The Nature of Custom: Custom’s Place in and (Legal) Regulation by International Law

The authors appositely defend CIL’s centrality in the international (legal) system: general regimes of international law regarding fundamental matters, ranging from “Statehood” to “State responsibility,” remain “in whole or in part primarily” governed by CIL (p. 11 n. 7). And, despite ours being “an age where treaties dominate,” CIL “continues to underpin the international legal system” (p. 17), thus constituting “a central source of law” (p. 23). The UNGA’s view that CIL identification is “of major importance in international relations” was justified, they add, given CIL’s “central role … in world affairs.” (p. 45). More specifically, CIL identification’s “credibility” and “predictability,” they state, are “essential” for international law (p. 299), and so is “[a]dherence” to secondary rules on CIL identification for grounding “any” primary rule’s “legitimacy” (p. 7).

The authors could elaborate on the importance they correctly attribute to these secondary rules (let alone on their “nature,” as discussed below). For instance, they characterize the “persistent objector rule” as one of “systemic significance,” given its “important function in the international legal system” (p. 282). Such characterization could a fortiori be made extensive to all the other secondary rules on CIL identification. Relatedly, since certain states consider “the persistent objector rule … inapplicable not only in relation to jus cogens, but also … to a broader category of ‘fundamental’ rules” (p. 288), if secondary rules on CIL identification, given their “systemic significance,” are a fortiori seen as part of that “broader category” of “fundamental” non-peremptory rules, they would likewise be incapable of being contracted out.

The authors observe that the regime of CIL identification is not only general but also “uniform,” despite “occasional suggestions” that “different approaches” to CIL identification exist, “depending on the subject matter”: “[t]he two-element approach applies … in all fields of public international law” (p. 64). Such uniformity, they add (taking a heuristic stance despite their choice to avoid theorization), precludes “persons holding different functions (for example, judge … or … adviser)” from adopting “different approaches” to CIL identification, on grounds of what the International Law Association (ILA) called an “observational standpoint” (p. 66). Of course, while the two-element approach is uniform and rigorous, it is not “rigid or dogmatic”; it is appropriately “flexible” and “does not impose a straitjacket” (pp. 66–67).

The authors carefully examine the link between international personality and CIL identification, notably dissociating subjecthood and custom-making power. They oppose the view that “participation of international organizations in the development of” CIL is somehow entailed by “their being subjects of international law, or … bound by … customary international law”: “other non-state actors,” such as armed groups, are “said to be subjects of international law,” but do not “contribute to the formation of the law” (p. 110 n. 61). Of course, “the international legal personality of international organizations suggests that practice may well be attributed to the organization as such” (p. 108).

CIL’s place at the intersection of written and unwritten law is addressed as well. CIL is characterized by the authors as part of “unwritten” international law (lex tradita): “Article 38.1 of the ICJ Statute lists … another source of unwritten international law,” namely “general principles of law” (p. 23). Yet, while CIL stems from a “unwritten” source, unqualifiedly contrasting it to “written” international law (lex scripta) risks neglecting not only that treaties may be orally concluded but also that written materials are increasingly relevant in CIL identification (as noted at p. 78).

General practice’s “internal” facet is also examined by the authors: “conduct within the State,” they note, “may also concern international law” and can constitute state general practice; consequently, a (previously commonly held) view confining such practice to conduct that “emanate[s] from organs competent to bind the State in its international relations” is “no longer seriously maintained” (p. 103). While the authors have engaged with the ILA work on CIL identification, engagement with more recent ILA work (on the “engagement of domestic courts with international law”), similarly finding that, unlike “traditional international law … primarily concerned with … obligations … implemented through conduct on the international plane … obligations are increasingly inward-looking,”Footnote ² could have enriched their analysis.

That the ILC’s “output did not lend itself to transformation into a convention” formed common ground between the ILC and states (p. 46 n. 102): the ILC, seeking “to find the right balance between helpful guidance and overly restrictive rule-making … would not be drafting a ‘Vienna convention on customary international law’” (p. 34). As ILC rapporteur, Wood refrained from characterizing the Conclusions as embodying “rules,” let alone legal ones (p. 51 n. 130).Footnote ³ Consequently, the ILC did not “directly” address the “question” of the Conclusions’ “own nature,” which was formulated as specifically concerning whether they “reflect or codify customary international law,” because it was considered “an essentially theoretical matter, without practical consequences” (id.). Yet, had rules on CIL (identification) been characterized as legal (lex lata), the ILC’s aim to “describe ‘the current state of international law on the formation and evidence of rules of customary international law’” (p. 34), reflecting “the existing position, without ‘progressive development’” (p. 52), could have plainly been characterized as stating lex lata.

