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A Mystery No Longer? Opinio Juris and Other Theoretical Controversies Associated with Customary International Law

Published online by Cambridge University Press:  16 October 2017

Omri Sender
Counsel, The World Bank.
Michael Wood
Barrister, 20 Essex Street, London(UK)
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Offering a current reflection on Raphael Walden's 1977 article, ‘The Subjective Element in the Formation of Customary International Law’, this contribution seeks to illustrate that considerable clarity has been achieved over the decades with regard to several long-standing questions associated with customary international law, not least those surrounding opinio juris. Accumulated practice and constructive scholarship have supplied insights into, and indeed answers to several of the controversies that have bedevilled the theory of this central source of international law. While it may inherently defy exact formulations, and some theoretical questions remain, customary international law is thus today not only as present in the international legal system as it has always been but is also better understood.

Celebrating 50 Years of Scholarship: Reflections on Key Articles from the First Five Decades
Copyright © Cambridge University Press and The Faculty of Law, The Hebrew University of Jerusalem 2017 

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1 Walden, Raphael M, ‘The Subjective Element in the Formation of Customary International Law’ (1977) 12 Israel Law Review 344 CrossRefGoogle Scholar.

2 Fidler, David P, ‘Challenging the Classical Concept of Custom: Perspectives on the Future of Customary International Law’ (1996) 39 German Yearbook of International Law 198 Google Scholar (borrowing the words of Sir Winston Churchill).

3 Sender, Omri and Wood, Michael, ‘The Emergence of Customary International Law: Between Theory and Practice’, in Brölmann, Catherine and Radi, Yannick (eds), Research Handbook on the Theory and Practice of International Lawmaking (Edward Elgar 2016) 133–59Google Scholar.

4 Thirlway, Hugh WA, International Customary Law and Codification (AW Sijthoff 1972) 47 Google Scholar.

5 Kadens, Emily and Young, Ernest A, ‘How Customary is Customary International Law?’ (2013) 54 William & Mary Law Review 885, 907Google Scholar.

6 Those who followed Walden in putting the subjective element under the microscope have certainly benefited from his account and analysis: see, for example, Mendelson, Maurice, ‘The Subjective Element in Customary International Law’ (1995) 66 British Yearbook of International Law 177208 Google Scholar; Elias, Olufemi, ‘The Nature of the Subjective Element in Customary International Law’ (1995) 44 International and Comparative Law Quarterly 501–20Google Scholar; Maluwa, Tiyanjan, ‘Custom, Authority and the Law: Some Jurisprudential Perspective on the Theory of Customary International Law’ (1994) 6 African Journal of International and Comparative Law 387410 Google Scholar; and Lepard, Brian D, Customary International Law: A New Theory with Practical Implications (Cambridge University Press 2009)Google Scholar. For a more comprehensive bibliography on the subjective element (‘acceptance as law’) see Michael Wood, Special Rapporteur, Fourth Report on Identification of Customary International Law (25 May 2016), UN Doc A/CN.4/695/Add.1. Tasioulas's ‘disjunctive account of opinio juris’ appears to offer an approach that is particularly compatible with that of Walden: Tasioulas, John, ‘ Opinio Juris and the Genesis of Custom: A Solution to the “Paradox”’ (2007) 26 Australian Year Book of International Law 199205 CrossRefGoogle Scholar.

7 Educated at Oxford and the London School of Economics, Walden worked in various legal and diplomatic positions for the Israel Ministry of Foreign Affairs from 1974 to 2000. He was Director of the Treaty Division (1974–80), Counsellor with the Embassy in Copenhagen (1980–84), Director of the International Law Division (1984–87), Minister and Deputy Head of Mission, Israeli Mission to the UN in Geneva (1987–91, 1993–97), and Ambassador to Eritrea (1997–2000). He was Deputy Agent in the Taba arbitration, and participated in many UN meetings and in the negotiations for the Peace Treaty with Egypt. Among his special interests within international law are ‘history and theory’: Byford, Jennifer (ed), Who's Who in Public International Law (Crestwall 2007) 419–20Google Scholar. When writing on this subject, Walden was Director of the Treaty Division of the Israeli Ministry of Foreign Affairs, a position that surely influenced his approach, even though it seems to have been his fascination with customary international law generally that led him to investigate the topic. The papers contain the usual caveat that ‘[t]he views expressed are wholly personal to the author’.

