1 Introduction

The interpretation of unwritten norms is fraught with difficulty. Without the settled text of a treaty, interpreters of unwritten norms are without a basic linguistic framework within which they may conduct their search for meaning. In such situations, the boundaries between the existence of a norm and the determination of its content can become blurred. Interpreters may return to the evidence of the norm’s existence (such as state practice) in order to determine its content or it may be that interpretation itself is part of the constitutive process of unwritten norms. This confusion is exacerbated by a lack of established methods and procedures for the interpretation of unwritten international law. Customary rules are, of course, a type of unwritten international law. Yet, custom is not alone in this category. There are other, even more nebulous, unwritten norms: general principles of law. While it is commonplace to speak of custom and general principles under the umbrella of ‘general international law’,Footnote ¹ it is unclear whether questions of interpretation are to be approached in the same manner for both categories of norms or whether custom and general principles may assist in the interpretation of one another.

My central objective in this chapter is to examine the interactions between these two categories of norms in the context of interpretation. More specifically, I consider whether (and if so to what extent) general principles of law may play a role in the interpretation of customary rules. In approaching this task, I have structured the chapter in three main sections. First, it is necessary to make explicit some assumptions and presumptions about the nature of the norms under examination and the potential relationships between them. To this end, in Section 2 I clarify how I understand general principles of law – that is, as a separate kind of rules distinct from both custom and principles in the true sense of the term. In essence, I view general principles as a distinct category of (mainly secondary) rules that come into existence as a result of a dynamic process of assertion and confirmation involving a range of actors, with a particularly prominent role played by courts and tribunals. Further, I briefly sketch how, on the basis of this understanding, I view the potential relationships between general principles of law and customary law. Building on this foundation, I then turn to examine two possible roles for general principles of law in the interpretation of customary rules. In Section 3 I consider whether, as a distinct category of rules, general principles of law may form part of the legal framework relied upon when interpreting international law generally, and custom in particular. Finally, in Section 4 I examine whether it is within the scope of general principles of law to play a part in the systemic interpretation of customary rules, both as the source of the rule on systemic interpretation and as other ‘relevant rules’ of international law. The core of the argument presented in this chapter is that general principles of law could be the source of many of international law’s general rules on interpretation, including those applicable to custom, and that an enhanced reliance on general principles could facilitate further coherence in the interpretation of customary rules.

2 A Theory of General Principles in International Law

Until rather recently, ‘general principles of law’ had received considerably less academic attention than treaties and custom. The inclusion of general principles of law in the programme of work of the International Law Commission (ILC) has, in no small part, contributed to a significantly heightened interest in the subject.Footnote ² Despite this substantial rise in scholarly debate on the topic, it would still be accurate to describe general principles as ‘the most enigmatic [of the] sources of international law’.Footnote ³ Simply put, there is more disagreement about more aspects of general principles than the other sources. The very nature of these norms, their functions, their creation, and their importance remain unsettled questions. It is, of course, likely that the ILC’s work and the body of literature that has followed and to come will help to solidify the foundations of general principles. Yet, at this stage, it remains necessary to explicitly articulate how I understand general principles and their place in international law. This is particularly pressing as my theory of general principles has aspects that contrast with the mainstream (if there is such a thing in this field), including some departures from the positions adopted in the work of the ILC.

In short, I view general principles of law as concrete rules of law, distinct from broader, value-laden principles in the strict sense of the term. General principles, in my view, are developed not (solely) from the practice of states or through transposition from domestic law. Instead, they develop (usually incrementally) as the result of a dynamic process of assertion and contestation by a range of international actors: certain ‘systemic officials’. Courts and tribunals play a particularly prominent role in the development of general principles, and recourse to this category of rules is typically made where there is the need for a secondary rule not found in the other sources. In the sections that follow, I expand on the idea that general principles are rules, before explaining the processes through which they may be ascertained. In both parts, I endeavour to clarify the key points of distinction and interaction between general principles, on the one hand, and principles stricto sensu and customary law, on the other.

3 General Principles as Part of the Interpretative Architecture of International Law

It has been argued that international law on interpretation has a ‘treaty focus’,Footnote ³⁸ meaning that ‘international legal interpretative doctrine remains firmly rooted in the law of treaties, even as scholars push for a wider lens’.Footnote ³⁹ Explaining this position, Duncan Hollis refers to the challenges present in the ascertainment of unwritten sources, which results in the central position of treaties, specifically the rules contained in Articles 31 and 32 of the Vienna Convention on the Law of Treaties (VCLT) of 1969,Footnote ⁴⁰ in the body of norms that guide the interpretation of international law.Footnote ⁴¹ Indeed, it is clear that the VCLT rules are core elements in the interpretative architecture of international law, with both judicial decisionsFootnote ⁴² and academicsFootnote ⁴³ asserting that they form part of customary international law. On the basis of the aforementioned understanding of general principles and customary law, it is arguable that such assertions are not commonly supported by reference to the required state practice and opinio juris. That being said, this is largely a question of semantics, as I would not question that these rules are part of general international law. Yet, from this starting point, two issues arise in relation to the interpretation of customary international law. First, whether the general international law equivalents of the VCLT rules apply to unwritten rules (in the same way as they do to treaties) and, second, whether there are other norms at play in the interpretation of international law.

Precisely mapping this interpretative infrastructure is a challenging task, particularly in the case of unwritten rules, where the lines between law ascertainment and content determination (interpretation) may appear difficult to draw.Footnote ⁴⁴ Nevertheless, as a preliminary matter, it should be made clear that interpretation of customary rules is not only possible, but necessary.Footnote ⁴⁵ While there are key differences between the interpretation of written and unwritten rules, the content of all rules will be expressed in written form of one kind or another. Naturally, the processes of ascertainment of unwritten rules may themselves involve interpretation of some kind, but this does not change the fact that the content of the rules will be subject to interpretation once they have been ascertained.Footnote ⁴⁶ Put differently, the criteria for the existence of a customary rules are distinct from the rules as such.Footnote ⁴⁷ The interpretation of custom should thus be viewed as ‘the act of determining/construing the content of customary rules the existence of which is unchallenged’.Footnote ⁴⁸

My central argument in this section is that general principles of law could play a key role when considering these questions in the context of interpreting customary rules. If, as asserted, general principles are a distinct category of (predominantly) secondary rules, they could serve as a basis for international law’s general rules of interpretation, in particular in the case of interpreting unwritten rules such as those of customary international law. Before expanding on this claim further, it is necessary to consider the normative composition of the interpretative architecture of international law. In seeking to describe this legal framework, authors and judges have deployed a range of terms to denote differences in the nature, function, and origin of norms that aid the process of interpretation.

4 General Principles, Custom, and Systemic InterpretationFootnote ⁹⁰

A particularly prominent tool in the interpretation of unwritten rules is the notion of systemic integration. More precisely, against the background of broader principles of coherence and consistency in the international legal system,Footnote ⁹¹ there is a specific rule of systemic interpretation. This rule requires that interpreters take into account ‘any relevant rules of international law applicable in the relations between the parties’.Footnote ⁹² The central objective of this rule of interpretation is coherence and consistency in the international legal system. The development of general principles of law, too, has links to such notions. This final section explores the interaction between this rule of systemic interpretation and general principles in international law. Once general principles are understood to be rules, two issues arise with regard to systemic interpretation: first, whether this rule on systemic interpretation can itself be considered a general principle and, second, whether and how the interpretation of customary rules occurs in light of general principles as ‘relevant rules of international law’.

5 Concluding Remarks

This chapter has explored the interface between two developing fields at the very foundations of international law: the nature of general principles of law and the interpretation of customary rules. Central to the ongoing discussion on the nature, function, and place of general principles in the international legal system is the question of the interaction between general principles and other international legal norms. Questions of interpretation are a prominent area of such interaction, and, assuming general principles to be secondary rules of international law, there is significant potential for the development of general principles of interpretation. For the most part, the discussions sketched in this chapter may appear to amount to an exercise in (inconsequential) labelling. Yet, when approaching a task as challenging as that of mapping the interpretative architecture of custom, the labels used matter. As important as customary rules are, it is inaccurate to use ‘customary international law’ as shorthand for any and all norms beyond treaties. There are clear distinctions to be drawn between the different categories of rules and principles that constitute ‘general international law’. This exploration of the role of general principles in the interpretation of custom illustrates the normative heterogeneity of general international law, providing a foundation for further examination of the precise contours of the structures that govern the interpretation of unwritten rules and other systemic questions of international law.