Significantly and consequentially, the authors, taking a step that the ILC (as noted above) was unwilling to take, go on to declare that the Conclusions embody “rules of recognition” and, as such, “are rules of law … best viewed as themselves forming part of customary international law” (id., emphasis added).

The statement’s significance lies in three important (largely theoretical) choices underpinning it.

First, they approach CIL identification as a matter governed by (legal) rules, notwithstanding their acknowledgement that, as Jean d’Aspremont notes, “whether the criteria of law-identification … constitute rules … or … practices” remains unsettled in “legal theory” (p. 51 n. 131).

Second, they characterize those legal rules as “rules of recognition,” instead of ‘“meta-law” to be traced to some Grundnorm’ (id.), thus relying on H.L.A. Hart’s instead of Hans Kelsen’s strand of positivism.Footnote ⁴

Third, they formally ground those legal “rules of recognition” on custom, yet without engaging with some of the vexed questions which affirmations of source-based legal rules on sources usually confront. The authors could have further strengthened their affirmation of custom-based legal “rules of (CIL) recognition” by considering certain preliminary questions. Notably, they could have considered whether postulating source-based legal rules on sources involves a petitio principii, presumably entailed by a source’s self-referential creation of rules governing that very sourceFootnote ⁵ (as would be the case with CIL on CIL identification). While, in practice, such self-referentiality has posed no problem (as evidenced by at least one international decision, not included in the authors’ comprehensive survey, relying on custom to formally ground legal rules on sources),Footnote ⁶ standard doctrinal theories have continued to deny the possibility of source-based legal rules on sources on grounds of such self-referentiality.Footnote ⁷

Furthermore, the statement not only is significant but also has a consequential (methodological) implication: substantiating the status of each of the rules set out in the Conclusions as rules of CIL proper would require, if the two-element approach is to be upheld, enquiring not only into their basis in “general practice” (of states) but also their acceptance as law (by states) on a one-on-one basis, as opposed to relying on formulations in pronouncements by (mostly international) courts and tribunals, notably the International Court of Justice (ICJ) (whose “approach,” the authors state, being “not in itself determinative” yet “fundamental to any understanding of the methodology to be applied … was adopted by the ILC and in this book” (p. 7)).

The authors deliver a resoundingly negative response to what has at least been (quite literally) a “one-and-a-half million” dollar question,Footnote ⁸ namely whether CIL can be interpreted (pp. 76–78): “[i]t is rather misleading to apply the notion of interpretation … to the unwritten rules of customary international law” (p. 76). Setting aside the authors’ unqualified assumption that unwritten rules cannot be interpreted (which would notably imply that oral treaties cannot be interpreted), interpretation is regarded by them as inherently problematic: they quote approvingly Charles de Visscher’s view that treaty law is exposed “to the difficulties of interpretation” (p. 21 n. 76, emphasis added). Leaving aside this general stance on interpretation, the authors are seemingly concerned that CIL interpretation might hollow out CIL identification: “identification of a rule of customary international law involves the determination of its scope and content, and once that determination has been made, what remains is the application of the rule to a given situation” (p. 77). Consequently, insofar as CIL’s “content” is determined upon its identification (for it “becomes established and its content made clear” when identified (p. 160, emphasis added)), no interpretation thereof can ensue thereafter, in the authors’ opinion. In short, it would appear as though, in the authors’ eyes, CIL interpretation (conceptually) poses a fatal threat to CIL identification, one which the latter can only overcome if the former is negated as a separate concept (as if their coexistence were a zero-sum game).

Yet, based on the authors’ own analysis, interpretation is not conceptually incompatible with determinations involved in CIL identification.

The authors’ denial of CIL interpretation does not imply CIL’s constitutive elements lack an interpretive aspect: such denial, they clarify, does not imply “State practice and evidence of opinio juris underlying a rule of customary international law cannot be interpreted: they may well need to be, so as to evaluate their relevance and significance” (p. 77). The “assessment of evidence” for each constitutive element involves an analysis of a CIL rule’s “overall context,” which “necessitates … taking into account ‘any underlying principles of international law that may be applicable’” (p. 81), “not unlike the rule of interpretation in the law of treaties” codified in Article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT) (id., n. 11). As for general practice, “[e]ach of the forms of practice listed in Conclusion 6 ‘is to be interpreted broadly ….’” (p. 122). As for opinio juris, under certain “circumstances … inaction may reasonably be construed as acceptance as law” (p. 190).