8 Walden, Raphael M, ‘Customary International Law: A Jurisprudential Analysis’ (1978) 13(1) Israel Law Review 86102 CrossRefGoogle Scholar.

9 ibid 97. In other words, ‘[f]or customary law to be generated, conduct must be treated as a standard for behaviour; this may take the form either of complying with an existing standard, or of creating a new one … What starts as an intention to create law, ultimately becomes a belief that the law in question exists … Thus this analysis has a flexibility which the usual doctrine of opinio juris lacks’: ibid 98.

10 Swaine, Edward T, ‘Rational Custom’ (2002) 52 Duke Law Journal 559, 569Google Scholar. The so-called paradox refers, of course, to the argument that a new rule of customary international law can never emerge if the relevant practice must be accompanied by a conviction that such practice is already law (see, for example, Taki, Hiroshi, ‘ Opinio Juris and the Formation of Customary International Law: A Theoretical Analysis’ (2008) 51 German Yearbook of International Law 450 Google Scholar).

11 It seems that there is nothing like Latin to provide a legal concept with an added ring of mystery. Tiersma has suggested that ‘a great majority of legal maxims are indeed in Latin, partly for historical reasons, but sometimes also to mask the fact that many of these maxims are self-evident banalities made to seem more impressive by being expressed in a dead language’: Tiersma, Peter, ‘The New Black's ’ (2005) 55 Journal of Legal Education 386, 397Google Scholar. Reisman has written, with reference to opinio juris in particular, ‘I warn my students that if they confront something in Latin, it is usually a signal that jurists are unsure of what they are talking about and are trying to conceal their confusion behind a solemn and pretentious Latin phrase’: Reisman, W Michael, ‘Jonathan I. Charney: An Appreciation’ (2003) 36 Vanderbilt Journal of Transnational Law 23 Google Scholar. Both are cited in Fellmeth, Aaron X and Horwitz, Maurice, Guide to Latin in International Law (Oxford University Press 2009) viii Google Scholar.

12 Statute of the International Court of Justice (entered into force 24 October 1945) 3 Bevans 1179, art 38.1(b). See also Santulli, Carlo, Introduction au droit international (A Pedone 2013) 50 Google Scholar (‘Le statut de la Cour internationale de Justice considère en son article 38 que la coutume est une pratique “acceptée”. Ainsi le statut rompt-il avec une tradition qui aimait présenter l'opinio iuris sive necessitatis comme la “conscience” d'obéir à une règle de droit’); Pellet, Alain, ‘Article 38’, in Zimmermann, Andreas and others (eds), The Statute of the International Court of Justice: A Commentary (2nd edn, Oxford University Press 2012) 819 Google Scholar (referring to the travaux préparatoires of art 38.1(b) of the ICJ Statute and to the practice of the Court when suggesting that ‘“acceptation” is not necessarily restricted to the will of the States but to an “acceptance”, which can be interpreted less strictly’); MacGibbon, IC, ‘Customary International Law and Acquiescence’ (1957) 33 British Yearbook of International Law 115, 129Google Scholar (‘[As compared with the term ‘opinio juris’,] [t]he phrase “accepted as law”, however, may admit of interpretation in senses which more accurately reflect the actual processes of evolution from practice or usage to custom’).

13 Thirlway (n 4) 55; this would be in addition to practice accompanied by opinio juris in its traditional sense.

14 Michael Wood, Special Rapporteur, Second Report on Identification of Customary International Law (22 May 2014), UN Doc A/CN.4/672, para 68; Report of the International Law Commission, Sixty-Eighth Session (2 May–10 June, 4 July–12 August 2016), UN Doc A/71/10, 75–115, containing the ILC's 16 draft conclusions, with commentaries, adopted on the first reading in 2016 (ILC Draft Conclusions). A second reading is expected in 2018.

15 Brownlie, Ian, The Rule of Law in International Affairs: International Law at the Fiftieth Anniversary of the United Nations (Martinus Nijhoff 1998) 21 Google Scholar. See also, for example, De Visscher, Charles, Theory and Reality in Public International Law (Princeton University Press 1968) 154 Google Scholar; International Law Association (ILA), London Conference (2000), Final Report of the Committee, Statement of Principles Applicable to the Formation of General Customary International Law (ILA London Statement of Principles) 30 (‘… in the real world of diplomacy the matter [of the subjective element in customary international law] may be less problematic than in the groves of Academe’).