1 Introduction

In its case law, the International Court of Justice (ICJ) has repeatedly suggested the idea that rules of customary international law (CIL) do not operate in a vacuum but, instead, are to be understood against the background of other rules of the international legal system.Footnote ¹ This observation, although somewhat unsurprising, shows that the sources of international law exist in close interconnection – something that is also visible if one looks at the rules of interpretation contained in Article 31 of the Vienna Convention on the Law of Treaties (VCLT). Accordingly, ‘any relevant rules of international law applicable in the relations between the parties’ must be taken into account, together with the context, when interpreting treaty provisions.Footnote ² The question addressed in this chapter is whether or not the same can be said of the interpretation of customary rules. In other words, if we look at the practice of international courts and tribunals, is it possible to reach the conclusion that CIL rules, too, must be interpreted with the cognizance of any relevant rules of international law applicable between the parties?

To answer this question, I have enquired into the practice of international courts and, in particular, into the ways in which, if at all, they use treaties and general principles of law – the other two sources of international law contained in Article 38 of the ICJ Statute – to interpret the content of customary rules. Here, following my previous writings on the subject of CIL interpretation, I understand interpretation as an act of discerning the content and scope of the customary rule, rather than interpreting the elements of custom – state practice and opinio juris. While in this chapter I engage in a critical reading of the judgments, I leave aside as being outside the scope of this chapter my musings on the interpretability of CIL.Footnote ³

According to the ILC’s draft conclusions on the identification of CIL, other rules, such as those contained in treaties, can aid determination of a customary rule’s existence.Footnote ⁴ Our focus, then, will be on studying the judgments of international courts with a view to demonstrating how they use treaties and general principles to establish the content of customary rules whose existence is not disputed by the parties.Footnote ⁵

To facilitate the analysis of the case law and the accompanying argumentation, the chapter has been structured in three sections. Section 2 unpacks the various meanings of systemic interpretation that can be found both in legal scholarship and in practice and which will be used as an analytical framework for the subsequent analysis. Sections 3 and 4 analyse the case decisions in which international courts and quasi-judicial bodies have used treaties and general principles of law in construing the content of customary rules. Section 5 examines case law in which the interpretation of a CIL rule is carried out by reference to the body of rules of which it is part.

2 The Multiple Meanings of Systemic Interpretation as an Analytical Framework

It is common practice among international courts and tribunals to use customary rules to interpret treaties. One frequently cited example is the Jan Mayen case, where the ICJ stated that Article 6 of the 1958 Convention on the Continental Shelf must be interpreted and applied by reference to customary law.Footnote ⁶ Pronouncements similar to those made in the Jan Mayen case usually fall within the ambit of Article 31(3)(c) of the VCLT, according to which ‘any relevant rules of international law applicable in the relations between the parties’Footnote ⁷ must be taken into account, together with context, in interpreting treaty provisions. This rule is usually referred to in case law and literature as the principle/rule of systemic interpretationFootnote ⁸ or systemic integration.Footnote ⁹ Confusingly, however, both of these terms have been used to convey not one but four different meanings.

Firstly, a textual analysis of Article 31(3)(c) shows this paragraph to be part and parcel of the core rule according to which treaty provisions must be interpreted in accordance with the ordinary meaning the words are to be given in their context and in the light of their object and purpose. In addition to context, Article 31(3)(c) states that any relevant rules of international law applicable in the relations between the parties must be taken into account in the determination of the ordinary meaning of the terms. As aptly noted by Tzekvelos, ‘Article 31(3)(c) expands the semantic field of the provisions of a convention’.Footnote ¹⁰ One example of this is when interpreters look at the use of the same term in another treaty that is in force between the parties and use it to further a particular understanding of the term. This is the first meaning of systemic interpretation.

The second meaning that has been ascribed to systemic interpretation is that of an interpretation of a rule against the background of the system of law as a whole. At the Institut de Droit International, this position was expressed by Verdross, who stated that ‘a treaty must be interpreted in light of general law and general principles of law’.Footnote ¹¹ According to the ICJ, ‘a rule of international law, whether customary or conventional, does not operate in a vacuum; it operates in relation to facts and in the context of a wider framework of legal rules of which it forms only a part’.Footnote ¹² Depending on how it is applied, this meaning of systemic interpretation may either converge with the first meaning – if the ordinary meaning is determined by reference to these other principles or rules – or go beyond it and result in what Alexy and Adler describe as the use of systemic arguments in interpretation.Footnote ¹³

This second meaning is markedly different from what Article 31(3)(c) prescribes,Footnote ¹⁴ which is to use any relevant rules for the determination of ordinary meaning in the light of context and of object and purpose. It is important to note that Article 31(3)(c) mentions the use of any relevant rules applicable between the parties and not any relevant rules generally. Only reference to all relevant rules would in fact suggest that the rules as a whole should be taken into account for interpretative purposes. According to this second understanding, systemic interpretation is essentially a use of systemic arguments that involves placing the rule in the system of international law as a wider form of context.Footnote ¹⁵

Another meaning of systemic interpretation is reflected in Verzijl’s dictum in the Georges Pinson Case, where he noted that every treaty must be considered to refer tacitly to general principles of international law for all those matters that it does not clarify in express terms.Footnote ¹⁶ This is different from systemic interpretation as meaning that other relevant rules must be used to determine the sense to be given to the words used in the treaty. Instead, it is a form of gap-filling, a technique to avoid a pronouncement of non liquet. And while Verzijl’s statement conditions recourse to general principles of law on the lack of an express resolution of the issue in conventional rules, systemic interpretation/integration has also been argued to support what essentially is an incorporation of extraneous rules and their application to the case, even when there is no clause providing for such reference.Footnote ¹⁷ The issue here is that the use of what is termed systemic interpretation would in this case be in violation of the obligation of good faith in interpretation.Footnote ¹⁸ Moreover, as Moreno-Lax rightfully points out, ‘Article 31(3)(c) VCLT should indeed be taken as a rule of interpretation, rather than a source of directly applicable law’.Footnote ¹⁹

Finally, systemic interpretation or integration has also been interpreted in a way that equates it with an interpretation that allows for co-ordination of norms in the case of indirect or, what de Wet and Widmar call, broad normative conflict.Footnote ²⁰ The case law of the ECtHR, which refers to it as harmonization, is a good example of this approach,Footnote ²¹ although, strictly speaking and interpreting the Article 31(3) holistically, it goes beyond what Article 31(3)prescribes. In the words of Tzevelekos, the European Convention on Human Rights ‘benefits from the latter [international law] through absorption of normative elements which, although absent from its “imperfect” text, are both complementary and necessary for the effective promotion of its special scopes’.Footnote ²²

To sum up, this means that other rules may essentially fulfil four different functions, all under the label of systemic integration: (1) other rules may serve as an aid to determine the ordinary meaning of the terms; (2) other rules may be used as a tool for systemic arguments; (3) other rules may be used as tools for gap-filling; or (4) other rules may be used as a tool for the resolution of a normative conflict. The latter two go beyond our understanding of interpretation. I will use this framework as a reference point for my analysis, in the following three sections, of case law where customary rules have been interpreted.

3 Interpretation of Customary Rules by Reference to Treaties

While, according to Article 38 of the ICJ Statute, international custom is a separate source of law, it exists in close interconnection with the other sources of international law. This is clear from the ILC’s draft conclusions, according to which ‘[v]arious materials other than primary evidence of alleged instances of practice accepted as law (accompanied by opinio juris) may be consulted in the process of determining the existence and content of rules of customary international law’.Footnote ²³ Such materials include ‘treaties, resolutions of international organizations and intergovernmental conferences, judicial decisions (of both national and international courts), and scholarly works’.Footnote ²⁴ The ILC then adds that ‘such texts may assist in collecting, synthesizing or interpreting practice relevant to the identification of customary international law, and may offer precise formulations to frame and guide an inquiry into its constituent elements’.Footnote ²⁵

According to an empirical study conducted by Choi and Gulati, treaties are the most frequently used materials for the identification of customary rules.Footnote ²⁶ Treaties can codify, crystallize and even generate customary rules.Footnote ²⁷ Treaties that codify pre-existent custom are known as declaratory, and whether a treaty is declaratory of CIL is determined by analysing the preamble of the treaty or by looking at the travaux préparatoires for confirmation that this was the intention of the parties.Footnote ²⁸ Most treaties, however, fall into the category of partly declaratory treaties,Footnote ²⁹ meaning that some provisions are codifications, whereas others are not.

