1 Introduction

A number of scholarly contributions on the theme have tackled the determinationFootnote ¹ of rules of customary international law (CIL) under the umbrella of the methodological dualism between induction and deduction.Footnote ² Induction indicates the method of extrapolating a general rule by observing specific instances of practice; deduction is instead the method whereby a specific rule can be inferred from generally accepted rules or principles.Footnote ³ ‘Filling lacunae’ by ascertaining rules of CIL is a canonical example of deduction. Accordingly, two main approaches have been described as underpinning the ascertainment of rules of CIL by interpreters. Pursuant to the former, a rule of CIL may be induced from patterns of state practice and opinio juris. This way of ascertaining rules of law proceeds from the observation of empirical facts and, via induction, finds rules of customary law which are created by the combination of the two constitutive elements.Footnote ⁴ As such, ‘lawyers move behind the law and cannot pretend to lead it’.Footnote ⁵ For the latter, instead, a rule of CIL may be (logically) deduced from the existence of axiomatic rules or principles of international law, for example the principle of sovereign equality between statesFootnote ⁶ or the principle of good faith.Footnote ⁷ This way of reasoning is based on the fundamental assumption that international law is a system of rules where claims to the existence of CIL rules draw justification from their coherence with other rules within the system.Footnote ⁸

However, the methodological dualism between induction and deduction is too ambitious and short-sighted at the same time. It is too ambitious, because it presumes that an extensive review of empirical elements would point to the existence of a legal rule presumably and incontrovertibly existing ‘out there’, ready to be singled out; and it is too short-sighted, because it disguises – as empirically or logically based – the argumentative nature of claims to existing rules of CIL and the role that judicial discretion plays therein. Interestingly, the methodological oscillation between induction and deduction may be portrayed as a struggle between a historical and a philosophical approach to the identification of rules. While the historical approach (induction) would point to the collection of facts as empirical evidence from which to extract a certain historical narrative, on the other hand the philosophical approach (deduction) would serve as an efficient short-cut to make a logically based descriptive claim of the law.Footnote ⁹ Importantly, both approaches strive to advance claims to scientific truths, thus leaving little space to the contestation of such findings.

In light of the foregoing, this chapter has a twofold aim. First, it recalibrates the debate surrounding the ascertainment of CIL towards an argumentative lens. Such a recalibration is conducive to illuminate the element of discretion involved in the ascertainment of rules of CIL, which remains controversially clothed in a method-focused debate. Importantly, this implies looking at potential rules of interpretationFootnote ¹⁰ of CIL not as a method to find the law ‘out there’, but rather as shared arguments to justify any claim to existing rules of CIL.

Secondly, this chapter clarifies an irony surrounding the determination of rules of CIL. If, on the one hand, illuminating the element of discretion defeats the idea of an entirely objective reality observable by courts; on the other hand, the authoritative verbalisation of such rules by courts is necessary for their materialisation and for their coming to fruition in the legal practice. In the absence of such authoritative verbalisation, there would hardly be any ‘rule’ of CIL; at best a rough idea of a metaphysical CIL. This is demonstrated by a number of casesFootnote ¹¹ in which, where applicable, courts have relied on prior judicial decisions ascertaining rules of CIL or of ‘soft law instruments’ codifying such rules qua written utterances on CIL.

This chapter is divided into four sections followed by a fifth conclusive one. Section 2 takes the cues from the recent work of the International Law Commission on the Identification of Customary International Law and considers the implications of the shift from a methodological to an argumentative lens for such identification. Section 3 presents a perusal of judicial decisions in the context of international criminal law illustrating the range of discretion exercised by judges in appraising evidentiary elements for the purposes of ascertaining rules of CIL. Section 4 reflects upon the role of courts for the materialisation of ‘rules’ of CIL and the correlated role that past judicial decisions play in the ascertainment of such rules. Finally, Section 5 draws conclusions.