16 ILC Draft Conclusions (n 14) 99, Conclusion 10 ‘Forms of evidence of acceptance as law (opinio juris)’.

17 ILC Draft Conclusions (n 14) 97, Conclusion 9 ‘Requirement of acceptance as law (opinio juris)’ and accompanying commentary.

18 See Dissenting Opinion of Judge Spender in Case concerning Right of Passage over Indian Territory (Portugal v India), Merits, Judgment of 12 April 1960 [1960] ICJ Rep 6, 99 (‘The proper way of measuring the nature and extent of any such custom, if established, is to have regard to the practice which itself both defines and limits it. The first element in a custom is a constant and uniform practice which must be determined before a custom can be defined’).

19 Bernhardt, Rudolf, ‘Custom and Treaty in the Law of the Sea’ (1987) 205 Recueil des Cours 267 Google Scholar. See also, for example, Villiger, Mark E, Customary International Law and Treaties: A Manual on the Theory and Practice of Interrelation of Sources (2nd edn, Kluwer Law International 1997) 1920 Google Scholar (‘there is much merit in qualifying verbal acts as State practice. First, and most important … States themselves as well as courts regard comments at conferences as constitutive of State practice’); Parry, Clive, ‘The Practice of States’ (1958) 44 Transactions of the Grotius Society 168 Google Scholar; Akehurst, Michael, ‘Custom as a Source of International Law’ (1977) 47 British Yearbook of International Law 1, 53Google Scholar (‘State practice means any act or statement by a State from which views about customary law can be inferred’); Müllerson, Rein, ‘On the Nature and Scope of Customary International Law’ (1997) 2 Austrian Review of International & European Law 341, 342Google Scholar (‘even if one would be eager to make a clear-cut distinction between ‘actual’ practice and other forms of practice (non-actual?) it is not easy and sometimes it is simply impossible’).

20 D'Amato, Anthony A, The Concept of Custom in International Law (Cornell University Press 1971) 88 Google Scholar. See also, for example, van Hoof, GJH, Rethinking the Sources of International Law (Kluwer Law and Taxation 1983) 108 Google Scholar; State v Petane, South African Supreme Court Decision (3 November 1987), ILDC 1348 (ZA 1987), paras 59F–G, 61D–E (‘customary international law is founded on practice, not on preaching … One must … look for state practice at what states have done on the ground in the harsh climate of a tempestuous world, and not at what their representatives profess in the ideologically overheated environment of the United Nations where indignation appears frequently to be a surrogate for action’).

21 ILC Draft Conclusions (n 14) 91, Conclusion 6 ‘Forms of practice’.

22 See also Müllerson, Rein, ‘The Interplay of Objective and Subjective Elements in Customary Law’, in Suy, Eric and Wellens, Karel (eds), International Law: Theory and Practice: Essays in Honour of Eric Suy (Martinus Nijhoff 1998) 161, 162Google Scholar (‘if only seizures, invasions, genocide and other similar acts were state practice then in some areas of international law (for example international humanitarian law) only so-called rogue states would contribute to the development of customary law … it would [also] increase even more the role of powerful states in the process of international law-making. Finally … in many areas of international law only a few states may have such [‘actual’] practice or states may become involved in ‘actual’ practice only occasionally’).

23 See, for example, Kunz, Josef L, ‘The Nature of Customary International Law’ (1953) 47 American Journal of International Law 662, 666CrossRefGoogle Scholar; Thirlway (n 4) 58 (‘State practice as the material element in the formation of custom is, it is worth emphasizing, material: it is composed of acts by States with regard to a particular person, ship, defined area of territory, each of which amounts to the assertion or repudiation of a claim relating to a particular apple of discord’).

24 Brownlie, Ian, ‘Some Problems in the Evaluation of the Practice of States as an Element of Custom’ (2004) 1 Studi di diritto internazionale in onore di Gaetano Arangio Ruiz 313–14Google Scholar.

25 ILC Draft Conclusions (n 14) 90, Conclusion 5 reads: ‘State practice consists of conduct of the State, whether in the exercise of its executive, legislative, judicial or other functions’.

26 Jennings, Robert Y, ‘The Identification of International Law’, in Cheng, Bin (ed), International Law: Teaching and Practice (Stevens & Sons 1982) 3, 5Google Scholar.

