1 Tit. ex corpore Ulpiani 1, 4. “…Mores sunt tacitus consensus populi longa consuetudine inveteratus.

2 D. 1.3.32.1. The translation is taken from The Digest of Justinian, translated by Charles Henry Monro (Cambridge U.P., 1904).

3 Lambert, , La Fonction du Droit Civil Comparé (1903) 125.Google Scholar

4 De Legibus ac Deo Legislatore (1612, ed. and trans, by Scott, J.B., Classics of International Law) vol. 2.Google Scholar

5 Ibid., at 527–529.

6 Ibid., at 545–553.

7 Ibid., at 564–566.

8 Ibid., at 553–561.

9 Ibid., at 527–528.

10 See infra from n. 13–14.

11 See infra from n. 47–49.

12 See Grotius, , De Jure Belli ac Pacis Prolegomena, paras. 11, 40Google Scholar; Book One, Ch. 18, para. 4; Wolff, , Jus Gentium Methodo Scientifica Pertractatum Prolegomena, para. 24Google Scholar; Vattel, , le Droit des Gens Introduction, paras 25, 26.Google Scholar

13 The Classical writers do not, on the whole, treat this point explicitly, but it is apparent from the general tenor of their references to custom that they do not contemplate the possibility of its binding non-consenting States. Thus Grotius writes (op. cit. Book One. Ch. 1, para XIV) “The Law which is broader in scope than municipal law is the law of nations; that is the law which has received its obligatory force from the will of all nations, or of many nations. I added ‘of many nations’ for the reason that, outside the sphere of the law of nature, which is also frequently called the law of nations, there is hardly any law common to all nations. Not infrequently in fact, in one part of the world there is a law of nations which is not such elsewhere …” Again, Wolff writes (op. cit., prolegomena, para. 24) “The customary law of nations is so called, because it has been brought in by long usage and observed as law.

It is also frequently called simply custom … Since certain nations use it one with the other, the customary law of nations rests upon the tacit consent of the nations, or if you prefer, upon a tacit stipulation, and it is evident that is is not universal, but a particular law …” And Vattel, op. cit., Introduction, para. 25, writes: “Certain rules and customs, consecrated by long usage and observed by nations as a sort of law, constitute the customary law of nations, or international custom. This law is founded upon tacit consent, or rather upon a tacit agreement of the nations which observe it. Hence it evidently binds only those nations which have adopted it and is no more universal than the conventional law”. And Von Martens writes in Summary of the Law of Nations (trans. by Cobbet, William, 1795)Google Scholar Introduction s. 3: “But on the other hand it is clear, that what is become a law between two or three, or even the majority, of the powers of Europe, either by treaty or from custom, can produce neither rights nor obligations among the others”.

14 Rachel, , De Jure Naturae et Gentium Dissertationes (Classics of International Law) vol. 2, p. 157.Google Scholar

15 See infra at n. 42–45.

16 Cf. Triepel, , “Les Rapports entre le Droit Interne et le Droit International” (1923) Hague Recueil vol. 1, pp. 73–121.Google Scholar See esp. at pp. 82–83.

17 Austin, , The Province of Jurisprudence Determined (Library of Ideas ed., New York, 1954) 142.Google Scholar

18 The distinction between Vereinbarung and Vertrag is discussed in Lauterpacht, H., Private Law Sources and Analogies of International Law 157.Google Scholar

19 Triepel, op. cit., at 83.

20 Strupp, , “Les Règles Générales du Droit de la Paix” (1934) 47 Hague Recueil vol. 1, p. 291.Google Scholar

21 Loc. cit.

22 An earlier version of this theory is contained in his Eléments du Droit International Public Universel, Européen et Américain (2nd ed. revised, Paris, Les Editions Internationales, 1930). This is a simpler account, and diverges from that discussed in the text chiefly in that at this stage of his thinking he still attributed importance to the distinction between traité-loi and traité contrat.

23 Ibid., at 301. Compare Rachel's similar argument quoted supra at n. 14.

24 Strupp, op. cit. supra n. 20, at 302.

25 Ibid., at 303.

26 Ibid., at 302.

27 Ibid., at 303–304.

28 Ibid., at 302.

29 Strupp, , Grundzüge des Positiven Völkerrechts (1932)Google Scholar cited in Strupp, op. cit. at 304.

30 Strupp, cites Oppenheim in Niemeyers Zeitschrift für internationales Recht (1915) XXV, p. 12Google Scholar as supporting this view of the place of usage. (Ibid., at 305).

31 Strupp, op. cit. at 302.

32 Ibid., at 306.

33 Loc. cit.

34 Ibid., at 310.

35 Ibid., at 309–310.

36 Ibid., at 310–311.

37 The discussion of Tunkin's views is based on his Theory of International Law (English ed., Harv. U.P., 1974).

38 Ibid., at 117–118.

39 Ibid., at 124.

40 Ibid., at 129.

41 Waldock, , “General Course in Public International Law” (1962) 106 Hague Recueil vol. 2, p. 52.Google Scholar

42 Falk, , “The New States and International Legal Order” (1966) 118 Hague Recueil vol. 2, p. 24.Google Scholar

43 They may, of course, repudiate individual rules. But this is not the same as repudiating existing customary law en bloc.

44 Kelsen, , Principles of International Law (2nd ed., 1967) 444.Google Scholar

45 It is arguable that all the fallacies of the modern ‘tacit consent’ schools of thought derive from their quite unnecessary identification of consent with contract.

