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    Heffes, Ezequiel 2018. Some Reflections on the Theory of Sources of International Law: Re-examining Customary International Law. Israel Law Review, Vol. 51, Issue. 3, p. 485.




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

1. Introduction

The reprinting of Raphael Walden's article, ‘The Subjective Element in the Formation of Customary International Law’, 1 forty years after its first publication, is a timely opportunity to examine briefly the current health of some of the long-standing theoretical controversies associated with customary international law, including opinio juris. For decades, these have provided scholars with a fertile research agenda, so much so that customary international law has been depicted as no less than ‘a riddle inside a mystery wrapped in an enigma’; 2 but such controversies have also proven to be, well, theoretical. They have not stood in the way of courts, practitioners and writers in regularly identifying and applying customary international law: the academic torment that accompanied this source of law in the books has not impeded it in action. 3 Such accumulated practice, for its part, has clarified much about the operation of customary international law and supplied both authors and practitioners with insights into, and indeed answers to several of the difficulties. As a result, customary international law today is not only as present in the international legal system as it has always been; it is perhaps also better understood than before.

2. The Subjective Element

Let us begin by looking at opinio juris, the subjective element of customary international law that Thirlway so memorably described as the ‘philosopher's stone which transmutes the inert mass of accumulated usage into the gold of binding legal rules’. 4 In taking on this subject, Walden joined a fascinating conversation spanning centuries of legal thought: the ‘extra ingredient’ necessary to transform a general practice into a binding rule, as Kadens and Young have argued, ‘has always been the central problem’ in theorising customary (international) law. 5 Walden grappled with the writings of such jurists as Ulpian and Suarez, Grotius and Vattel, as well as Tunkin, Kelsen and Lauterpacht, in identifying and dissecting two central theories that have previously been put forward to explain the nature of the subjective element. The first, that of tacit consent, he found to have the merit of recognising custom as law-creative (as opposed to strictly declarative of pre-existing law) but ultimately defective in implying that states cannot be bound by customary rules to which they have not agreed. He appreciated the second theory, or rather theories, based on the classical doctrine of opinio juris, as providing a criterion for distinguishing between customary international law and mere usage; but he rejected it, too, because it treated custom as merely declarative and not also constitutive of rules of law. 6 Writing as a practitioner informed by, inter alia, the development of the customary international law of the continental shelf and the recognised potential role of United Nations (UN) resolutions in the customary process, 7 Walden then proposed, in a companion paper published in the Israel Law Review in 1978, his own approach to the subjective element of customary international law. 8 Borrowing from HLA Hart's theory of the ‘internal aspect’ of rules (and following in Thirlway's footsteps), he there suggested that the troubled concept of opinio juris should be reformulated to acknowledge that the subjective element accompanying the relevant practice ‘may be, not a belief that the practice is already legally binding, but a claim that it ought to be legally binding’. 9 By this he tackled head-on the so-called ‘opinio juris paradox’ that authors have described ‘in loving detail’; 10 held opinio juris to be an observable fact rather than an indiscernible inner feeling; and, ultimately, treated the enigmatic Latin phrase 11 as just another juridical term in the service of mediating between real-world facts and the law.

Such an approach does appear to correspond with the development of much of contemporary customary international law, and is compatible with the wording of the almost century-old formula in the Statute of the International Court of Justice (ICJ Statute), which refers to international custom as a general practice that is ‘accepted as law’. 12 It also makes it possible to take account of early practice in assessing the requirement of generality, as ‘has undoubtedly been the practice of international tribunals faced with problems of the existence or non-existence of rules of customary law’. 13 Furthermore, this approach is in line with the current work of the International Law Commission (ILC) under the heading ‘Identification of Customary International Law’. While the ILC has been able to avoid some of the theoretical debates connected with the formation of customary international law given its focus on identification, it has recognised that in practice the two cannot always be considered in isolation and partly for that reason has opted to refer to the subjective element primarily by the term ‘accepted as law’ in the ICJ Statute (with opinio juris retained in parentheses, given its prevalence in legal discourse). 14 The Commission has also offered a non-exhaustive list of potential forms of evidence of acceptance as law, having surveyed numerous decisions that have once again illustrated that establishing opinio juris has not ‘present[ed] as much difficulty as the writers have anticipated’. 15 The list refers to public statements made on behalf of states, official publications, government legal opinions, diplomatic correspondence, decisions of national courts, treaty provisions, conduct in connection with resolutions adopted by an international organisation or at an intergovernmental conference, and inaction (under certain circumstances). 16 Guidance has also been provided on how to distinguish between acceptance as law and other motives that may accompany a certain practice. 17 All of this suggests that the subjective element is not (or is perhaps no longer) an elusive, intangible notion in dire need of deciphering.