2 Revisiting Old Myths: From Epistemological Methods to Argumentative Strategies

The work of the International Law Commission (ILC) on the identification of CILFootnote ¹² intervenes in the debate about the determination of CIL rules by tackling the long-standing question of the ‘methodology’ that interpreters ‘must’Footnote ¹³ apply to identify such rules. Indeed, the international law literature has repeatedly emphasised the difficulties linked to the determination of rules of CIL. One of such difficulties rests with the fact that evidence of state practice and of opinio juris may be interpreted differently by different courts, may be considered quantitatively insufficient to prove the existence of customary rules or to be regarded as conclusive of such an existence. Different types of practice may be taken into account, as well as different methods may be employed in this identification activity. This point was expressed by Judge Tanaka in his dissenting opinion in the seminal judgment in the North Sea Continental Shelf cases:

In the face of such difficulties, the ILC has laid down preliminary conclusions seeking ‘to offer practical guidance on how the existence of rules of customary international law, and their content, are to be determined’.Footnote ¹⁵

Two points are in order here. First, the ILC conclusions make reference to two types of activities: one ascertaining the existence of a rule of CIL, which, from a formal point of view, was created by state practice and opinio juris; the other determining the content of such an identified rule. Although both these activities are interpretive in character, they concern two ontologically different dimensions: that of law-ascertainment and that of content-determination.Footnote ¹⁶ While the former articulates itself along elements that are constitutive ingredients to a claim to an existing customary rule, the latter typically hinges on interpretive strategies such as the textualist, intentionalist and purposivist.Footnote ¹⁷ It is germane to acknowledge that while the ascertainment of rules of CIL is ingrained in a vigorous doctrinal convergence towards the two-pronged structure of state practice and opinio juris, albeit identifiable via different methods, the content-determination activity appears fuzzier and is indeed a dimension where the exercise of discretion by interpreters is left most unrestrained. This chapter primarily focuses on the law-ascertainment activity.

Secondly, by offering such preliminary conclusions, the ILC seemingly perpetuates two intrinsically entangled myths, namely the myth of a universal methodology to explore and assess state practiceFootnote ¹⁸ and opinio juris; and the myth of a hypothetical ‘out there’ where to identify already existing rules of CIL.Footnote ¹⁹ The idea of these being myths stems from a sceptical conception of interpretation, defined as an act consisting in ascribing, as a matter of choice, normative meaning to texts as well as in engaging in legal constructions, especially when no text to interpret in the former sense is available. Indeed, legal construction is particularly relevant in the context of ascertaining rules of CIL as, by definition, such rules are unwritten or, rectius, ‘unexpressed’, and are made expressed though the ascription of a normative meaning to empirical facts.Footnote ²⁰ Such definition of interpretation may be further reduced by accepting that also texts are no more than facts and therefore interpretation is no more than an act of legal construction of facts bearing a normative meaning. As a consequence, law is a set of interpretive practices in which judges play a central role in constructing the object to interpret.

Against this sceptical understanding of interpretation, the problématique of reiterating these legendary beliefs essentially rests with the normative view which produces the empirical facts upon which to substantiate the existence of a certain CIL rule. Indeed, state practice and opinio juris do not exist, under these labels, in the empirical world out there, but are an interpreter’s intellectual construction. As such, they are first identified, selected, assessed and categorised like relevant by the interpreter, as a reflection of his/her normative ideology.Footnote ²¹ In other words, the selection and assessment of practice and opinio juris are but the result of an exercise of discretion, which looms in every act of legal interpretation.

The ILC Report does not consider this stage of construction of relevant facts, but rather assumes that state practice and opinio juris are given, intelligible to interpreters in equal terms.Footnote ²² However, this position has largely displayed its limits,Footnote ²³ in that legal interpretation entails a subjective choice of the judge between different possible interpretive outcomes and, thus, it cannot be retained watertight to an interpreter’s own normative stance vis-à-vis international law as a legal order and its function.Footnote ²⁴ Once assumed that interpreters contribute themselves to construct the object of interpretation, professing that interpreters operate a finding exercise of legal rules appears a commitment of faith more than anything else. As such, questions pertaining to the law-ascertainment and the content-determination of rules of CIL are inescapably accompanied by rival ideologies about the ontology of interpretation in international law and, more broadly, about international law as a legal order.

In light of the foregoing, the ILC conclusions are worthy of reflection beyond the myth’s objectivity and ‘out-there-ness’ in the ascertainment of rules of CIL it seemingly reiterates. Rather, by moving away from understanding law-ascertainment and content-determination as a finding exercise, one could appreciate the ILC draft conclusions as directives constraining the interpreters’ range of discretion in the context of justification. In other words, evidence of state practice and opinio juris are used to justify the claim to existing rules of CIL, not to find them. Looking at induction and deduction as argumentative strategies entails that interpreters of international law lay down norm-descriptive statements about the law that require justification in order to be accepted as correct.