CIL once identified is also interpretable: insofar as “any conflict between a general principle of law and a rule of customary international law is to be resolved by applying the generally accepted techniques of interpretation and conflict resolution in international law” (p. 26) (paraphrasing a proposition made by the ILC in its work on “General principles of law” (id., n. 100), which the authors do not contest), CIL in itself (as opposed to its constitutive elements) is assumed to be interpretable. So is it according the ICJ: “[r]ules which are identical in treaty law and in customary international law are also distinguishable by reference to the methods of interpretation and application” (p. 77 n. 159)Footnote ⁹ (one among various “[o]ccasional references to the interpretation” of CIL by the ICJ which the authors discard, without specific reasons, as reflecting “an imprecise use of language” (id.)).

Thus, the authors’ denial of CIL identification proves merely (and largely) nominal for they do not deny CIL identification involves interpretation nor seem to deny CIL interpretation may take place independently of CIL identification. The only material reason for opposing CIL interpretation’s independent status from CIL identification probably lies in the authors’ (justified, though not fully elaborated) concern over jeopardizing the two-element approach’s (appropriate) application. Notably, the two-element approach would not be (properly, if at all) applied if, as Panos Merkouris postulates, the “interpretation of customary rules is not bound by the straitjacket of ‘practice’ and ‘opinio juris’” (id., n. 160), a proposition which the authors contest due to its arguably “excessive” reliance on “deduction” (and which, at face value, would in fact lead to the two-element approach’s non-application to CIL interpretation). Indeed, the authors object to CIL interpretation on grounds that its (presumed) predominantly “deductive” nature might distort CIL identification’s (presumed purely “inductive”) nature: for them, “depending on what rules of interpretation … apply, an ‘interpretation’ of customary international law may result in unsupported conclusions as to the content of the customary rule in question, for example, by introducing an excessive measure of deduction” (id., emphasis added). Yet, such non-application (or misapplication through “excessive … deduction”) of the two-element approach is not entailed by (every method of) CIL interpretation. In addition, if the formation of custom (as source of CIL) and the application of resulting CIL—as a form of (source-based) law capable of creation, interpretation, application, modification, termination, etc., like any other form of (source-based) law—are clearly distinguished (a distinction not precluded by the ILC’s focus on “identification”), differences in the two-element approach’s use for identification or interpretation purposes may be probably set aside without imperiling its proper application. Furthermore, admitting previously identified CIL’s interpretability is demanded by the fact that, in practice, disputes may not concern whether a CIL rule exists (a matter of CIL identification) but rather its precise content and scope (a matter of CIL interpretation).

The Structure of Custom: Custom’s (Two) Constitutive Elements

The two-element approach to CIL identification, requiring general practice and acceptance of that practice as law, lies at the Conclusions’ core and stands in stark contrast to certain “modern approaches” amounting to “a single-element theory, … de-emphasizing or dismissing one or other of the two elements” (p. 73). CIL, “[i]n the absence of practice, … would be a misnomer,” and “[t]o deny the necessity of opinio juris … would … deny the crucial distinction … between custom … and mere usage” (p. 74). The bi-elemental nature of CIL identification implies it partakes in the commutativity of (mathematical) dyadic operations:Footnote ¹⁰ the order of the operands does not change the result or, as the authors put it, “there is no mandatory order in which one should determine whether both constituent elements are established” (p. 91). While such commutativity is not problematic, a certain lack of precision in how the two constituent elements are approached may be so (and probably reflect certain conceptual insufficiencies in the Conclusions and the materials underpinning them). For instance, the view that “the same action may serve as evidence both of State practice and opinio juris,” provided each constitutive element is “assessed separately” (p. 92, emphasis added), while conceptually impeccable, risks verging on the artificial. Indeed, crucially, it may fail to fully assuage concerns over “double-counting,” a “concern” discarded by the authors as having “no real substance: the ascertainment of a general practice is not simply a question of numbers” (id.): yet, in holding that “citing the same conduct or material when assessing both elements does not eliminate either element as a necessary condition for determining the existence of a rule of customary international law” (id.), the authors have not ruled out the possibility (theoretical though it may be) that one single “conduct or material” is so claimed to satisfy the two-element approach as forcefully as they could have.