At the same time, because treaties and custom originate from two different processes of law creation,Footnote ³⁰ treaties do not absorb CIL even when they codify it. This means that custom does not cease to exist upon being codified; it runs its course parallel to the treaty.Footnote ³¹ This is what the ICJ conveyed through the following statement in the Nicaragua case:

Moreover, as the ICJ argued, custom continues to apply even between the states that are parties to the treaty.Footnote ³³ This means that while different in nature and form, custom and treaty often overlap in substance. But treaties can also crystallize emergent customs,Footnote ³⁴ where crystallization means that the custom was in statu nascendi when the treaty was drafted but became a customary rule subsequently.Footnote ³⁵ They can also generate custom, where a provision created in the process of the drafting of a treaty becomes customary because it is widely followed even by non-parties.Footnote ³⁶

According to the ILC, all three types of treaties – those that codify custom, those that crystallize custom and those that generate custom – can be used in the process of CIL identification. Yet, as the ILC itself points out, a distinction needs to be drawn between the use of conduct in relation to treaties as state practice, where behaviour such as voting patterns is used as evidence for one of the elements of CIL, and the use of treaties as a reflection of CIL,Footnote ³⁷ where actual treaty provisions are used as the container of a customary rule, because they have codified, crystallized or generated a customary rule.

The use of treaties for what is, or what in some cases the courts frame as, interpretation of customary rules in international courts and quasi-judicial bodies tends to fall into two categories: (1) the use of treaty provisions to interpret CIL; or (2) the use of elements from treaty interpretation to interpret CIL.

In none other than the Nicaragua case, where the ICJ established the relationship between treaties and custom, it also noted that ‘while the Court has no jurisdiction to consider that instrument [the Charter of the Organization of the American States] as applicable to the dispute, it may examine it to ascertain what light it throws on the content of customary international law’.Footnote ³⁸ This statement was made in connection with the question of whether the lawful use by a third state of collective self-defence depended on a request from the attacked state.Footnote ³⁹ After examining the provisions of the OAS Charter and the Inter-American Treaty of Reciprocal Assistance, the ICJ concluded that there was no rule allowing for the exercise of collective self-defence without a prior request made by the attacked state.Footnote ⁴⁰ In this case, the court frames its reasoning as a form of interpretation. Yet, it ends up applying the requirement contained in the OAS Charter and the 1947 Rio Treaty, according to which the measures of collective self-defence must be taken at the request of the attacked state. This led to the following conclusion:

In other words, the ICJ engaged in a form systemic integration as a gap-filling exercise bordering on law-making.

The second case under analysis is the Nuclear Weapons Advisory Opinion,Footnote ⁴² which concerned the permissibility of the use and threat of use of nuclear weapons. The issue was controversial because international humanitarian law, including customary rules, lacked specific rules that would govern the use or threat of use of nuclear weapons in particular. One of the members of the bench, Judge Guillaume, appended a separate opinion, in which he stated that the rules of the jus ad bellum – in this particular case Article 51 of the UN Charter – could provide a clarification of the rules of the jus in bello.Footnote ⁴³ To answer the question on the extent to which the use or threat of use of nuclear weapons was permitted in international law, Judge Guillaume, largely following the ideas set out in the main advisory opinion, emphasized that given the content of Article 51, according to which nothing shall impair a state’s right of self-defence, the use of nuclear weapons is allowed.Footnote ⁴⁴

Judge Guillaume further opined that customary rules of humanitarian law must also be ‘completed by reference to the rules concerning the collateral damage which attacks on legitimate military objectives can cause to civilian populations’,Footnote ⁴⁵ as contained in the Additional Protocol to the Geneva Conventions. Therefore, the only prohibition in customary humanitarian law was on the use of weapons that could not distinguish between civilian and military targets, which, according to the judge, was not necessarily the case with nuclear weapons.Footnote ⁴⁶

In both cases there was a legal gap, which was resolved by what was framed as an interpretative exercise and which, if accepted as such, would fall under the third meaning of systemic interpretation outlined in Section 2. Whereas in the first case the customary rules on self-defence were completed with the requirement of a request on the basis of a reference to the OAS conventions, in the second case treaty rules were used to make the argument that rules of customary international law do not prohibit the use or threat of use of nuclear weapons. In both cases, the answer was sought outside CIL or its constituent elements – there was no mention of either state practice or opinio juris – instead, the reasoning was framed as an attempt at clarification or explanation, both of which are more akin to interpretation rather than orthodox ascertainment of customary rules. While in its draft conclusions the ILC established that treaties may ‘assist in collecting, synthesizing or interpreting practice relevant to the identification of customary international law, and may offer precise formulations to frame and guide an inquiry into its two constituent elements’,Footnote ⁴⁷ it is clear that the function of treaties in these cases goes beyond this and extends to interpretation lato sensu of the content of CIL in a way that completes it.

Two other cases, both from the practice of international and internationalized criminal courts, show a different facet of systemic interpretation. In The Prosecutor v. Kunarac the Trial Chamber of the International Criminal Tribunal for the Former Yugoslavia (ICTY) sought to establish the definition of torture under customary international law.Footnote ⁴⁸ After noting that the definition given in the Convention against TortureFootnote ⁴⁹ could be taken as representing customary international law, the Trial Chamber stated that Article 1 of the Convention against Torture could nevertheless be used as an interpretational aid.Footnote ⁵⁰ In a similar vein, in The Prosecutor v. Chea the Pre-Trial Chamber of the Extraordinary Chambers in the Courts of Cambodia (ECCC) found that the expression ‘other inhumane acts’ in connection with crimes against humanity was likely a CIL rule and then stated that:

In these cases, other treaty provisions seem to have been used for construing the meaning of the customary rule, which comes closest to the first meaning of systemic interpretation described in Section 2.

Another pair of cases – or two individual opinions, to be more precise – illustrate how elements of treaty interpretation have been used in an interpretative argument on customary rules. The first example is the dissenting opinion of Judge Sørensen in the North Sea Continental Shelf case,Footnote ⁵² where he noted that:

Unlike in the previous pair of cases, where treaties themselves were used to construe and complete the content of customary rules, in this case Judge Sørensen, while not actually making an interpretation on the basis of elements of treaty interpretation, clearly advocated in its favour, but only in cases where the treaty codifies, crystallizes or generates rules of CIL. In such a case, the argument goes, judges may resort to considering even the preparatory work of the treaty.

The second example is Judge Shahabudeen’s dissent from the Decision on Interlocutory Appeal from the ICTY in Hadžihasanović. In this case the tribunal was called upon to determine whether a superior could be punished under the principle of command responsibility for acts committed by subordinates prior to the assumption of command.Footnote ⁵⁴ When determining the scope of action of the customary principle of command responsibility, Judge Shahabudeen argued that ‘any interpretation [of the customary rule] can be made by reference to the object and purpose of the provisions laying down the doctrine’Footnote ⁵⁵ – Articles 86 and 87 of Protocol I Additional to the Geneva Conventions.

A final example that illustrates this approach is the judgment of the Appeals Chamber of the ICTY in The Prosecutor v. Aleksovski.Footnote ⁵⁶ In examining the standard of control for the purposes of establishing the international character of an armed conflict, the Appeals Chamber opined that:

In other words, the Appeals Chamber favoured the overall control test applied in The Prosecutor v. Tadić over the effective control test applied in Nicaragua and used the purpose of Geneva Convention IV as an argument to buttress its position.Footnote ⁵⁸ This statement evokes, to some extent, Article 32 of the VCLT, according to which supplementary means of interpretation may be used to confirm the meaning arrived at through the general rule. However, it is not so much a supplementary means of interpretation, as foreseen in Article 32, but rather the consistency of the chosen standard with the purpose of a treaty that is being used here to confirm the choice in favour of the overall control standard. At the same time, the argument relies on the need to ensure normative harmony or consistency between the chosen standard and the convention in relation to which it applies. All in all, in terms of qualification, here it seems that the courts are engaged in an operation akin to that of systemic interpretation in its second meaning – as a tool for crafting systemic arguments.

