Customary law, general principles of international law recognised by civilised nations and equity are generally considered as three different and autonomous types of norms. This is not surprising since these latter have quite different formation methods and roles in the international order. However, a deeper analysis rapidly shows more similitudes and connections between these concepts than one might have thought. Traditional distinctions often do not consider the existing points of contact of these notions and their relationship. In practice, there seems to be a real convergence between these three notions. More than autonomous concepts, it is submitted here that custom, general principles of international law recognised by civilised nations and equity are convergent and complementary notions.
I. What are they?
Customs, general principles of international law recognised by civilised nations and equity are all mentioned by article 38 of statute the International Court of Justice. According to this provision, international custom and general principles are both formal sources of international law.
According to the Max Planck Encyclopedia of International Law, “The expression ‘customary international law’ concerns, on the one hand, the process through which certain rules of international law are formed, and, on the other, the rules formed through such a process.”. If the existence of customary international law as a source of law has been contested in the past, this is hardly the case today.
General principles of international law recognised by civilised nations, for their part, were traditionally considered as principles common to all or most national systems, that is,legal principles that most domestic orders share. They therefore do not directly emanate from the international order but are incorporated in it. It is worthwhile noting that for some authors, general principles can be found in the nature of the international community itself; in international legal logic. They are principles inherent to the nature of the international order. French doctrine therefore generally differentiates general principles de et du droit international. While the former refers to general principles of international law shared by all or most national systems, the latter concerns principles specific to the international legal order. The “principe généraux du droit international” are however different from those mentioned in article 38 1(c) of the Statute of the ICJ on which we focus on that post.
Finally, equity is mentioned in article 38(2) according to which “This provision shall not prejudice the power of the Court to decide a case ex aequo et bond, if the parties agree thereto.”, in other words to decide in equity. Francesco Francioni defines this latter as following: “In its most general meaning it refers to what is fair and reasonable in the administration of justice”. Equity is generally divided between three functions: equity infra legem, an aid to interpret the law, equity praetor legem, a tool to fill the gap in law, and equity contra legem, a modifier of the law.
As per their definitions, customs, general principles of international law recognised by civilised nations and equity already appear to be quite different concepts. Such a conclusion is reinforced by the study of their respective functions and normative values.
II. The differences between customs, general principles and equity
An obvious difference between customs, general principles of international law recognised by civilised nations and equity lie in their respective roles on the international plane. Petersen tried to distinguish custom and general principles of international law recognised by civilised nations on the ground that rules “[…] prescribe concrete conduct. They can thus easily be identified by analyzing a conduct related practice. Principles (…) do not refer to a certain conduct, but to a specific objective”.
Indeed, general principles of international law recognised by civilised nations do not prescribe conduct as such but have for objective the functioning and the preservation of the international legal order. The travaux préparatoire of article 38 clearly shows that the general principles was intended to fill the gap in international law and avoid non liquet. Cherif Bassiouni noted that they also are an aid for interpretation, ‘[a] means for developing conventional and customary international law’ and even a ‘modifier’ of these latter. Such principles bring flexibility and practice directions in the functioning of the international order.
As the case for general principles, equity does not prescribe conducts but aims to bring fairness and justice in proceedings. The prevailing view holds – and this marks an important difference with customs and general principles- that equity is not a formal source of international law. It is true that Article 38 of the Statute does not list this latter amongst the other formal sources. For some authors, however, ‘[t]o a large extent the question whether equity is a formal source of international law is a purely verbal question: (…) it is an undeniable fact that international tribunals often apply equity’. Indeed, on multiple occasions, the ICJ and its predecessor, the Permanent Court of International Justice (PCIJ), seemed to resort to equity in its findings, even if not explicitly mentioning it or explaining its reasoning for that matter. Nonetheless, it would be wrong to consider that this situation proves that equity is a source of international law. As noted by Alain Pellet, ‘In all [cases where equity was referred to] considerations based on equity can either be analysed as the material source of customary (and treaty) rules, or as a description of the content of the rule itself […]’. It makes perfectly sense for international courts to frame their analysis in terms of equity when the rules they interpret and apply refers to equity. Moreover, Courts can also resort to equity, labeled infra legem in such a situation, in order to interpret and apply existing rules. For Pierre Maire Dupuy, equity ‘[…] intervient en particulier, à titre d’inspiratrice et de guide, pour aider à l’adéquation d’une règle abstraite à une situation concrète’. In its commentary of the Draft articles on succession of States in respect of matters other than treaties that it elaborated, the International Law Commission (ILC) explained for example that “[…] the principle of equity is more a balancing element, a corrective factor designed to preserve the « reasonableness » of the linkage between the movable State property and the territory.” . Equity therefore appears as a safeguard preventing a too rigorous application, not intended and unfair of international norms. As Lauterpacht puts it, ‘The fact that a Tribunal is bound to apply the law does not necessarily mean that it must apply it uncritically.’.
As we can see, customs, general principles of international law recognised by civilised nations and equity can be discerned on the ground of their function and their characterisation as a source of international law. Despite such differences, these latter appear closer than one may prima facie consider.
III. The close relationship between customs, general principles and equity
Customs, general principles and equity actually entertain a close relationship. As seen above, equity considerations can be embodied in customary or treaty rules and even general principles.