The nature and modalities of consent involved, if any, in determinations of acceptance as law underscore various other issues addressed in the Conclusions and by the authors.

Consent plays an important role, generally and in CIL identification: consent, the authors reason, underpins what the authors call CIL identification’s “moral dimension” (pp. 5–7). In their discussion of such “moral dimension,” they agree with W. Michael Reisman that “[i]nternational law is based on consent” and, consequently, that finding CIL to have “formed” through “state practice” not “accompanied by the necessary attitudes” would involve a “profoundly immoral” outcome, tantamount to “try[ing] to impose a contract on a party that had never agreed to it” (p. 6, emphasis added). The forecited passage illustrates the ambiguity of portraying consent both as “attitude” (which may or may not be consensual) and (contract-like) “agreement” (which epitomizes consent). Indeed, consent involves “will,” and the latter can have a (merely) factual or juridical facet: on one hand, as the Permanent Court of International Justice stated, in international law, “municipal laws are merely facts which express the will … of States” (p. 127, emphasis added);Footnote ¹¹ on the other hand, as the authors observe, “[t]he separate legal personality of international organizations makes them capable … of expressing a ‘juridical will’ …, including acceptance as law” (p. 196, emphasis added).

Yet, precisely how acceptance as law involves consent is not addressed in a straightforward manner by the Conclusions or the authors. The authors refer to “the subjective element of acceptance of the practice as law” as a “subjective sense of a legal obligation or a correlative legal right” (p. 169, emphasis added) and note that “a State … may assert or acknowledge a legal right or obligation under customary international law” via “many vehicles” (p. 179, emphasis added). Such “subjective sense” may further take negative form: “an express disavowal of legal obligation (‘opinio non-juris’), … when States … apply … rules … not because they are binding but as a matter of policy” (p. 171 n. 57). The forecited references to acceptance as law as assertion, acknowledgment, disavowal, or, in general, as “subjective sense,” do not indicate whether juridically significant “will” (or “consent” proper), is involved. Nevertheless, that acceptance as law may be properly “consensual” is sometimes implied by the authors, as exemplified by their clarification that “inaction as evidence of acceptance as law has to be assessed with great circumspection” for “silence by no means always denotes consent” (p. 190, emphasis added), further implying that, in the final analysis, given the ultimate absence of any distinction between (in)action and evidence thereof (as implied by the Conclusions’ deliberate omission of “any distinction between an act and evidence thereof” (p. 121)), silence itself may constitute consent.

Whether, in addition to the consensual character (vel non) of acceptance as law, the modality of acceptance as law need be abstract or concrete and collectively or individually manifested raises various unresolved issues, on which the authors’ analysis touches yet without reaching definitive findings (beyond those which the Conclusions already—and also, at times, inconclusively—suggest).

Acceptance as law, according to the ICJ’s widest articulation thereof, implies that “general recognition that a rule of law or legal obligation is involved” (p. 175, emphasis added).Footnote ¹² And, insofar as a general practice is subject to a “legal obligation,” as the ICJ observes, that general practice “is rendered obligatory by the existence of a rule of law requiring it”Footnote ¹³ (p. 174, emphasis added). The former formulation is sufficiently wide as to comprise the legal consequence inferred by the latter: what is accepted as “law” might be a “rule of law” or a “legal obligation” and these two objects of acceptance as law may be separately analyzed, even if normally “rules of law,” being typically normative, create obligations, and every “legal obligation” is (directly or indirectly) grounded on a rule of law. These separate, yet closely interrelated, legal categories can be best addressed by the (traditional, albeit often misused) distinction between “source of law” and “source of obligation,” as succinctly argued below.

The Functions of Custom and General and Particular CIL: A Difference of Nature, Not of Degree

Particular CIL identification involves what the authors call a “stricter application of the two-element approach” (p. 84 n. 27, emphasis added), yet this formulation probably understates the difference between general and particular CIL identification at play. While “stricter” suggests a difference of “degree,” on closer inspection, what emerges are differences of “nature,” which, without calling into question the uniformity of the two-element approach, can be best approached as stemming from the different functional nature of particular custom, by contrast to general custom. The distinction between source of law and source of obligation may elucidate such functional differences.