The implications of a recalibration from a methodological to an argumentative lens are manifold. First, it entails looking at opinio juris and state practice as corroborative or evidentiary elements, rather than truly constitutive or formative ones. Importantly, their persuasive strength rests on the fact that they are traditionally accepted as necessary ingredients to a claim to existing rules of CIL. As questions about the existence and content of CIL rules are addressed within an argumentative framework, it follows that, by way of legal justification, these findings need to persuade that they are correct.Footnote ²⁵ Secondly, understanding the ascertainment of rules of CIL as a finding exercise rather than an argumentative activity suggests that there is one objectively correct rule to which general practice and opinio juris point. Conversely, argumentation, as a process of justification, is premised upon the idea that potentially a range of different hypotheses about existing rules of CIL can be justified and regarded as correct in law.Footnote ²⁶ By admitting that different simultaneous plausible interpretations of facts and legal rules are possible, the argumentative lens emphasises the subjective element involved in the ascertainment of rules of CIL and, as such, it embraces rather than negating the diverse and competing normative views informing interpretation in international law. Thirdly, a recalibration from a methodological to an argumentative framework entails that criteria (or meta-rules) envisaged as a universal methodological roadmap to the ‘identification’ of rules of CIL – for example those proposed by the ILCFootnote ²⁷ – are instead arguments restraining the discretion of interpreters – with special regard to courts – that is, what it can be considered and how much weight shall be given to these elementsFootnote ²⁸ in determining the existence and the content of rules of CIL.Footnote ²⁹ Against this backdrop, the point is not to establish the appropriate method to identify customary international rules existing out there, but rather to establish the range of discretion which a court can possibly exercise in order for the ascertainment of rules of CIL to be reasonable and not to result in arbitrary adjudication.

3 Judicial Discretion in the Ascertainment of CIL: Clues from the Practice

The preceding sections have attempted to problematise the myth of epistemological methods reiterated in the scholarly debate on the determination of rules of CIL. In the wake of this, a twist to an argumentative lens is suggested to illuminate the element of discretion in legal interpretation, typically left in the background. Discretion, in the context of legal interpretation too, is not a concept of easy definition. One tentative definition has been provided by Cass R. Sustein as ‘the capacity to exercise official power as one chooses, by reference to such consideration as one wants to consider, weighted as one wants to weight them’.Footnote ³⁰ In Sustein’s view, ‘[a] legal system cannot avoid some degree of discretion, in the form of power to choose according to one’s moral or political convictions. … [T]he interpretation of seemingly rigid rules usually allows for discretion. But a legal system can certainly make choices about how much discretion it wants various people to have’.Footnote ³¹

Typically, in a legal order, courts are afforded some degree of interpretive discretion, enabling judges to make a choice between possible interpretive outcomes. The international legal order is no exception to this. For instance, Article 38(2) of the Statute of the International Court of Justice provides a useful illustration of the discretion vested in the court by state parties, in that it acknowledges the non-prejudiced ‘power of the Court to decide a case ex aequo et bono, if the parties agree thereto’. Likewise, in the Continental Shelf case, the International Court of Justice (ICJ) expressly recognised its power to discretionary choices: ‘when applying positive international law, a court may choose among several possible interpretations of the law the one which appears, in the light of the circumstances of the case, to be closest to the requirements of justice’.Footnote ³² Indeed, past judicial decisions on points of CIL are a good terrain to explore the way in which courts exercised discretion in the assessment of evidence of state practice and opinio juris. Qualities typically associated with rules of CIL such as repetition, generality, uniformity and duration, as well as the weight to allocate to opinio juris as compared to state practice were laid down and elaborated in judicial decisions. Arguably, these case-law-based criteria are an expression of how discretion is channelled into legal argumentation and enables the exercise of discretion by a judge to appear rationalised, rather than arbitrary, in that they offer a range of arguments that a court may put forward to justify a certain holding.

This section considers some judicial decisions, as well as separate opinions laid down by the International Criminal Tribunal for the Former Yugoslavia (ICTY) established by UN Security Council resolutions under Chapter VII.Footnote ³³ Looking at these decisions is particularly appropriate for the purposes of this contribution, given the tribunal’s mandate to apply rules that had, ‘beyond any doubt’, crystallised into CIL.Footnote ³⁴ The purpose of showcasing these judicial decisions is to illustrate, by reference to practice, the range of approaches exhibited by judges in the ascertainment of rules of CIL. Arguably, such a variation cannot be adequately explained by the methodological dualism between induction and deduction, as the evaluation of evidentiary elements supporting the existence of a rule of CIL is far from incontrovertible. After all, what judges do is to argue in favour of an interpretation rather than another based on certain elements of state practice and opinio juris.Footnote ³⁵ As such, statements about the existence of a particular rule of CIL are argumentative in nature and seek to persuade a certain audience of their correctness.