4 Interpretation of Customary Rules by Reference to General Principles

While it seems that general principles have no role to play in the identification of CIL, in the practice of courts, especially criminal courts, it can be seen how the two can be embedded in judicial reasoning, often to the point of confusion. A few words, then, must first be said about the conceptual embeddedness and difference between general principles and custom.Footnote ⁵⁹

References to principles in the case law of international courts frequently cause confusion because it is unclear whether they are referring to fundamental principles of international law or those belonging to a branch of international law or to general principles of law recognized by civilized nations.Footnote ⁶⁰ The latter, in turn, may be variously defined as domestic law principles that are common to all/most States, such as estoppel;Footnote ⁶¹ as natural law principles, such as equity or considerations of humanity;Footnote ⁶² as principles that originate from international relations;Footnote ⁶³ or as ‘general propositions underlying the various rules of law which express the essential qualities of juridical truth itself’.Footnote ⁶⁴ Because of this lack of agreement or clear-cut definition of (general) principles, their analysis is often embedded with that of custom.Footnote ⁶⁵ This is seen especially where ad hoc courts and tribunals declare their aim to be the establishment of customary international law at a particular point in time, whereas in actual fact they are surveying the domestic legislation of states, not as state practice but for the purpose of finding a common denominator in the definition of specific crimes.Footnote ⁶⁶

The embeddedness between general principles and custom can be seen not only in case law but also in those writings of legal scholars that propound a broad understanding of the concept of customary international law.Footnote ⁶⁷ The confusion is due to, or at least so it seems, the early practice of the Permanent Court of International Justice (PCIJ). For instance, in the Lotus case the PCIJ observed that ‘rules of law binding upon States therefore emanate from their own free will as expressed in conventions or by usages generally accepted as expressing principles of law’.Footnote ⁶⁸ What adds to the confusion is that in its more recent practice, the ICJ seems to have requalified as CIL norms that it previously considered to be general principles.Footnote ⁶⁹

The absence of a clear-cut distinction can even be found in the conclusions of the ILC. The ILC’s stance is that the ‘“rules” of customary international law … may be referred to as “principles” because of their more general and more fundamental character’.Footnote ⁷⁰ In other words, it is admitted that general principles can be of a customary origin (also given that CIL is part and parcel of general international law) and are distinguished from regular customary rules by possessing a higher degree of abstractness.Footnote ⁷¹ The practice of international courts and tribunals seems to support this view, at least to a certain degree.Footnote ⁷² If, however, a neat distinction is maintained between general principles and CIL, then, given the former’s high degree of generality, general principles constitute a residual category, acting as a filler when the other two sources – treaties and CIL – are unable to resolve the dispute.Footnote ⁷³

According to Judge Trindade of the ICJ, general principles of law ‘orient the interpretation and application of the norms and rules of this legal order, be they customary or conventional’.Footnote ⁷⁴ The ICTY seems to share this view. In The Prosecutor v. Kupreškić,Footnote ⁷⁵ the Trial Chamber, by taking into account other principles, expressed its clear support for a systemic approach when examining the prohibition of attacks on civilian populations. The Trial Chamber argued that to establish ‘the scope and purport’Footnote ⁷⁶ of the customary rules on the requirement of proportionality between collateral damage and direct military advantage and the prohibition of the use of indiscriminate means or methods of warfare, it was necessary to interpret them by reference to elementary considerations of humanity, which the Trial Chamber framed as being ‘illustrative of a general principle of international law’.Footnote ⁷⁷

In The Prosecutor v. Furundžija, the Trial Chamber had to decide on the definition of rape and the forms of behaviour that fell under this offence and, in particular, whether oral penetration could qualify as rape.Footnote ⁷⁸ The Trial Chamber firstly stated that the prohibition of rape in armed conflict had evolved into a norm of customary international law,Footnote ⁷⁹ yet found that international law (whether treaty or custom) contained no definition of rape.Footnote ⁸⁰ Subsequently, it scrutinised national legislation and, as a result, established that while national laws generally converged around the definition of rape as ‘the forcible sexual penetration of the human body by the penis or the forcible insertion of any other object into either the vagina or the anus’,Footnote ⁸¹ there were discrepancies concerning whether oral penetration qualified as rape or a different type of sexual assault.Footnote ⁸² The question of whether the definition of rape included or excluded oral penetration was decided by reliance on the principle of respect for human dignity as ‘the essence of the whole corpus of international humanitarian law as well as human rights law’.Footnote ⁸³ The inclusion of oral penetration in the definition of rape, both as a treaty and a customary prohibition, was preferred because this solution appeared to be ‘consonant with this principle’.Footnote ⁸⁴ Judging from these two cases, it appears that general principles of law are used mainly for making systemic arguments that would determine the scope of the customary rule, thus falling under the second meaning of systemic interpretation.

An example of how the language of systemic interpretation can be misused is found in Case 002, where the ECCC Trial Chamber stated that ‘as recognised by the Pre-Trial Chamber, having regard to general principles of law can assist when defining the elements of an international crime, where that crime has otherwise been recognised in customary international law’.Footnote ⁸⁵ After surveying the legislation of different countries, the Trial Chamber concluded that the mens rea of murder as a crime against humanity included dolus eventualis.Footnote ⁸⁶ While announcing what appears a systemic approach towards the interpretation of the elements of the crimes found in CIL, the Pre-Trial Chamber ended up surveying the legislation of states and, given that the majority included dolus eventualis as the mental element for this crime, established that this had been the mens rea for murder prior to 1975.

5 Interpretation of Customary Rules by Reference to the System of Rules as a Whole

To complement the case law analysed in the previous sections, where other rules were used to construe custom, mention should lastly be made of a case where judges relied on a system of rules in its entirety to interpret the customary rule on attribution. In The Prosecutor v. Tadić, when arguing against the use of the Nicaragua effective control test in this case, the Appeals Chamber made the overarching argument that ‘a first ground on which the Nicaragua test as such may be held to be unconvincing is based on the very logic of the entire system of international law on State responsibility’.Footnote ⁸⁷ It subsequently added that:

In addition, it stated that the same logic had to apply to the questions on the appropriate control test.Footnote ⁸⁹ Overall, when looking at the argument advanced by the Appeals Chamber, it appears to conflate teleological considerations with systemic ones in order to show why the effective control test is at odds with the whole system of state responsibility. In subsequent paragraphs, the Appeals Chamber examined state practice and opinio juris to show how the effective control was not rooted in practice.Footnote ⁹⁰ However, this was done only subsequently and as an additional argument to the first teleological-systemic argument advanced, which means that it supports the argument being made here that ascertainment of state practice and opinio juris cannot be considered the only methods of determining the content of CIL rules in the case law of international courts and tribunals. This is an instance of a classic systemic argument in interpretation combined with teleological considerations, which shows that systemic arguments in the wider sense are not alien to the interpretation of customary rules.

6 Conclusion

Systemic treaty interpretation is ever-present in the practice of international courts and tribunals. While the term does not at first glance appear equivocal, it has in fact been used to refer to four different operations. Firstly, it may denote the act of using other rules as an aid to determine the meaning of treaty terms. Secondly, it can mean the use of systemic interpretative arguments. Thirdly, it has been used to refer to the act of gap-filling and, lastly, to the act of resolving conflicts between norms.

As this chapter has shown, systemic interpretation is also not foreign to CIL rules and, as with treaties, different variants of systemic interpretation have emerged in the practice of international courts. The most common is the use of systemic interpretative arguments, where other rules or the rules of the system taken as a whole are used to construe the content of a customary rule.

That systemic interpretation is equally capable of applying to customary rules not only advances our understanding of CIL interpretation but is further proof of the enduring interconnection between CIL and the two other sources of international law – treaties and general principles of law.

1 Introduction

The risk of possibly conflicting norms of customary international law (CIL) has received increased attention in recent international legal scholarship and practice.Footnote ¹ In the absence of commonly accepted or authoritative rules of conflict that may mitigate the ramifications which stem from a possible clash of opposing obligations under CIL, legal scholars and practitioners alike almost instinctively turn towards competent (judicial) authorities, thereby seeking advice on how to strike a balance between conflicting norms in conformity with applicable legal frameworks and regimes. The emergence of this ‘legal dilemma’ hence calls for, and even heralds, an established and accepted modus operandi to which international judges and adjudicators – as competent authorities – can resort in relevant situations. This basic constellation of juxtaposing norms pointing in different directions is of significant relevance, most importantly with regard to the operations of the International Court of Justice (ICJ), designated as ‘the principal judicial organ of the United Nations’Footnote ² and competent to adjudicate on ‘any question of international law’.Footnote ³

The tension inherent in the coincidence of contrary, yet equally valid and established, norms of CIL has hence become an increasingly real issue in recent years. For example, it arises in relation to the scope and application of ratione personae immunity from prosecution and adjudication enjoyed by heads of state, heads of government, and ministers of foreign affairs for international (core) crimes. The underlying conflict has become especially manifest in the Al BashirFootnote ⁴ case at the International Criminal Court (ICC). The case concerns acts committed by the former Sudanese president Omar Al Bashir, who has been charged with genocide, crimes against humanity, and war crimes in the context of the situation in Darfur – referred to the International Criminal Court by the United Nations (UN) Security Council under its Chapter VII powersFootnote ⁵ – and is the subject of two arrest warrants issued in recent years.Footnote ⁶ Beyond the question of Al Bashir’s imputed individual criminal responsibility for these international crimes, this case subsequently triggered a series of interrelated legal and legislative developments in a number of states parties to the Rome StatuteFootnote ⁷. Most importantly, they concerned constitutional issues flowing from the fact that Al Bashir had sojourned on the territory of states parties to the Rome Statute notwithstanding the pending arrest warrants and related legal obligations under the Statute.Footnote ⁸ Accordingly, the case of Al Bashir has offered an enduring illustration of some of the inherent tensions that might arise between two possibly conflicting, yet imperative, norms of CIL:Footnote ⁹ on the one hand, the established principle of ratione personae immunity applicable to heads of state, heads of government, and ministers of foreign affairs;Footnote ¹⁰ and, on the other, the imperative to end impunity for international (core) crimes by facilitating individual criminal responsibility under international law.