27 North Sea Continental Shelf, Judgment of 20 February 1969 [1969] ICJ Rep 3, 43 para 74; see also ibid Separate Opinion of Judge Ammoun, 124; ibid Dissenting Opinion of Judge Lachs, 230; ibid Dissenting Opinion of Judge Sørensen, 244. The Inter-American Court of Human Rights has similarly held that ‘it is not essential that the conduct should be practiced over a specific period of time’: Baena Ricardo and Others v Panama (2003 Inter-Am Ct HR), Judgment of 28 November 2003, IHRL 1487, para 104; ILC Draft Conclusions (n 14) 94, Conclusion 8(2) reads: ‘Provided that the practice is general, no particular duration is required’.

28 See the much debated article by Cheng, Bin, ‘United Nations Resolutions on Outer Space: “Instant” International Customary Law?’ (1965) 5 Indian Journal of International Law 23, 37Google Scholar (‘there is no reason why an opinio juris communis may not grow up in a very short time among all or simply some Members of the United Nations with the result that a new rule of international customary law comes into being among them. And there is also no reason why they may not use an Assembly resolution to “positivise” their new common opinio juris’).

29 See also ILC Draft Conclusions (n 14) 94, Conclusion 8 Commentary; van Hoof (n 20) 86 (‘customary law and instantaneousness are irreconcilable concepts. Furthermore, it is detrimental to the effective functioning of international law, as an ordering and regulating device, to water down the meaning of its sources to almost the vanishing point’).

30 Wood (n 14) 55–57.

31 ILC Draft Conclusions (n 14) 87–88, Conclusion 4 ‘Requirement of practice’, and accompanying commentary.

32 ibid.

33 Kolb, Robert, ‘Selected Problems in the Theory of Customary International Law’ (2003) 50 Netherlands Yearbook of International Law 119, 128Google Scholar.

34 See, for example, Kopelmanas, Lazare, ‘Custom as a Means of the Creation of International Law’ (1937) 18 British Yearbook of International Law 127, 129–30Google Scholar; Kelsen, Hans, ‘Théorie du Droit International Coutumier’ (1939) 1 Revue Internationale de la Théorie du Droit 253, 263Google Scholar (stating a position that he later abandoned); Guggenheim, Paul, ‘Les deux éléments de la coutume en droit international’, in Rousseau, Charles (ed), La Technique et les Principes du Droit Public: Etudes en l'Honneur de Georges Scelle (Librairie Générale de Droit et de Jurisprudence 1950) 275, 280Google Scholar; D'Amato, Anthony, ‘Customary International Law: A Reformulation’ (1988) 4 International Legal Theory 1 Google Scholar (‘My work was considered radical by other scholars; with the passage of time I have reluctantly concluded that it may not have been radical enough. Instead of trying to work within the notion of opinio juris, I should have discarded it entirely’).

35 See, for example, Bin Cheng, ‘Epilogue’, in Bin Cheng (n 26) 203, 223 (‘The main thing, therefore, is to recognise that usage (consuetud o) is only evidential, and not constitutive, of what is commonly called “international customary law”, however else one may wish to label it’); Guzman, Andrew T, ‘Saving Customary International Law’ (2005) 27 Michigan Journal of International Law 153–54Google Scholar; Lepard (n 6).

36 Lukashuk, Igor I, ‘Customary Norms in Contemporary International Law’, in Makarczyk, Jerzy (ed), Theory of International Law at the Threshold of the 21st Century (Kluwer Law International, 1996) 488, 493Google Scholar.

37 Wolfke, Karol, Custom in Present International Law (2nd edn, Martinus Nijhoff 1993) 4041 Google Scholar.

38 Weil, Prosper, ‘Towards Relative Normativity in International Law?’ (1983) 77 American Journal of International Law 413, 435CrossRefGoogle Scholar.

39 Jurisdictional Immunities of the State (Germany v Italy: Greece intervening), Judgment [2012] ICJ Rep 99, 122, [55]; see also Tomka, Peter, ‘Custom and the International Court of Justice’ (2013) 12 The Law & Practice of International Courts and Tribunals 195, 197Google Scholar (‘In fact, the Court has never abandoned its view, firmly rooted in the wording of the Statute, that customary international law is “general practice accepted as law”’).