In the seminal Erdemović case, the ICTY Appeals Chamber was to consider whether, under CIL, duress would allow a complete defence to a soldier charged with the killing of civilians.Footnote ³⁶ To this purpose, national courts’ decisions and state legislations were examined. Yet, the threshold beyond which such evidence suffices to demonstrate the existence of a rule of CIL lies within the discretion of an interpreter. For instance, the joint separate opinion of Judges McDonald and Judge Vohrah, appended to the judgment is a good illustration of how elements of state practice and opinio juris are hardly incontrovertible and can be differently appraised by different interpreters.

This holding considered the defence’s survey, in its Notice of Appeal, of

Judges McDonald and Vohrah finally concluded that ‘[n]ot only is state practice on the question as to whether duress is a defence to murder far from consistent, this practice of States is not … underpinned by opinio juris’,Footnote ³⁹ since ‘the decisions of these tribunals [the post–World War Two military tribunals] or those of other national courts and military tribunals constitute consistent and uniform State practice underpinned by opinio juris sive necessitates’.Footnote ⁴⁰

The approach of Judges McDonald and Vohrah can be contrasted with the declaration of Judge Robinson to the Appeal Judgment in the Furundžija case,Footnote ⁴¹ in which the judge considered that ‘[a] global search, in the sense of an examination of the practice of every state, has never been a requirement in seeking to ascertain international custom, because what one is looking for is a sufficiently widespread practice of states accompanied by opinio juris. … [I]t is accepted that such [national] decisions may, if they are sufficiently uniform, provide evidence of international custom’.Footnote ⁴²

This strikes a significant discrepancy between the approach of Judges McDonald and Vohrah, in upholding an extensive empirical test, as formulated by the ICJ in the cited North Sea Continental Shelf cases, for ascertaining the existence of a rule of CIL, and Judge Robinson who instead submitted that a wide (‘global’) test has never been the requirement, but rather a sufficiently widespread practice. The threshold of empirical evidence demanded by the two approaches is expression of the range of discretion available to the interpreter when engaging in the ascertainment of rules of CIL.

Moreover, judges have granted a different weight to state practice and opinio juris for the purposes of establishing rules of CIL. One such illustration is offered by the Kupreskić caseFootnote ⁴³ in which the ICTY Trial Chamber acknowledged that opinio juris may play a primary evidentiary role at the expense of state practice.Footnote ⁴⁴

The ICTY Trial Chamber further elaborated on the formation of a rule of CIL prohibiting reprisals against civilians by reference to ‘widespread opinio necessitatis’ … ‘confirmed, first of all, by the adoption, by a vast majority, of a Resolution of the UN General Assembly in 1970 which stated that “civilian populations, or individual members thereof, should not be the object of reprisals”’ and by the high number of states that have ratified the First Protocol.Footnote ⁴⁶ The reference to manifold instruments such as the above mentioned UN General Assembly (UNGA) resolution of 1970, a Memorandum of the International Committee of the Red Cross (ICRC) of 7 May 1983, the pronouncement of ICTY Trial Chamber I in Martić, ‘substantially upholding such a rule’,Footnote ⁴⁷ shows the intention of the chamber to find ample corroboration to its claim to the existence of a rule of CIL. This overview, in the Kupreskić case, finally led the chamber to conclude that ‘the demands of humanity and the dictates of public conscience, as manifested in opinio necessitatis, have by now brought about the formation of a customary rule also binding upon those few States that at some stage did not intend to exclude the abstract legal possibility of resorting to the reprisals under discussion’.Footnote ⁴⁸

In the Furundžija case, the ICTY Trial Chamber was to establish the customary character of the prohibition of torture in time of armed conflict. The chamber found that ‘the broad convergence of international instruments and international jurisprudence demonstrates that there is now general acceptance of the main elements contained in the definition set out in article 1 of the Torture Convention’.Footnote ⁴⁹ In particular, indication of the customary character of the prohibition of torture in time of armed conflict was inferred from the number of ratification of relevant international treaties, as well as in the lack of opposing claims by states purporting the contrary.Footnote ⁵⁰ This finding was finally sealed by reference to relevant ICJ judicial decisions.Footnote ⁵¹