This cursory sketch of two CIL norms in possible conflict with one another points, however, to a more structural tension – namely the question of applying and reconciling (seemingly) opposing legal principles in concrete circumstances. Thus far, international courts and tribunals, including the ICJ, have regularly refrained from outlining, conceiving, and imposing coherent analytical and prescriptive means of establishing an equilibrium between (partially) opposing norms of CIL in situ. Yet, devising such means – i.e. the creating transparent and selective rules of collision – could arguably assist international judges in striking a reasonable and credible balance in reconciling conflicting norms of CIL in concrete cases. In this chapter it is submitted that in such circumstances international adjudicative bodies could contemplate resorting to the German constitutional law principle of practical concordance (praktische Konkordanz) and thereby draw on a legal methodology that has become well established and is regularly applied by the German Federal Constitutional Court when ruling on fundamental rights.Footnote ¹¹ The principle is applied in cases where there is a conflict between fundamental norms of equal (constitutional) rank, neither of which can be fully or partially overridden.Footnote ¹² A core objective of practical concordance is thus the careful balancing of the two legal norms so as to allow each to be as fully effective as possible. Given that any legal arrangement needs to be assessed in context, practical concordance denotes essentially both a relationship of qualified precedence and a method of resolving conflicting legal standards – that is, establishing a specific rule of conflict in situ.

This chapter’s objective is threefold. Firstly, it discusses an example of a dichotomous conflict of CIL norms – namely, between the rule on immunity ratione personae and the rule on individual criminal responsibility. Secondly, the notion of practical concordance is introduced and then (re-)conceptualized as a form of an adjudicative reasoning that international courts and tribunals, including the ICJ, can apply to (better) address situations in which there is a potential conflict between CIL norms. Thirdly, the chapter will offer some tentative thoughts on how to address possible ramifications arising for international judges called upon to adjudicate in cases where such conflicts occur and thereby outline how practical concordance might assist in addressing underlying legal dilemmas.

2 The Interplay between Personal Immunity and Individual Criminal Responsibility

There has been much scholarly debate around the controversial question of immunity ratione personae, or personal immunity from criminal jurisdiction. It appears to be a well-established principle of CIL that heads of state, heads of government, and foreign ministers are considered to enjoy extensive immunity as individuals during their time in office, such immunity covering both private and official conduct.Footnote ¹³ It implies that since these officeholders represent and even incarnate the state throughout their tenure, any affront to their dignity and inviolability should, by extension, amount to an affront to the state they represent.Footnote ¹⁴

This understanding of immunity ratione personae was confirmed the International Court of Justice in the Armed Activities case, affirming that ‘it is a well-established rule of international law that the Head of State, the Head of Government and the Minister for Foreign Affairs are deemed to represent the State merely by virtue of exercising their functions, including for the performance, on behalf of the said State, of unilateral acts having the force of international commitments’.Footnote ¹⁵ Further substantiation of immunity ratione personae for heads of state as CIL can be found in the Vienna Convention on the Law of Treaties.Footnote ¹⁶ Codifying pre-existing CIL,Footnote ¹⁷ the Convention states: ‘In virtue of their function and without having to produce full powers, the following are considered as representing their State: (a) Heads of State, Heads of Government and Foreign Ministers, for the purpose of performing all acts relating to the conclusion of a treaty.’Footnote ¹⁸

The notion of immunity ratione personae under CIL becomes particularly relevant in matters concerning international criminal justice and related questions of jurisdiction. In the Arrest Warrant case, the ICJ examined whether individuals entitled to personal immunity might be charged and subsequently prosecuted in domestic courts for having allegedly committed international crimes. It clarified that ‘in international law it is firmly established that … certain holders of high-ranking office in a State, such as the Head of State, Head of Government and Minister for Foreign Affairs, enjoy immunities from jurisdiction in other States, both civil and criminal’.Footnote ¹⁹ As far as criminal proceedings before foreign courts were concerned, however, the court underlined that foreign affairs ministers would enjoy full and unlimited ratione personae immunity, elaborating that:

Confirming and further substantiating the applicability of immunity ratione personae in proceedings before domestic courts, the ICJ acknowledged and, moreover, clarified in what circumstances the personal immunity enjoyed by an officeholder would not preclude criminal prosecution. This would be the case if the proceedings took place in their home country, or if the home state decided to waive their immunity. Beyond that, once an individual ceased to hold office, they would no longer enjoy the privileges and immunities accorded by international law in third states. Thus, provided it had jurisdiction under international law, a foreign court could try a former foreign affairs minister of another state for acts allegedly committed prior to or after their term in office, as well as for acts alleged to have been committed in a private capacity during the minister’s term in office.Footnote ²¹

In an obiter dictum, the court pointed out that both incumbent and former foreign affairs ministers could be pursued in criminal proceedings before ‘certain international criminal courts, where they have jurisdiction’.Footnote ²² In particular, this would apply to the International Criminal Tribunal for the Former Yugoslavia, the International Criminal Tribunal for Rwanda,Footnote ²³ and – though still in the process of being created at the time the judgment was rendered – the International Criminal Court. It was also noted that the latter’s founding legal document, the Rome Statute, expressly provided that: ‘Immunities or special procedural rules which may attach to the official capacity of a person, whether under national or international law, shall not bar the Court from exercising its jurisdiction over such a person.’Footnote ²⁴ Although this differentiation in the application of ratione personae immunity between the international and domestic spheres expressed in an obiter dictum was not unequivocally endorsed by the entire bench,Footnote ²⁵ the judgment pointed to a tacit consensus in statu nascendi, according to which certain and future international criminal courts and tribunals with the necessary jurisdiction over international core crimes might indict and prosecute both incumbent and former holders of the highest offices of state. Accordingly, the ratione personae immunity to which such individuals would otherwise be entitled in connection with such offences would no longer apply.

As a relatively new development, this re-configuration of the legal and conceptual understanding of the role and functions of ratione personae immunity under (customary) international law has become the object of increasingly close scholarly scrutiny in recent years, particularly in relation to heads of state. Historically, heads of state have been seen as enjoying considerably broad, if not absolute, immunity from foreign jurisdiction – in part, reflecting and reinforcing the prevailing belief in states’ sovereign immunity. However, a tangible decline in the deference paid to incumbents of highest state offices and the concomitant replacement of the former absolute immunity by a more qualified and conditional approach towards immunity, as well as an increasing emphasis on functional rather than personal status as the referential frame for according immunity, have, in sum, led to the above-described legal and conceptual re-alignment. As a result, ratione personae immunity for heads of states is now scrutinized in a far more critical manner. That said, the degree and extent of this change, and thus of the re-conceptualization of the scope of ratione personae immunity in respect of both substance and application have remained rather unclear.Footnote ²⁶ This may in part be due to the fact that the legal construct of ratione personae immunity resembles a sui generis phenomenon: accordingly, analogies with the legal categorization of other state offices – such as, most importantly, diplomatic agents – are far from ideal, as they do not fully account for the specificities of personal immunity. Given that only a few international courts have thus far addressed the jurisdictional implications of ratione personae immunity in a comprehensive and authoritative manner, its legal status – both internationally and in domestic contexts – is still being dealt with primarily as a matter of CIL.Footnote ²⁷ Thus, while neighbouring foundational principles of international law, such as the sovereign equality of states or sovereign immunity, are clearly helpful in delineating the scope and nature of immunities ratione personae, those principles are insufficient to arrive at a comprehensive and contemporary determination of the formers’ status under CIL.