Moreover, it is perfectly possible for a general principle to evolve in a customary rule. As Giorgio Gaja puts it, « Certain principles asserted by the ICJ have developed into principles pertaining to customary international law. », as the recent ICJ’s advisory opinion on the Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 demonstrate about the principle of self-determination. For Combacau and Sur, general principles are simply « subsidiary » and « transitory » norms, bound to become customary or treaty rules. According to some authors, equity contra legem may as well lead to an evolution of international law. For Francesco Francioni, « Equity may […] anticipate the crystallization of the law and provide the rational and ethical justification for its transformation. ». Rather than autonomous concepts, customary law, general principales and equity actually interact with each other.
Furthermore, even if the formation methods of customs, general principles and equity appear prima facie quite different, States’ opinions and beliefs seem to play an important role with respect to each of them. As highlighted by Article 38 1 (b), custom is constituted by two elements: a material element, that is the state practice, and a psychological one, that is the opinio juris. This latter is the belief that a practice was rendered obligatory by the existence of a rule of law. If there is no mention in Article 38 of such an opinio juris criterium concerning general principles of international law recognised by civilised nations, Simma and Alston rightly point out that the drafters of Article 38 were opposed to the recourse to subjective or jus naturale argument to justify the existence of general principle. On the contrary, nothing shows that the drafters were opposed to the recourse to objective criteria, such as “some sort of general acceptance or recognition by States”. For Christina Voig, such a general acceptance is materialised by a “‘common legal conscience’; an opinio juris communis”.
If equity were to be considered as an independent source of law, it may be argued that it would equally be based on beliefs shared amongst States. Recourse to equity should not be interpreted as a blank check. Equity should be subjective in the sense that used to take into consideration special circumstances in individual cases and attenuate the effect of the law in such cases. However, it would not be reconcilable with the positivist nature of the international society to let a court’s or a tribunal’s own standards determine what is fair. On the contrary, the content of equity should be found in the opinion of States, in the common judgement of what is fair or not. Therefore, equity also seems to build on the opinion of law (opinio juris) of States and not about the existence of a legal obligation but about how the law should be applied, or a dispute should be settled.
Consequently, the formation of customary law as well as general principle and, in a way, equity all assign an important role to opinio juris. As we can see, customs, general principle and equity are not so far apart. These norms incline towards each other.
The relationship among customs, general principal and equity is also influenced by their respective roles. Indeed, these latter must not be seen as autonomous; instead they should be seen as complementary. General principles and Equity are used to correct the shortcomings of other sources of law, especially customs. Customary law is indeed often less easy to determine and interpret than treaty law. If customs are unclear, or even do not exist, general principles will intervene to allow judges to reach an answer in the case before them. Equity will, for its part, most of the time intervene when a strict application of a norm would tend to lead to unfair and unjust results. Therefore, rather than autonomous norms, customs, general principles and equity appear interconnected and mutually reinforcing.
More on the formal sources of international law:
 Tullio Treves, « Customary International Law », Max Planck Encyclopedia of Public International Law, 2006, para 1.
 M.Akehurst ‘Equity and General principles of law’, International and Comparative Law quarterly, 1976.
 See J. Combacau et S. Sur, Droit international public, Montchrestien, 2008, p. 109. For these authors, general principles are transposed from different intern legal orders : « […] il semble établi que ces principes sont transposés de divers droits internes ».
 D. Anzilotti, Cours de Droit international (Paris: Recueil Sirey, 1929) 117
 Francesco Francioni, « Equity in International Law », Max Planck Encyclopedias of International Law, 2013.
 M.Akehurst ‘Equity and General principles of law’, International and Comparative Law quarterly.
 Niels Petersen ‘Customary law without custom? rules, principles, and the role of state practice in international norm creation’, 2008, AM. U. INT’L L. REV, p. 302.
 See Simma and C. Bassiouni ‘A functional approach to « general principles of international law’, 1990, Michigan journal of International law, p. 775.
 Ibid., p. 776.
 See Alain Pellet, « « Article 38 » », in A. Zimmermann et al. (éds.), The Statute of the International Court of Justice: A Commentary, Oxford Commentaries on International Law, Oxford, New York, Oxford University Press, 2012, §146 ; Francesco Francioni, « Equity in International Law », op. cit. §4.
 M.Akehurst ‘Equity and General principles of law’  International and Comparative Law quarterly
 See Hudson in The diversion of Water from the Meuse Case PCIJ Reports, Series A/B N°70; North Sea Continental Shelf case, ICJ Reports 1969; Georges Pinson (France) v. United Mexican States.
 See Alain Pellet, « « Article 38 » », in A. Zimmermann et al. (éds.), The Statute of the International Court of Justice: A Commentary, Oxford Commentaries on International Law, Oxford, New York, Oxford University Press, 2012, p. 785-786.
 Ibid, p. 787, §146.
 Pierre-Marie Dupuy, Droit international public, 2008, p. 373.
 Yearbook of the International Law Commission 1976, Vol. II, Part 2, p. 132.
 H. Lauterpacht, International Law: Being the Collected Papers of Hersch Lauterpacht, Cambridge University Press, 1975, p. 235.
 Giorgio Gaja, « General Principles of Law. », Max Planck Encyclopedias of International Law, 2020, §24.
 Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965, Advisory Opinion, ICJ, (2019) §24.
 J. Combacau et S. Sur, Droit international public, op. cit. p. 108.
 Ibid., p. 110.
 Francesco Francioni, « Equity in International Law », op. cit.
 North Sea Continental Shelf case, ICJ Reports 1969
 B.Simma and P.Alston, ‘The sources of human rights law: custom, jus cogens, and general PRINCIPLES ‘,1988-1989, Aust. YBIL 82, p. 102.
 F Christina Voigt, ‘The Role of General Principles in International Law and their Relationship to Treaty Law’, 2008, Nordic journal on Law and Justice, p. 8.