This overview of judicial pronouncements suggests that judges play a fundamental role in the ascertainment of CIL. In particular, judges’ verbalisation of ‘rules’ of CIL in judicial decisions appear a propaedeutic step for making such rules materialise in an authoritative form and bringing them to fruition in legal practice. Courts’ engagement in such verbalisation may also be determinant to assess the interpretive steps (meta-rules) claimed to have been adopted for the ascertainment of such rules and possibly challenge them. As recalled earlier, judges may engage in the formal ascertainment of rules of CIL, as well as in the determination of their substantive content.Footnote ⁵² While for the former, state practice and opinio juris occupy a prominent role in legal argumentation, for the latter courts are seemingly inclined to refer to existing written formulations as bearing normative value. In fact, reference to existing written formulations allows a court to articulate an interpretation of the content of existing rules of CIL in a more persuasive way.

4 The Materialisation of ‘Unexpressed’ Rules and the Role of Past Decisions

Based on the judicial decisions considered thus far, at least two factors have played a role in allowing the interpreter to modulate the range of discretion: first, the threshold of empirical evidence required for a claim to CIL; second, the more or less weight that an interpreter may attribute to state practice and opinio juris as evidentiary elements. In addition, one may consider factors which instead appeared to constrain a judicial exercise of discretion. For instance, the following examples show that prior written formulations of unexpressed rules – first and foremost, although not exclusively, judicial decisions – were typically relied upon in international adjudication.

In the recent Chagos Advisory Opinion,Footnote ⁵³ the ICJ was to determine ‘when the right to self-determination crystallised as a customary rule binding on all States’.Footnote ⁵⁴ After recalling the trite adage that ‘custom is constituted through general practice accepted as law’, the court turned to the UNGA resolutions to survey the evidence of state practice, which it considers relevant and determinant for sealing the customary nature of the right to self-determination, notably resolutions 637 (VII)/1952, 738 (VIII)/1953, 1188 (XII)/1957 and 1514 (XV)/1960. The court regarded this latter as ‘a defining moment in the consolidation on State practice on decolonization’ clarifying ‘the content and scope of the right to self-determination’.Footnote ⁵⁵ In ascertaining the customary character and the substantive contours of the right to self-determination, the court thus deferred to UNGA resolution 1514/1960 not only as declaratory of the existing customary right to self-determination,Footnote ⁵⁶ but also to determine ‘the content and scope of such a right’,Footnote ⁵⁷ namely to interpret such a right.Footnote ⁵⁸ Unsurprisingly, such material is used by the court to justify the claim of ascertained rules of CIL having a certain meaning.

In the Rwamakuba case,Footnote ⁵⁹ the International Criminal Tribunal for Rwanda (ICTR) Appeals Chamber was confronted with the question whether joint criminal enterprise was an existing mode of liability under CIL, whereby conviction of an individual was permissible. The chamber approached the question by reference to state practice and opinio juris, but instead of engaging with these elements, it upheld the finding in the Tadić Appeals Judgment pursuant to which the participation to a common plan to commit a crime against humanity was criminalised under CIL before 1992.Footnote ⁶⁰ The ICTY Appeals Chamber has placed similar reliance in other cases on proceedings held following World War II, including the proceedings before the International Military Tribunal and before tribunals operating under Allied Control Council Law No 10 (‘Control Council Law No 10’), as indicative of principles of CIL at that time.Footnote ⁶¹

Similarly, in the Kayishema & Ruzindana case, the Appeals Chamber considered the principle of the right to a fair trial as ‘part of customary international law … embodied in several international instruments, including Article 3 common to the Geneva Conventions [See Čelebeći Appeal Judgment, §§138 and 139]’.Footnote ⁶² In the Hadžihasanović et al case, the ICTY Appeals Chamber considered that ‘to hold that a principle was part of customary international law, it has to be satisfied that State practice recognised the principle on the basis of supporting opinio juris’.Footnote ⁶³ By reference to the ICJ judicial decisions concluded that ‘Article 3 common to the Geneva Conventions of 1949, which has long been accepted as having customary status [See Corfu Channel, Merits, I.C.J. Reports 1949, p. 22, and Military and Paramilitary Activities in and against Nicaragua, I.C.J. Reports 1986, pp. 112 and 114].’ In the same case, the Appeals Chamber found ‘that the customary international law rule embodied in Article 3(e) is applicable in all situations of armed conflict [international and non-international], and is not limited to occupied territory [Kordić Appeals Judgement, §78 (“[t]he prohibition of plunder is general in its application and not limited to occupied territories only”)]’,Footnote ⁶⁴ and that, as such, ‘violations of the prohibition against “plunder of public or private property” under Rule 3(e) entail, under customary law, the individual criminal responsibility of the person breaching the rule’.Footnote ⁶⁵ Similarly, in the Tadić Appeal Judgment, the ICTY Appeals Chamber found case law to be reflective of CIL.Footnote ⁶⁶