The last twenty-five years have thus seen growing unease in international legal scholarship over unconditionally according almost unlimited personal immunity, given the emergence of an array of international crimes and the growing number of capable international adjudicative bodies having jurisdiction to rule on them.Footnote ²⁸ The concern to guarantee human rights protection has led to increasing deliberation on the possibility of legally challenging the conduct of heads of state, heads of government, and ministers of foreign affairs, including, in a limited number of cases, also in the courts of foreign states and, in particular, in a growing number of international criminal courts and tribunals, with a view to possibly overriding the previously accepted conceptualization of personal immunity conferred on those selected individuals under CIL.

Assuming that immunity for heads of state, heads of government, and ministers of foreign affairs is still – notwithstanding the aforementioned efforts, most notably on the part of the International Law Commission,Footnote ²⁹ to codify it – primarily based on CIL, no clear indication has been given as to how this principle may be properly reconciled with other, potentially conflicting customary norms that have the same legal rank. This is most importantly the case when the principle of ratione personae immunity comes up against that of individual criminal responsibility, aimed at ending impunity for international core crimes. This latter principle has two fundamental dimensions: firstly, it confirms that individuals can be held accountable directly under international law for certain criminal offences, thereby becoming subjects of international law and bypassing the state as an intermediary. Secondly, it implies that individuals may incur personal culpability for these specific offences and can therefore be held accountable for their conduct within the international arena.

Outside the laws of war – in which context individual combatants, are to be held liable, albeit to a limited degree, in the case of violations of international humanitarian law in their home jurisdictions – individual criminal responsibility has been recognized under CIL for offences such as piracy, crimes against peace, and crimes against humanity, and has subsequently been affirmed for other kinds of crimes in various international treaties that address the protection of human rights, both in times of war and peace.Footnote ³⁰ Conversely, certain international crimes that have been codified in international treaties are considered to have (subsequently) entered the corpus of CIL – for example, the crime of genocide.Footnote ³¹

Individual criminal responsibility as a norm of CIL has its foundations in the so-called Nuremberg Principles, espoused in the wake of World War II. On 11 December 1946, the United Nations General Assembly unanimously adopted Resolution 95(I),Footnote ³² in which it ‘[a]ffirms the principles of international law recognized by the Charter of the Nuremberg Tribunal and the judgment of the Tribunal’.Footnote ³³ Simultaneously, the General Assembly instructed the body that later became known as the International Law Commission to draft guiding principles essentially reflecting the Nuremberg and Tokyo Charters’ foundational provisions and the jurisdictional mandates of their respective tribunals. During the ILC’s deliberations, the question of the need to determine whether the rules applied by both adjudicative bodies already constituted applicable principles of international law arose. Given that this was the interpretation of the UN General Assembly in its Resolution 95(I), the legal nature of the principles was considered sufficiently established. During the following years, a growing number of domestic criminal courts of different kinds referred to the Nuremberg Principles, thereby endorsing and accepting their nature and scope as general principles of (international) criminal law. For example, in the Eichmann case, the District Court of Jerusalem emphasised that Resolution 95(I) showed the Nuremberg Principles to be part of CIL.Footnote ³⁴ Following similar reasoning, the International Criminal Tribunal for the Former Yugoslavia confirmed the overall customary law status of individual criminal responsibility in Tadić.Footnote ³⁵

The notion of individual criminal responsibility therefore lies at the very core of the international criminal justice system. However, it increasingly depends upon interactions with cognate legal concepts and other sub-realms of international law. Therefore, how individual criminal responsibility develops in the future will be a function of related legal principles such as an (international) duty to prosecute for defined criminal offences; international legal rules on immunities and amnesties; and the likelihood of co-operation between international criminal courts and tribunals, domestic justice systems, and states more broadly. The actual extent to which individual criminal responsibility might thus (still) be evaded through the application of particular legal principles – such as ratione personae immunity – has therefore remained a controversial and steadily evolving phenomenon, featuring in an ever-increasing number of cases before international criminal courts and tribunals – for example, TaylorFootnote ³⁶ and Kallon and KamaraFootnote ³⁷ before the Special Court for Sierra Leone and KaradzicFootnote ³⁸ before the International Criminal Tribunal for the Former Yugoslavia.

It is this complex legal entanglement that is at the root of the somewhat uneasy interplay and interdependence between ratione personae immunity and individual criminal responsibility as potentially conflicting CIL norms. As explained in Section 1, this predicament has been notably highlighted by the case of (former) Sudanese president Al Bashir and the charges brought against him before the International Criminal Court for his alleged role in the Darfur conflict.

3 Introduction of Practical Concordance into International Adjudication

The notion of practical concordance has its origin in German constitutional law; it can be thought of as a guiding (legal) principle and is particularly characteristic of human rights jurisprudence. Deriving from the Latin concodare (to agree), the principle of practical concordance is regularly applied as a means of arriving at a legal equilibrium in human rights cases in which there is a risk of conflict between constitutional norms of equal rank, neither of which should take precedence over the other and both of which call for protection. In relation to possibly conflicting constitutional principles, the intention is to strike a deliberate balance that allows both legal principles to produce their effects as harmoniously and fully as possible. Typically applied to concrete constitutional scenarios, in which possibly conflicting fundamental norms need to be reconciled, practical concordance is informed by case-specific logic, in which legal interests and preferences need to be interpreted in the light of other constitutional principles and purposes. It therefore represents a conditional and, possibly, temporary prioritisation between norms of equal constitutional rank. As a distinct method of reconciling conflicting constitutional norms by balancing them against one another and scrutinising them vis-à-vis other legal considerations, practical concordance can thus be conceived as a specific rule on collision. This is not to say, however, that practical concordance is to be applied unconditionally; it is not applicable in cases in which the legal norm in question might collide with higher-ranking or more specific norms, or in which the norm might contradict general legal norms and principles – the colliding norm would in such cases lie outside practical concordance’s scope of application.Footnote ³⁹

The notion of practical concordance owes its place in the German constitutional law debate above all to the German academic Konrad Hesse, who defined the principle as a method according to which ‘[c]onstitutionally protected legal interests must be co-ordinated with each other in the solution in such a way that each of them becomes reality. … [L]imits must thus be set for both legal interests so that each may attain optimal effectiveness.Footnote ⁴⁰ According to the definition provided by Hesse, the realization of one legal interest shall not be at the expense of the other, which also excludes an over-hasty weighing or accommodation of conflicting legal positions. In that sense, practical concordance is geared towards an overall imperative of constitutional unity and thus of both coherence and impartiality, the ultimate aim being to establish a lasting balance while at the same time furthering both legal interests at issue. In this respect, practical concordance could be considered an emanation of the principle of proportionality. Expressed differently, practical concordance realizes two seemingly different, yet inherently interlinked, objectives – namely, to reconcile conflicting norms through a harmonized interpretation and to facilitate an equilibrium among norms.

This explains the widespread attention and approval given to practical concordance in domestic constitutional debates in several (mostly German-speaking) countries. The principle is today generally recognized as a constitutionally grounded balancing mechanism between potentially conflicting legal norms, the aim of which is to achieve an equilibrium between those norms and thereby generate a state of practical concordance. The concrete scope and character of practical concordance has been delineated first and foremost by the German Federal Constitutional Court in its fundamental rights jurisprudence.Footnote ⁴¹ In general, the principle serves in cases of collision between fundamental rights that are guaranteed without qualification and is thus applicable to a broad range of fundamental rights collisions and interrelated conflicting constitutional principles. It has been questioned, however, whether the factual application of practical concordance in the sense of a systemic method of interpretation can place limits (possibly unintended) on the protective scope normally inherent in an unconditionally guaranteed fundamental right. If the conflicting constitutional provision amounts to a limitation in itself, this may justify the imposition of limits, provided the norm collision concerned is addressed and balanced in the spirit of practical concordance. On this particular issue, constitutional jurisprudence has remained rather patchy.

The foundational idea on which practical concordance is grounded also constitutes the basis for what other constitutional scholars, such as Peter Lerche, have identified as the components of an appropriate balancing of conflicting, yet constitutionally protected and relevant, interests as a way of legitimizing contingent restrictions on fundamental rights.Footnote ⁴² This approach lends itself to situations in which the absence of an explicit authorization to restrict fundamental rights may need to be compensated by imparting constitutional legitimation. The rationale implicitly underlying practical concordance has subsequently been adopted by constitutional orders outside Germany – for example, in FranceFootnote ⁴³ and Switzerland,Footnote ⁴⁴ where the notion has been taken up, sometimes unchanged, in both constitutional law scholarship and fundamental rights jurisprudence.