40 ILC Draft Conclusions (n 14) 82, Conclusion 2 ‘Two constituent elements’.

41 Reisman's words may come to mind here (n 11) 24: ‘Just as it would be intellectually dishonest and profoundly immoral to try to impose a contract on a party that had never agreed to it, it is intellectually dishonest and immoral to try to reach the same result by pretending that a customary international rule has been formed, without systematically determining that state practice accompanied by the necessary attitudes has generated a customary rule’.

42 Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 8 July 1996 [1996] ICJ Rep 226, 254–55, [70] (‘The Court notes that General Assembly resolutions, even if they are not binding, may sometimes have normative value. They can, in certain circumstances, provide evidence important for establishing the existence of a rule or the emergence of an opinio juris’).

43 See, in particular, ILA London Statement of Principles (n 15) 61 (‘Resolutions accepted unanimously or almost unanimously, and which evince a clear intention on the part of their supporters to lay down a rule of international law, are capable, very exceptionally, of creating general customary law by the mere fact of their adoption …’). This proved highly contentious when the London Statement was adopted.

44 Charter of the United Nations (entered into force 24 October 1945) 1 UNTS XVI.

45 Tomka (n 39) 211 (adding that ‘[i]n the end, it is the “general practice accepted as law” that constitutes the source of custom, but determining that States accept a certain General Assembly resolution as normative will be important evidence implying that concordant practice is accepted as law’). See also Higgins, Rosalyn, Problems and Process: International Law and How We Use It (Clarendon Press 1994) 28 Google Scholar (‘one must take care not to use General Assembly resolutions as a short cut to ascertaining international practice in its entirety on a matter – practice in the larger world arena is still the relevant canvas, although UN resolutions are part of the picture. Resolutions cannot be a substitute for ascertaining custom: this task will continue to require that other evidences of state practice be examined alongside those collective acts evidenced in General Assembly resolutions’); Schwebel, Stephen M, ‘The Effect of Resolutions of the UN General Assembly on Customary International Law’ (1979) 73 Proceedings of the Annual Meeting (American Society of International Law) 301 CrossRefGoogle Scholar (‘It is trite but no less true that the General Assembly of the United Nations lacks legislative powers. Its resolutions are not, generally speaking, binding on the States Members of the United Nations or binding in international law at large. It could hardly be otherwise. We do not have a world legislature … not a phrase of the Charter suggests that it is empowered to enact or alter international law’).

46 ILC Draft Conclusions (n 14) 106, Conclusion 12(1) ‘Resolutions of international organizations and intergovernmental conferences’.

47 ILC Draft Conclusions (n 14) 112, Conclusion 15 ‘Persistent objector’.

48 Stein, Ted L, ‘The Approach of the Different Drummer: The Principle of the Persistent Objector in International Law’ (1985) 26 Harvard International Law Review 457, 463Google Scholar.

49 Michael Wood, Special Rapporteur, Third Report on Identification of Customary International Law (27 March 2015), UN Doc A/CN.4/682, paras 86–87. As emphasised in the ILA London Statement of Principles (n 15) 27, there are no decisions that challenge the rule.

50 Wood, ibid para 87 fn 212; Danilenko, GM, Law-Making in the International Community (Martinus Nijhoff 1993) 112 Google Scholar (‘the possibility of effective preservation of the persistent objector status should not be confused with the legally recognized right not to agree with new customary rules’).

51 Green, James A, The Persistent Objector Rule in International Law (Oxford University Press 2016) ix Google Scholar.

52 Wolfke, Karol, ‘Some Persistent Controversies regarding Customary International Law’ (1993) 24 Netherlands Yearbook of International Law 1, 2Google Scholar.

53 Sender, Omri and Wood, Michael, ‘Custom's Bright Future: The Continuing Importance of Customary International Law’, in Bradley, Curtis A (ed), Custom's Future: International Law in a Changing World (Cambridge University Press, 2016) 360Google Scholar.

54 Treves, Tullio, ‘Customary International Law’, in Wolfrum, Rüdiger (ed), Max Planck Encyclopedia of Public International Law (Oxford University Press 2012) para 4Google Scholar; Sur, Serge, International Law, Power, Security and Justice: Essays on International Law and Relations (Hart 2010) 167 Google Scholar (‘[G]rand doctrinal conceptions have clashed over international custom. It has polarised debates, syntheses and hypotheses and thereby creates a sort of microcosm of the principal debates in the field of international law’).