At a very first glance, the ascertainment of rules of CIL, more than any other ambit, seems to confirm the tenets of a legal realist approach to law. If law is fact, namely the law which is applied in practice by courts, what else than ‘finding’ rules of CIL can prove that such rules are brought to ‘reality’ through judicial pronouncements? Indeed, the ascertainment of ‘unwritten law deriving from practice accepted as law’Footnote ⁶⁷ entails important juristic and epistemological implications. From a juristic standpoint, the ascertainment of rules of CIL consists in an act of interpretation carrying with itself claims of formal and substantive validity. From an epistemological point of view, the act of ascertainment presupposes that rules of CIL exist ‘out there’ and that an interpreter may bring them to perceived ‘cognition’ or to ‘reality’, hence to fruition of actors in the international legal practice.

In relation to this, two entangled questions are in order. First, what kind of act is the act of ascertaining rules of CIL? It is argued that this is an act of legal construction that is adjudicative, not cognitive, in nature.Footnote ⁶⁸ Second, are interpretive utterances claiming the existence of CIL norm-descriptive or norm-expressing statements? In Alf Ross’ view, judicial decisions may be considered as norm-descriptive statements about the law, as opposed to deontic rules, which are norm-expressive statements of the law.Footnote ⁶⁹ More precisely, the written formulation of rules of CIL in judicial decisions provides these rules with an authoritative text constituted by the written utterances of what the court ascertained as existing rules of CIL and what it interpreted as their normative meaning. This owes to, among other things, the nature of international law, and law more generally, as a learned profession in which participants – including courts – articulate verbal/written expressions about the formal and substantive validity of the law.Footnote ⁷⁰ Importantly, such verbalisation stems from an evaluative process – entrenched in an exercise of discretion – channelled through the judges’ normative ideologyFootnote ⁷¹ about what they believe exists – or should exist – as a matter of legal rules, universally binding qua CIL. Within this learned profession, judicial decisions constitute authoritative statements on rules of CIL, embedding a standard of correctness.Footnote ⁷² As such, this actual formulation of rules of CIL in their form and content is necessary in order for ‘rules’ as such to materialise, as well as to formally and substantively challenge such rules on the basis of a cognised formulation. Even more so, if courts claim to have found rules of CIL based on state practice and opinio juris. Whether those verbal expressions truly reflect existing law is arguably irrelevant as long as those expressions are accepted as correct.

As such, judicial decisions verbalising rules of CIL fall short to be considered as purely norm-descriptive statements on the law, as they embed the (deontic) expression of rules of CIL. In other words, sentences which formulate unexpressed norms are ‘secretely prescriptive’,Footnote ⁷³ as they pretend to be describing existing law but are actually constructing new rules.

To illustrate this ambiguity, one may refer to the ILC Report on the identification of CIL mentioned above, whose proposed meta-rules are not laid down in a vacuum. Rather, they considerably draw from ICJ pronouncements determining the qualities of the constitutive elements of CIL, that is, the criteria necessary to claim the existence of a CIL rule. For instance, in the commentary to Draft Conclusion 2, the ILC maintains the same criteria for the identification of rules of customary law as those established by the ICJ in its judicial decisions:

The ample reliance on these judicial decisions suggests that criteria determined therein have been accepted as correct. In particular, criteria such as ‘settled practice’ or ‘consistent practice of the majority of the States’, found in judicial decisions inasmuch in the report of the ILC, stem from the discretion that a court enjoys in the adjudication of legal issues – that is, they are set forth according to the discretion which the court considers it is able to exercise – and have the power to limit or further enlarge the measure of discretion afforded to the judge in later cases. The ILC Report sanctions the criteria relevant for the ascertainment of rules of CIL that have been considered persuasive. Furthermore, the determination by the ILC that the test to ascertain the existence of a rule of CIL ‘must always be: is there general practice accepted as law?’ is eloquent for the constraint to interpretive discretion which the ILC conclusions, too, seek to place onto subsequent interpretive authorities.