Although international law recognizes the existence of conflicting norms and provisions and has long-developed rules of conflict at its disposal, including general principles of law such as lex specialis derogat legi generali,Footnote ⁴⁵ – the tension between conflicting CIL norms may lead to profound legal dilemmas when adjudicative bodies, including the ICJ, are confronted with such conflicts. As the corpus of international law expands and becomes more fragmented, this legal-theoretical problem is likely to have ramifications and repercussions in both scholarly discourse and judicial practice. In the absence of authoritative rules of conflict that address this enduring tension, practical concordance can be used by international judges, alongside existing methods of interpretation such as the principle of effectiveness, systemic integration, or the principle of harmonization,Footnote ⁴⁶ as a distinct analytical lens through which they can balance conflicting legal norms instead of preferring one norm to another one.

Section 4, sketches a possible way of introducing practical concordance into the legal toolkit employed by judges of international courts, including the ICJ. This transposition exercise seeks not to delineate yet another legal-methodological concept that can be regularly applied in a formal manner but rather to draw intellectual inspiration from the principle of practical concordance. The aim is to provide a tangible dimension and a name to the dilemma of reconciling conflicting CIL norms. The exercise aspires to raise awareness of this distinctive quandary among adjudicators, with a view to contributing to their reasoning and encouraging reflection on possible ways of establishing an equilibrium of legal interests in the concrete cases they handle.

4 ‘What a Judge Gotta Do’: Thoughts on Reconciling Conflicting CIL Norms at the ICJ

So, how can the judges of international adjudicative bodies make use of practical concordance in their daily work on the bench, and in particular at the ICJ? This section will propose various adjudicative ‘lenses’ whereby practical concordance might possibly contribute towards a more comprehensive assessment.

In recent years, international judges have increasingly become an object of research in themselves for international legal scholars.Footnote ⁴⁷ Matters such as (legal) socializationFootnote ⁴⁸ or international judges’ role in constituting court authorityFootnote ⁴⁹ have featured extensively in recent academic writings. These studies have shown that international judges carry with them an individualized legal fingerprint and trajectory, which determines the (legal) reasoning they pursue in any given case before them. Recalling that the principle of practical concordance has its origins in German constitutional law – and has thus far been mostly restricted thereto – it is no surprise that the concept has not (yet) expanded more broadly across international legal scholarship and penetrated the everyday practice of international judges.Footnote ⁵⁰ Mindful of this caveat, the following thoughts are but a tentative and initial attempt to introduce the notion of practical concordance into contemporary international legal thinking.

In brief, practical concordance may fulfil a threefold function for international judges called upon to reconcile conflicting CIL norms. Firstly, it may serve as a methodological-analytical prism through which the customary norm(s) in question may be viewed, scrutinized, and de-constructed while being mindful of the need to achieve an equilibrium of norms. This implies that the substantive scope of each norm will necessarily be circumscribed. It could be said that this readiness to engage in a balancing process requires a mindset similar to that displayed when judges rule ex aequo et bono.

Secondly, although the above-mentioned equilibrium of norms is to be sought in situ – that is, pursued by international judges in the concrete cases before them – applying practical concordance at the same time requires the broader picture to be taken into consideration – that is, international judges must ponder the results of weighing different norms against one another and take account of the legal effects that might result across various (sub-)branches of international law. This applies particularly to the ICJ, which, as the ‘principal judicial organ of the United Nations’Footnote ⁵¹ is of fundamental significance in the future development and practice of international law. With each decision they render, the ICJ judges address not only the legal interests at issue in the case before them but also the repercussions this may have for the domain of international law more broadly. Practical concordance could be regarded as a further aid in performing this role, accommodating international judges’ dual role as, on the one hand, authoritative arbitrators and legal interpreters in any given case before them and, on the other hand, international legal norm entrepreneurs, entrusted with developing international law in accordance with a constantly changing legal-political environment.

Thirdly, the application of practical concordance both enables and obliges international judges to take a ‘think outside the box’ approach when addressing legal dilemmas arising from the concurrence of possibly conflicting CIL norms and thereby to engage in judicial creativity. When applied in a given case, practical concordance requires established norms of (customary) international law to be balanced against one another in order to find an equitable solution to a previously disregarded legal dilemma. For this, interdisciplinary thinking and unorthodox legal approaches are needed. It also offers a way of upholding past precedence while striving for de lege ferenda.

Practical concordance should not therefore be regarded as a sophisticated and novel concept in legal theory. Rather, it denotes a distinct methodological lens through which cases that deal with conflicting CIL norms may be viewed, analysed, and resolved – hence putting a strong emphasis on considerations of balancing and compensation. Whether, to what extent, and in what way practical concordance may make inroads into the practices of international courts, especially the ICJ, remains to be seen. The compensatory character of practical concordance, which seeks an accommodation between prima facie contradictory CIL norms could emerge as a potential game-changer particularly whenever the ICJ is called upon to provide advisory opinions. It could, for example, help in answering the question of how to reconcile the ratione personae immunity enjoyed by heads of state, heads of government, and ministers of foreign affairs under international law with the pursuit of individual criminal accountability for international core crimes.

How, to what extent, and with what results might practical concordance be used by ICJ judges? While it might seem a somewhat vain exercise to ponder potential decisions in abstracto, and without wishing to interfere with judicial autonomy, a brief thought experiment will help to show how practical concordance could be applied to resolve conflicting CIL norms.

For this thought experiment, let us suppose that the ICJ is to render an advisory opinion at the request of the United Nations General AssemblyFootnote ⁵² on whether, how, and under what conditions two prima facie conflicting CIL norms–ratione personae immunity and individual criminal responsibility – may be reconciled. Our illustration will involve the (fictious) case of a sitting head of state accused of being responsible for an international crime falling within the jurisdiction of an international criminal tribunal legally empowered to adjudicate on such a situation. Faced with this (simplified) scenario, an international judge will need to take two initial steps. The first will be to analyse each (customary) norm and define its scope of application and the legal interests it protects, so as to distil the legal dilemma posed by the concurrence of the legal interests each norm seeks to protect. The second step will consist in delineating each norm’s legal nucleus – that is, the normative core to be preserved and without which the norm in question would become meaningless. This exercise will necessarily need to be placed within the context of the concrete case at hand and thus take account of the notional and circumstantial specifics of the case.

Taking this line of thinking a step further and applying it to the norms of ratione personae immunity and individual criminal responsibility, the establishment of an equilibrium between the norms would mean that neither norm is applied unrestrictedly, yet their overall applicability is upheld. As regards ratione personae immunity, this balancing could lead to confirmation that, for example, heads of state continue to enjoy extensive privileges, in particular before domestic courts, and that their previously unconditional immunity could be curtailed only where the individual is charged with a core crime of concern to the international community as a whole and this charge is made before a competent international criminal court or tribunal with the necessary jurisdiction. This qualified understanding of ratione personae immunity is the direct result of its being confronted with the other norm in question – namely, individual criminal responsibility. Although a bedrock principle in individualizing international justice, allowing the actual perpetrators of atrocious crimes (humans as opposed to states) to be held accountable, this principle would have to comport with the importance accorded to individuals who enjoy ratione personae immunity on account of their unique role as state representatives. For its part, therefore, the principle of individual criminal responsibility will have to be curtailed so that this limited circle of officeholders are prosecuted only before a competent international criminal court or tribunal with the necessary jurisdiction and only for a crime of concern to the international community as whole.

This (hypothetical) act of balancing can truly be considered an instance of practical concordance as the international judge will have engaged in a deliberate process of weighing both norms against one another and safeguarding each by ensuring that it remains intact and is not compromised or conceptually subordinated to the other. It should be pointed out that the balancing process could also lead to a different result, depending on the circumstances of the case. In our illustration, it can be claimed that both norms conserve their overall legitimacy and efficiency in protecting the fundamental legal interest(s) to which they relate and thus the substantive validity of those norms. In concreto, it would mean that a competent international criminal tribunal is required to prosecute a sitting head of state who enjoys ratione personae immunity save where that head of state is charged with crimes of concern to the international community as a whole. Although in this case the pendulum could be said to have swung more towards one of the two norms, the other norm’s overall applicability and validity in similar or different constellations is not undermined.

5 Conclusion

The present contribution has sought to introduce the notion of practical concordance into international adjudication and, in particular, into the everyday practice of international judges, thereby making the principle applicable in situations where possibly conflicting norms of CIL must be reconciled in situ. Far from a full-fledged legal-theoretical concept, practical concordance as presented here is rather a state of mind conducive to establishing an equilibrium between legal norms of equal rank and a distinct methodological approach in addressing legal dilemmas. It is hoped this contribution will spur further reflection on how to confront cases in which norms of CIL may conflict with one another, and thus point to the greater leverage offered by a more nuanced weighing of contradictory legal norms and principles.