Particular CIL identification in the absence of publicly available general practice, for instance, is articulated by the authors based on its “opposability.” For instance, insofar as no requirement of public availability exists, the authors envisage the possibility of custom based on general practice, which, albeit not “confidential,” is “known among only some or even two States [that] may contribute to the development of a rule of particular … customary international law, opposable to those States alone” (p. 133, emphasis added).

The notion of source of obligation, whose legal effects are often described through the notion of “opposability,” may shed light on the distinct nature of particular CIL rules, by elucidating particular custom’s predominant function in ways the latter notion does not. The authors state that “[r]ules of particular customary international law evolve from a general practice among two or more States that is accepted by them as regulating their relations inter se” (pp. 290–91, emphasis added). Consequently, they add, a particular CIL rule does “not bind other States that have not participated in the practice or otherwise assented to be bound thereby” nor “creates neither obligations nor rights for third States” (p. 291, emphasis added), leaving the states involved in a “position … similar to that set out in [VCLT] articles 34–38” (id., n. 7). Consequently, unlike general CIL, “[e]ven a rule of particular customary international law of a wide scope (such as a regional one) could thus not bind States seemingly encompassed by it without their acceptance” (p. 297, emphasis added). Of course, particular CIL rules remain “part of—and ultimately derive their validity from—the general international legal system” (p. 291). That particular custom is characterized as predominantly operating in the capacity of “source of obligation” (which is consistent with the concrete legal effects suggested by the authors’ propositions) need not detract from their (potentially concurrent) character as “source of (particular rules of) law” (of which the authors remind, by noting the continuing appurtenance of particular CIL rules (like general CIL rules) to international law). Indeed, as the ICJ stated in Right of Passage, general practice (like that bilaterally developed by India and Portugal, the parties to that case) can be one which has been “accepted as law by the Parties and has given rise to a right and a correlative obligation”Footnote ¹⁴ (p. 293, emphasis added).

That consent to a particular custom, primarily operating as source of obligation, is involved probably gives more definitive expression to the authors’ proposition that an “adaptation of the two-element approach” is involved where particular CIL identification is undertaken, which “highlights the need to ascertain clearly which States have participated in the practice and accepted it as governing the relations between them” (p. 297, emphasis added). Such “relations” most probably take the form of obligational legal relations, involving “a right and a correlative obligation.” Particular CIL, like general CIL, would not require “express” consent on the part of all states bound by the obligations arising thereunder, as observed in the authors’ statement that the “stricter” application of the two-element approach “brings to the fore the role of acceptance as law (be it express or tacit)” (id., emphasis added). However, more than a “stricter” application of the two-element approach, what particular CIL identification requires is determining that concrete legal obligations, as opposed to only (abstract) legal rules, requiring “acceptance … of each of the States in question” (id., emphasis added) exist. Two other features of particular CIL discussed by the authors lend further support to approaching it as a source that primarily operates in an obligation-creating, rather than an exclusively lawmaking, capacity (thus underscoring that a difference of functional nature, rather than of degree, is at play): the authors, in spite of the fact that “there would seem to be no relevant practice,” significantly suggest that particular CIL rules, unlike general CIL rules, can be contracted out, insofar as “a State may cease to be bound by a rule of particular customary international law … if it ceases to be associated with the ‘limited number of States’ concerned” (id., emphasis added); and, where particular CIL is invoked, “the party seeking to rely on the rule will have to establish that it exists and … is opposable to the State in question,” thus showing that particular CIL, in the authors’ view, “is quite different from general customary international law, for which no such burden exists before an international tribunal”, because, the authors add, apparently “the jura novit curia principle does not apply” to particular CIL (p. 298, emphasis added).

Concluding Reflections

This review has demonstrated that, despite the authors’ deliberate avoidance of theorization, some of their most significant contributions, addressing issues insufficiently discussed by the Conclusions, unavoidably involve choices of methodological and theoretical stances, potentially carrying consequential (even practical) implications. It has also examined how custom’s structure is articulated and, in particular, precisely how acceptance as law may involve consent. And it has argued that what underpins the distinction between general and particular CIL identification are not differences of “degree” (as suggested by merely characterizing the two-element approach’s application as “stricter”), but rather of “nature” (of a functional character), and that the latter differences are best captured by asking whether particular custom predominantly operates as “source of obligation” rather than only as “source of (particular) law.”

In sum, the authors’ work provides an indispensable companion to the Conclusions and, where it leaves room for analytical elucidation, a starting point for future enquiries into custom’s nature, structure, and functions.