46 See the passage from the Digest cited supra at n. 2.

47 For accounts of the general attitude towards custom adopted by the historical school, see Gény, , Méthode d'Interprétation et Sources en Droit Privé Positif (2nd ed., 1954) vol. 1, pp. 340 ff.Google Scholar; Lambert, , La Fonction du Droit Civil Comparé (1903) 127 ff.Google Scholar; Kosters, , Les Fondements du Droit des Gens, (Bibliotheca Visseriana vol. IX) 201–202Google Scholar; Allen, , Law in the Making (7th ed.) 87–97.Google Scholar

The attribution of the origin of the concept of opinio juris (or opinio necessitates) to the historical school is made by Guggenheim; see “L'Origine de la Notion de Opinio Juris sivé Necessitatis comme deuxième élément de la Coutume dans l'Histoire du Droit des Gens” in Mélanges Basdevant 280; and “Contributions à l'Histoire des Sources du Droit des Gens” in (1958) 94 Hague Recueil vol. 2, p. 52ff. According to him the concept was first used in connection with international custom by Rivier, Alphonse, who wrote (Principes du Droit des Gens (1896) vol. I, p. 35)Google Scholar “La coutume ou l'usage des nations est la manifestation de la conscience juridique internationale qui s'opère par des faits répétés continuellement avec la conscience de leur nécessité”. According to D'Amato, on the other hand, it was Gény who first used the concept of opinio juris in connection with international law. See D'Amato, , The Concept of Custom in International Law (Ithaca, N.Y., Cornell U. P., 1971) 48–49.Google Scholar

Allen, (op. cit. at 137) attributes the concept of opinio necessitatis to Blackstone, and Parry accepts this attribution (Sources and Evidences of International Law, The Melland Schill Lectures (Manchester U.P. 1965) 61). This attribution, if correct, would refute Guggenheim's claim, but in fact, although the germ of the idea is to be found in Blackstone, it is not developed, and above all, the terminology of “opinio necessitatis” is not used by him. What Black-stone actually wrote is: — “Customs, though established by consent, must be when established compulsory; and not left to the option of every man, whether he will use them or no. Therefore a custom, that all the inhabitants shall be rated towards the maintenance of a bridge, will be good; but a custom, that every man is to continue thereto at his own pleasure, is idle and absurd, and indeed, no custom at all.” (Blackstone, Book One, Sec. 3, para. 98).

48 Quoted by Allen, , Law in the Making (7th ed.) 88.Google Scholar

49 Kosters, , Les Fondements du Droit des Gens 202.Google Scholar

50 Loc. cit.

51 Lauterpacht, , “General Rules of the Law of Peace”, in Collected Papers, vol. 1, pp. 179–444.Google Scholar Originally published in French in (1937) 62 Hague Recueil vol. 4, pp. 99–419.

52 Ibid., at 237.

53 Ibid., at 239.

54 Ibid., at 238. A similar suggestion was made by Manley Hudson in his working paper on Art. 24 of the Statute of the International Law Commission. (A/CN. 4/16 (1950) ILC Yearbook vol. II, p. 25ff) In his list of the elements of customary international law he included “(c) Conception that the practice is required by, or consistent with prevailing international law”. In the ensuing debate, ((1950) ILC Yearbook vol. 1, p. 4ff) this was criticised by a number of members of the Commission. Yepes said that if custom had to be consistent with international law, then it ceased to be a source for that law; he added subsequently that custom could depart from prevailing international law; otherwise it had no raison d'être.

Amado said that custom was the primordial source of law, but to judge from subheading (c), it had ceased to be so. Scelle said it was somewhat contradictory to state on the one hand that custom was the basis of law, and on the other hand that it had to be consistent with law.

In the face of these criticisms, Hudson redrafted subheading (c) to read as follows: — “conception by the States engaged that the practice is not forbidden by prevailing international law”. (A/CN.4/R.1 — (1950) ILC Yearbook vol. 1, p. 275 footnote). However, in the debate on the revised draft, (ibid., at 275fT.) this met with equally strong criticism, Amado said that he disliked this formulation; custom need not necessarily be in harmony with pre-existing international law. He suggested replacing the words by “recognition of the custom by State practice”). In the end, it was decided to delete the section defining the elements of customary international law from the ILC's annual report.

55 Lauterpacht, op. cit., at 238.

56 Scelle, , “Règles Générales” (1933) 46 Hague Recueil vol. 4, p. 434.Google Scholar

57 Lauterpacht, , “International Law: The General Part” in Collected Papers vol. 1, pp. 1–178.Google Scholar The quotation appears on pp. 63–64.

58 Kelsen, , Principles of International Law (2nd ed.) 440.Google Scholar

59 E.g., what Falk refers to as the “rules of the game”. See Falk, , The Status of Law in International Society (Princeton U.P., 1970) 52–53Google Scholar, and see also Falk, and Black, , eds., The Future of the International Legal Order (Princeton U.P., 1969) 67–68.Google Scholar

60 Kunz, “The Nature of Customary International Law” (1953) 47 AJ.I.L. 662.Google Scholar

61 Ibid., at 667.

62 Loc. cit.

63 E.g. Strupp and Tunkin. In his proposed list of elements of customary international law, Hudson included under subheading (d), “general acquiescence in the custom by other states”. In the ensuing debate, in reply to a query by Scelle, Hudson said he thought that the elements mentioned in subheading (c) and subheading (d) corresponded to opinio juris. Thus his definition combined, somewhat inconsistently, both a declaratory and a consensual definition of opinio juris.

64 See supra at nn. 45–47.