1 Introduction

Interpretation is ubiquitous in everyday life. We constantly interpret a variety of objects, including texts, signs, gestures, works of art, intentions, and human practices and interactions. Interpretation is central to the practice of international law, too. Arguing about international law’s content is the everyday business of international lawyers, and this often includes arguing about the existence and content of norms of customary international law (CIL). Although a number of scholars recognise that CIL can be interpreted,Footnote ¹ disagreements remain as to the precise methods, modalities, and extent of CIL interpretation. Such disagreements are born of a common concern to secure competently made, coherent, and accurate interpretations of CIL, given the latter’s non-textual nature.

My aim in this chapter is to explore in a preliminary manner two related questions regarding CIL interpretation: (1) is it necessary, or even possible, to strive towards coherence in the interpretation of CIL? And (2) are there any possible indicators of (in-)coherence in that respect? Providing answers to these questions depends on how one understands coherence in the first place, including its relation to legal reasoning. A substantial part of the chapter will therefore deal with that as well.

Coherence is here envisaged as a state of axiological compatibility existing among a set of independent beliefs, statements, reasons for action, and so on, which are expressed at any level of abstraction. The degree of compatibility of the set is a function of the existence of internal structures of mutual, reciprocal support among the elements of the set, so that some of the elements are supported by other elements and vice versa.Footnote ²

Based on this understanding, I make three broad claims. First, coherence is related to, yet still independent of, concepts like consistency, correctness, and comprehensiveness (and its corollary, predictability); and, moreover, it has two dimensions (one substantive, the other methodological) which are both relevant in law (Section 2). Second, this makes coherence particularly relevant to legal reasoning, where its dual dimension translates into a similar dual role that is also at once substantive and methodological (Section 3). Given this, coherence in law is more than a simple goal or a results-oriented ideal. It is also a method of constructing one’s reasoning and of deliberating about one’s interpretative choices. It is therefore both possible and, indeed, necessary to strive for coherence in the interpretation of CIL if one wishes to claim authority for, and persuade others of, one’s interpretation. Third, striving for coherence in the interpretation of CIL means being cognisant of three kinds of processes – namely, framing, contextualising, and iteration/reflexivity (Section 4). These can serve as indicators of (in-)coherent interpretations; however, they themselves are interpretative in nature and therefore can be subject to debate and reasonable disagreement.

2 The Independent Concept of Coherence

We tend to place value on coherence because it implies that something, or someone, makes sense and is intelligible. Being incoherent, by contrast, causes frustration and confusion. Coherence is thought to be a desirable attribute to have in virtually every aspect of one’s life. The legal field is no exception, where there exists a vast literature on coherence in law in general and a large consensus that coherence suits law and legal reasoning particularly well.Footnote ³ However, coherence remains a largely under-theorised concept in the field of international law. One can, for example, find passing references to coherence in the ILC’s work on the fragmentation of international law. Therein, the ILC indicates that there exists a link between coherence and the principle of systemic integration under Article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT).Footnote ⁴ Beyond that, however, coherence seems to be generally regarded by the ILC as co-terminous with mere legal security and predictability, which is to say it is seen as a formal principle devoid of its own independent content.Footnote ⁵ One sees a similar picture in other areas of international law, notably international investment law, where concerns of incoherence in arbitral awards have been raised for years alongside concerns about a perceived lack of judicial consistency, correctness, and predictability.Footnote ⁶ Here, too, coherence is regarded as valuable, yet the content given to it is often interchangeable with that of legal certainty, predictability, and legal authority.Footnote ⁷

Therefore, before we can ponder the role of coherence in interpreting CIL, one key question must be answered: what is coherence and what is its role in legal reasoning more generally? This section seeks to answer the first half of that question (what is coherence?) by demarcating coherence’s scope from other, related concepts like legal consistency, correctness, and comprehensiveness (and its corollary predictability).Footnote ⁸ The goal is not simply to reduce confusion by disentangling the meaning of concepts but also to set proper expectations of the concept of coherence in the process. The second half of the question (the role of coherence in legal reasoning) is dealt with in Section 3.

3 Practical Legal Reasoning and the Dual Role of Coherence

Legal reasoning is the principal means through which lawyers argue about the existence and content of law. This makes it natural to enquire about the role that coherence may have to play therein. Yet, answering this question presupposes answering a more fundamental question first: what kind of reasoning is legal reasoning? There are two candidate-types to consider in this respect: the theoretical and the practical. A fundamental difference between the two relates to their respective conditions of validity and the outcomes that each type of reasoning seeks to achieve.

Starting from the proposition that, much like other kinds of normative reasoning, legal reasoning is practical rather than merely theoretical, this section argues that seeing legal reasoning as practical leads to the conclusion that coherence has a dual role to play therein, a role that is at once substantive and methodological. The section concludes by hinting at three interpretative processes acting as possible indicators of (in-)coherence in legal reasoning, termed ‘framing’, ‘contextualisation’, and ‘iteration’/‘reflexivity’. Their importance for the interpretation of CIL is subsequently examined in Section 4.

4 Indicators of (In-)Coherence and the Interpretation of CIL

There exists a link between coherence’s dual role in legal reasoning and the interpretation of CIL. In particular, the three interpretative processes of framing, contextualising, and reflecting can act as indicators of the coherence or incoherence of one’s proposed interpretation of a CIL norm. The word ‘indicators’ is key, however, as it points to something less than a determinative test. Indeed, it is argued here that acts of framing, contextualising, and reflecting are themselves interpretative in nature and their successful deployment can therefore be the subject of debate and reasonable disagreement. Moreover, my listing of the three processes in the above manner does not mean to indicate a strict separation between them or their strictly sequential application in practice. Rather, I see these processes as three interdependent facets of the same interpretative operation.

An additional clarification must also be offered before proceeding. Perhaps unsurprisingly, the very term ‘interpretation’ can itself be subject to interpretation and to different understandings in different fields. In a broad sense, any act of apprehension of meaning and any ‘reading’ of a situation, no matter how commonplace, intuitive, or unmediated, can be said to involve interpretation.Footnote ⁴⁹ This is not the common understanding of interpretation in international law. Interpretation in the latter is often understood in a narrower sense predicated on the existence of doubt about the meaning or the proper application of a pre-existing rule.Footnote ⁵⁰ In this narrower sense, CIL interpretation can be seen as taking place only after a CIL rule has been identified;Footnote ⁵¹ whereas the broader sense would see interpretation as permeating the entire lifecycle of CIL, from its formation and identification to the apprehension of its meaning now and in the future. The latter approach thus leaves open the interesting, but still somewhat controversial, possibility of state practice being a subject of interpretation also.Footnote ⁵²

In what follows, I have in mind the broader sense of interpretation just identified. I understand interpretation as elucidating the content of legal norms, but also as a reasoned ‘reading’ of a situation populating it with meaning. I recognise that my use of the term has implications about the way one understands the concept of law itself. However, I will not offer any arguments on that here, nor, more generally, on the plausibility of shifting to an interpretative conception of international law.Footnote ⁵³

5 Conclusion

This chapter has argued that it is both necessary and possible to strive towards coherence in the interpretation of CIL, once coherence is properly understood as an independent concept having both a substantive and a methodological dimension. This dual dimension of coherence is critical to law and legal reasoning. It means that coherence in law is more than a mere goal or a vague, results-oriented ideal. It is also a method of constructing one’s reasoning and of deliberating about one’s interpretative choices. The dual dimension is especially important when reasoning in practical settings (and legal reasoning is one such example), where one commits to action and where interpretability of ends is a core aspect of the process. The chapter has concluded by arguing that striving for coherence in the interpretation of CIL means being cognisant of three kinds of processes – namely, framing, contextualising, and iteration/reflexivity. These can serve as indicators of the coherence or incoherence of proposed CIL interpretations. However, the three processes are themselves interpretative in nature and therefore may be the subject of debate and reasonable disagreement.

To be sure, thinking about coherence, legal reasoning, and interpretation in the manner argued here raises fundamental questions about the nature of international law and the prevailing doctrine of sources. These are important and heavily implicated issues in the present analysis, which, however, I have not begun to address in this chapter. What can be said for the moment is that, if persuasive, the present analysis gives us at least an impetus to re-examine key tenets of legal positivism in international law and to re-assess the degree to which they comport with how international actors seem to reason and argue about international law in practice.