sine qua condition for the conviction that such usage is binding. . . .’96
Many arguments have been developed to circumvent this problem.97 There is, for example, a tempting invitation by Hans Kelsen to accept that states act in error in making new customary law. However, this invitation is quickly withdrawn,
‘because this “norm” does not exist during the procedure of custom-formation.’98 The usability of the belief is thereby conditioned upon the truth of the belief.
It bears pointing out, however, that ‘really is law’ is a different concept to ‘states really believe it to be law’ and this, again, is different from ‘states express their belief that it is law’ (Section 3.3.3).99 Kelsen later modified his views on the subjective element (he had originally rejected a need for opinio iuris)100 to the point that states ought only to believe in the existence of a norm, not just a legal norm.101 But this misses the point: the subjective element, formulated as opinio iuris, is necessary to distinguish between a factual pattern of behaviour and customary norms.
Hugh Thirlway and Raphael Walden widen the concept to include both the
93 Peter Benson, Franỗois Gộny’s doctrine on customary law, 20 Canadian Yearbook of Inter- national Law 1982 (1983) 267–281.
94 D’Amato (1971) supra note 15 at 73: ‘Here, opinio iuris is at worst a harmless tautology.’
95 Kelsen (1939) supra note 12 at 263.
96 Benson (1983) supra note 93 at 276.
97 Nearly every monograph and many articles on customary law contain descriptions of those efforts; e.g.: Jo Lynn Slama, Opinio iuris in customary international law, 15 Oklahoma City Uni- versity Law Review (1990) 603–656 at 620–625; Sienho Yee, The news that opinio iuris ‘is not a necessary element of customary [international] law’ is greatly exaggerated, 43 German Yearbook of International Law (2000), 227–238 at 231–234.
98 ‘puisque cette “norme” n’existe pas encore tant que dure la procedure de la création coutumiere’;
Kelsen (1939) supra note 12 at 263.
99 Akehurst (1977a) supra note 15 at 36.
100 Kelsen (1939) supra note 12 at 264.
101 Kelsen (1952) supra note 83 at 307.
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belief that the practice is required by law (whether erroneously or not – opinio iuris) and the belief that practice ought to be law (opinio necessitatis).102
Only if the view that the custom should be law has the effect of making it law (provided it is coupled with sufficiently general usage), can subsequent practice be coupled with the correct view that the custom is law.103
A supplementary charge as against this theory is that the subjective element can never be a psychological study of the minds of states, because this is an impossible anthropomorphism.104 To evade the charge by pointing to the will of decision- makers is not to have seen the charge for what it is – that we cannot attribute subjective will (or belief) to a juridical person like the state. Decision-makers are not the state; their psychological states are not the state’s, even as a collective.105