3. The Objective Element

The element of practice, which ‘both defines and limits’ customary international law, 18 has not been free of controversy either. The role that verbal acts play in the formation of customary rules, in particular, once sharply divided those who thought it ‘legally unacceptable to exclude communications, written and spoken words, from the world of State practice’, 19 and those who considered that ‘a State has not done anything when it [merely] makes a claim’. 20 The former have clearly had the upper hand: it is widely accepted today that practice may take a wide range of forms, including both physical and verbal acts (and, under certain circumstances, inaction). 21 Such an approach, which finds much support in judicial practice, recognises that states exercise their powers in various ways and do not confine themselves only to some types of act. The alternative is too restrictive, especially as action may at times consist solely in statements (for example, a protest by one state addressed to another) and because accepting such a view may be seen as encouraging confrontation and even the use of force. 22

Writers have also been divided on the question of whether practice may be relevant for the purposes of customary international law only when it relates to a situation at the international level and to some actual incident of making a claim (as opposed to assertions in abstracto). 23 However, such positions seem by now to have been abandoned. The development of international human rights law, for example, has illustrated that conduct within the state (such as a state's treatment of its own nationals) may also be relevant to international law, and that it may very well be that ‘the materials not related to sudden crises are more likely to represent a mature and consistent view of the law’. 24 It is no longer seriously contested, moreover, that relevant practice may emanate from any organ of the state, and not only from those authorised to represent the state in its international relations. 25

The length of time required for the formation of a rule of customary international law has also been the subject of disagreement. Some, attached perhaps to a conception of custom in domestic societies (or, perhaps, to an outmoded view of customary international law), have argued that ‘[c]ertainly practice over a more or less long period is an essential ingredient of customary law’. 26 The jurisprudence of the ICJ, however, has clarified that there is no specific requirement with regard to how long a practice must exist before it can ripen into a rule of customary international law. In the oft-cited words of the North Sea Continental Shelf judgment, ‘the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law’ where a general practice that is accepted as law may be observed. 27 On the other hand, it is equally clear by now that, despite continuing academic fascination with the term, 28 there is no such thing as ‘instant custom’. 29

Another dispute that belongs in the past concerns the extent of consistency that ought to be observed before a certain practice may be said to be general. It is widely held that absolute uniformity is not required, and that some inconsistencies and contradictions are not necessarily fatal to a finding of ‘a general practice’. Here, too, the case law of the ICJ and of other courts and tribunals, and the work of authors and of bodies such as the International Law Association and the ILC, have provided useful guidance, including on how variations in the relevant practice are to be assessed. 30

There is also general agreement with respect to the proposition that the acts of entities other than states (and in some circumstances intergovernmental organisations) – such as non-governmental organisations, non-state armed groups, transnational corporations and private individuals – do not count for the formation or identification of customary rules (but may have an important indirect role). 31 Official statements and publications of the International Committee of the Red Cross (ICRC), for example, may serve as helpful records of relevant practice (and acceptance as law) and play an important role in shaping the practice of states, but they are not practice that itself gives rise to or reflects customary international law. 32

4. The Significance of the Two Constituent Elements

Controversy has also surrounded the very need for the two constituent elements in the formation and identification of customary international law. Several writers, in particular, have argued that customary rules need not all be pressed into ‘the Procrustean bed of traditional practice and opinio iuris’. 33 Some have argued that widespread and consistent state practice may alone suffice for constructing customary international law, 34 while others, straying even further from the standard notion of customary law, were willing to relax the practice requirement to a vanishing point and concentrate instead on opinio juris. 35 As advocates of ‘modern’ custom, the latter have sought to turn the ascertainment of customary rules into a normative exercise rather than an empirical one, ‘attach[ing] greater relative weight to what ought to be than to what is’, 36 particularly in the fields of international humanitarian law, human rights law and environmental law.

However, the conceptual stretching espoused by single-element approaches results in a distortion or even a denial of customary international law, and risks severely undermining its stability and legitimacy. As Wolfke explained: 37

Without practice (consuetudo), customary international law would obviously be a misnomer, since practice constitutes precisely the main differentia specifica of that kind of international law. On the other hand, without the subjective element of acceptance of the practice as law the difference between international custom and simple regularity of conduct (usus) or other non-legal rules of conduct would disappear.

In any event, such a ‘veritable revolution in the theory of custom’ 38 has gained no traction with states and no significant following among practitioners. The ICJ ‘has repeatedly laid down … [that] the existence of a rule of customary international law requires that there be “a settled practice” together with opinio juris’. 39 The ILC, too, has recently confirmed that, in all fields of international law, ‘t[o] determine the existence and content of a rule of customary international law, it is necessary to ascertain whether there is a general practice that is accepted as law (opinio juris)’. 40 Such authoritative determinations, and even more so their reception by states, have made it clear that alternative approaches to the formation and identification of customary international law are, essentially, policy proposals. As such they may be instructive, but they remain policy, not law. 41

5. The Role of UN General Assembly Resolutions

Another theoretical controversy that seems to have been largely put to rest concerns the ability of resolutions adopted by a body composed of states, in particular the UN General Assembly, to create, simply by their adoption, rules of customary international law. That such resolutions may sometimes have a significant role as evidence of, or impetus for customary international law is widely accepted. Some, however, have taken this potential ‘normative value’ 42 of resolutions to mean that, albeit very exceptionally, they are capable of giving rise to customary international law by the mere fact of their adoption. 43 Such a position runs counter to the terms of the UN Charter, 44 which does not provide the General Assembly with any such power, and to the basic two-element approach, which requires not only a sense of a legal right or obligation but also a general practice embodying it. In Tomka's words: 45

The resolution does not have any legal force of its own, and it must be considered whether there is indeed a general view, held by States, that the resolution expresses a binding rule of international law, such that instances of State practice in accordance with that rule could be said to be motivated by that rule.