The spurious nature of judicial decisions ascertaining rules of CIL as merely norm-descriptive statements is further exacerbated by the sceptical understanding of interpretation discussed above, looking at it as an argumentative art rather than an exact science. In fact, courts ascertaining rules of CIL operate an existential interpretationFootnote ⁷⁵ and may not be regarded as performing a merely declaratory function. Although this outlook bears the marks of legal realism,Footnote ⁷⁶ it is not limited to it. Admittedly, even Hans Kelsen argued that ‘the function of adjudication is constitutive through and through’ and ‘the judicial decision is itself an individual legal norm’.Footnote ⁷⁷

5 Conclusions

Qua unwritten by definition, CIL seems to appertain more to a metaphysical dimension than to the world of reality. In this scenario, the judge seemingly plays an intermediary role between the metaphysical dimension of intangible CIL and the world of reality in which rules materialise through the pronouncements of the judge. As such, courts may be seen as bringing CIL to real life – as opposed to a metaphysical dimension – drawing from a world of hypothetical rules of CIL. In ascertaining the existence of such rules, and formulating their content,Footnote ⁷⁸ courts lay down written utterances of otherwise unwritten ‘law’ presumably existing ‘out there’. In other words, the route from the metaphysical space to the world of reality channelled by courts enables the materialisation of rules (verbalised in written utterances), the scrutiny of the methods and criteria (meta-rules) used to ascertain such rules, as well as the evaluation of the evidence that a court considered.

Courts are in a special position to pronounce such statements because of the authority typically vested in them within a legal order. As argumentative strategies, induction and deduction enable courts to portray the ascertainment of CIL as an act of finding, which does not depend on an exercise of discretion, but rather sets the interpreter in the context of exploring an objective reality. This ascertainment confers to CIL an aura of objectification and divests it of the potential criticism as judge-made law. As such, interpretation – which entails a discretionary choice between possible interpretive outcomes – is perceived as an act of cognition rather than adjudication. Discretion not only lies in the power to make such a choice, but also in formulating a hypothesis about a presumably existing rule of CIL, as a reflection of, inter alia, the ideal of international legal order that a court seeks to realise, as well as in regarding certain principles of international law as axiomatic. Accordingly, a judge may do away with the principle of sovereign equality between states less easily than – say – with the principle of responsibility to protect, depending on which normative ideology he/she would present as axiomatic.

Against this background, this chapter has revisited the methodological dualism between induction and deduction as applied in the context of the ascertainment of rules of CIL. Revisiting such dualism came with suggesting embracing an argumentative lens. Like shifting lenses may entail empowering or disempowering one’s sight, similarly, twisting a methodological focus, which has featured the legal discourse on the identification of CIL, towards an argumentative lens may entail that elements which previously appeared obfuscated become more candid and vice versa.

It has been contended that while the methodological lens obscures the range of discretion exercised by the court in the ascertainment of rules of CIL, the argumentative lens sheds light on it, insofar as a claim of the existence such rules necessarily entails the selection and assessment of state practice and opinio juris which is far from being incontrovertible. The cursory survey of judicial decisions, primarily drawn from the field of international criminal law, has sought to show the different argumentative strategies whereby judges evaluated ‘evidentiary elements’ (state practice and opinio juris). Whether and how judges engage in the argumentative strategies of induction or deduction of existing rules of customary law is after all a discretionary choice. Yet, judicial decisions verbalising rules of CIL are necessary for the materialisation of such unexpressed rules in an authoritative form, as well as for the contestation of such rules, based on the arguably identified form and content. As such, courts play a fundamental role to nurture the myth of rules of CIL as an empirically based discovery rather than a discretion-centred activity.

The ample reference to prior judicial decisions corroborates the fundamental role played by courts in interpreting the world of facts bearing a normative significance (‘practice accepted as law’) and in verbalising ‘rules’ of CIL. In other words, courts are in a special position as interpreters, insofar as their pronouncements are understood as authoritative statements on the law embedding a standard of correctness, upon which actors in a legal field can rely, and which seemingly motivates actors to reiterate the myth of rules of CIL existing ‘out there’.