The ILC's unequivocal statement that ‘[a] resolution adopted by an international organization or at an intergovernmental conference cannot, of itself, create a rule of customary international law’ 46 reflects the current views of states.

6. The Persistent Objector Rule

The ILC's draft conclusions under the heading of ‘Identification of Customary International Law’ include a provision on the persistent objector rule, according to which a state that objected to a rule of customary international law while that rule was in the process of formation is not bound by the rule for as long as it maintains its objection. 47 This is another issue that has proved to be less contentious than in the past, when it was suggested that the rule ‘played a surprisingly limited role in the actual legal discourse of states’. 48 In fact, judicial proceedings, in particular, furnish a number of instances where states have sought to rely on the rule (and courts and tribunals have acknowledged its existence). 49 In addition, there is other state practice in support of the rule, such as official government statements that recognise it explicitly. 50 An initially sceptical author who not long ago set out to write a monograph denying the existence of the rule found, as his research progressed, that ‘the more I read and the more deeply I delved into state practice, the more support for the rule I found … the rule does exist and is, moreover, well worth examining’. 51 In the recent debates on the matter in the ILC and in the Sixth Committee of the General Assembly, the Commission's draft conclusion stating the rule was widely supported and met with only limited opposition.

7. Conclusion

It is no longer (if it ever was) accurate to assert that ‘in customary international law nearly everything remains controversial’. 52 As the brief survey above indicates, considerable clarity has been achieved over the decades, and some long-standing questions – much like the controversy as to whether customary international law still has any role to play in modern international law 53 – do appear to have been settled. Throughout this time, customary international law has very much retained its core elements and characteristics.

This is not to say that all issues concerning the formation and identification of customary international law have now been resolved or are straightforward. Given custom's inherent qualities, such questions as the exact moment at which a customary rule comes into being or the number of states required for a given practice to be recognised as general, cannot be answered in the abstract. Theoretical discussions on customary international law are also bound to continue since they are intimately ‘connected with ideas about law in general and of international law in particular’. 54 Furthermore, as customary international law adapts to the changing circumstances of the international society that it is meant to regulate, new issues arise and require elucidation. For example, the ways in which and the extent to which the practice of international organisations may contribute to the creation and expression of customary rules may well benefit from further clarification.

Theoretical controversies are not inherently pernicious. On the contrary: with customary international law, as with any other subject, they are often instrumental in achieving greater understanding and agreement. Scholarship that remains in touch with reality and is committed to solving actual legal problems, like that of Walden, will doubtless continue to have a particularly important role in making customary international law more tangible and comprehensible. The latter are attributes of utmost importance to any law, not least one which underlies the international legal system as a whole and is a central means of bringing under one legal regime all members of a large and heterogeneous international community.

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

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 (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) 133–59.

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

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

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 ; Elias, Olufemi, ‘The Nature of the Subjective Element in Customary International Law’ (1995) 44 International and Comparative Law Quarterly 501–20; 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 ; and Lepard, Brian D, Customary International Law: A New Theory with Practical Implications (Cambridge University Press 2009). 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 .

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) 419–20. 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 .

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, 569. 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 ).

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, 397. 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 . Both are cited in Fellmeth, Aaron X and Horwitz, Maurice, Guide to Latin in International Law (Oxford University Press 2009) viii .

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 (‘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 (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, 129 (‘[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 . See also, for example, De Visscher, Charles, Theory and Reality in Public International Law (Princeton University Press 1968) 154 ; 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 . 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 (‘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 ; Akehurst, Michael, ‘Custom as a Source of International Law’ (1977) 47 British Yearbook of International Law 1, 53 (‘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, 342 (‘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 . See also, for example, van Hoof, GJH, Rethinking the Sources of International Law (Kluwer Law and Taxation 1983) 108 ; 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, 162 (‘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, 666; 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 313–14.

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, 5.

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, 37 (‘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, 128.

34 See, for example, Kopelmanas, Lazare, ‘Custom as a Means of the Creation of International Law’ (1937) 18 British Yearbook of International Law 127, 129–30; Kelsen, Hans, ‘Théorie du Droit International Coutumier’ (1939) 1 Revue Internationale de la Théorie du Droit 253, 263 (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, 280; D'Amato, Anthony, ‘Customary International Law: A Reformulation’ (1988) 4 International Legal Theory 1 (‘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 153–54; 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, 493.

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

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

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, 197 (‘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 (‘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 (‘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, 463.

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 (‘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 .

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

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) 360.

54 Treves, Tullio, ‘Customary International Law’, in Wolfrum, Rüdiger (ed), Max Planck Encyclopedia of Public International Law (Oxford University Press 2012) para 4; Sur, Serge, International Law, Power, Security and Justice: Essays on International Law and Relations (Hart 2010) 167 (‘[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’).