It seems that everything has been said ‒ and written ‒ about the rules of interpretation of international treaties and about their codification in Articles 31 to 33 of the 1969 Vienna Convention on the Law of Treaties (VCLT). Each new addition to the existing literature on this subject is therefore faced with the suspicion ‒ and with the risk ‒ of being a mere restatement of things already stated eloquently by the most prestigious scholars and practitioners of international law.

But the book written by Eirik Bjorge, Evolutionary Interpretation of Treaties, and recently published by Oxford University Press, is far more than a simple restatement.

It is true that the book echoes some of the views developed in 2013 by Special Rapporteur James Crawford in his book entitled State Responsibility, including Crawford's assumption that the issue of evolutionary interpretation is ‘essentially one of the correct application of VCLT Article 31’. This assumption is both the starting point and the conclusion of Bjorge's book. But the author's ambition is to give flesh to this statement by defining precisely the concept of ‘evolutionary interpretation’ – ‘a situation where account is taken of the meaning acquired by the treaty terms when the treaty is applied’ (p. 59) – and by measuring its usefulness as an interpretative tool of international treaties.

This ambition may appear, at first sight, a modest one, as the author himself admits that ‘there is nothing exceptional about evolutionary interpretation’ (p. 141). Nevertheless, the reader should not be discouraged by this blunt admission, which Eirik Bjorge himself describes as the main thesis of this book: ‘The main thesis of this book can be summarized in the following way: there is nothing special about the evolutionary interpretation of treaties as compared with other types of interpretation’ (p. 5).

In all fairness, there is much more to the concept of evolutionary interpretation than this ‘main thesis' suggests. In fact, there is more to the concept than the title of the book itself (The Evolutionary Interpretation of Treaties) suggests.

In fact, despite starting from a seemingly simple proposition (which could be summarized as ‘Article 31 of the VCLT sometimes prescribes an evolutionary interpretation of treaties and sometime doesn't’), the author uses a method of ‘progressive encirclement’ (an expression often used to describe the intellectual process of Article 31 of the VCLT itself) to get to the heart of the book's thesis. In four dense but concise chapters, the author covers some of the most debated questions in international law: whether different methods of interpretation should apply to different types of treaties; what should be understood as the common intention of the parties; the retroactive application of treaty provisions; the application of the concept of jurisdiction ratione temporis by international courts. Sometimes, the book tends to orbit a little too far away from its stated objective, and the reader may think that the author in fact has a more ambitious goal: to address in a comprehensive fashion how the interpretation of international treaties is globally affected by the passage of time. But this impression is simply the consequence of the progressive approach chosen by the author. If one accepts the need to follow the reasoning explained in the introduction of the book, then the four chapters fall into place and eventually contribute to the completeness of the analysis.

Eirik Bjorge's progressive encirclement of evolutionary interpretation starts with a plain rejection of the concept of ‘fragmentation of international law’. For the author, the unity of international law is a fact, which has gained considerable recognition over recent decades, up to the point where it has become ‘the new orthodoxy’. More importantly for the subject of the book, the author endorses the view that the means of interpretation contained in the Vienna Convention are universal, i.e. that they are applicable to all branches of international law, from investment treaties and the WTO Agreements to human rights law. In fact, ‘the unity of the interpretation process' (p. 3) derives from the fact that all treaty interpreters are confronted with the same fundamental task, i.e. to determine the common intention of the parties, which is eventually the ‘very aim of the whole process': ‘Article 31, as it was finally agreed upon, does not implicitly recognize intentions as just another (unmentioned) means of interpretation; rather it recognizes intention as the very aim of the whole process' (p. 63).

Intuitively, it may seem that the contemporaneous interpretation of a treaty, i.e. an interpretation which aims at discerning the exact meaning of the terms at the time of drafting, may seem more appropriate in order to bring to light what may have been the original intention of the drafters at the time of consent. Eirik Bjorge however endorses the opinion of the International Law Commission that: ‘the intention to be established is not a separately identifiable original will, and the travaux préparatoires are not the primary basis for determining the presumed intention of the parties' (p. 60). Or, as stated by judge Higgins in Kasikili/Sedudu, the object of treaty interpretation is ‘not to discover a mythical “ordinary meaning” within the Treaty’, rather the object of this exercise is ‘to give flesh to the intention of the parties, to decide what general idea the parties had in mind, and then make reality of that general idea’ (p. 63). For this purpose, the author argues, an evolutionary interpretation of the treaty terms may be as legitimate as a contemporaneous one.

The author goes on to demonstrate that evolutionary interpretation too is universal. In support, he quotes a range of rulings from numerous international tribunals, which, in very diverse situations, chose this path. This universality stands in contrast to the idea that evolutionary interpretation should be reserved to certain types of treaties, such as human rights treaties for example, whose object and purpose of safeguarding the position of the individual in a given legal-societal context seems to require such an evolutive interpretation. An obvious example of this is the European Court of Human Rights' famous statement in ‘Tyrer’ that: ‘The Convention is a living instrument, which, as the Commission rightly stressed, must be interpreted in the light of present-day conditions'.

All these citations end up serving the author's main thesis that the choice between an evolutionary interpretation and a contemporaneous one is, in fact, not left to interpreters: By correctly analyzing the facts and by correctly using the interpretative tools provided for in Articles 31 to 33 of the VCLT, treaty interpreters must necessarily deduce which type of interpretation (evolutionary or contemporaneous) should apply. Whenever the modern meaning of a treaty term is to be preferred over its original meaning, it is because the parties so intended. A logical consequence is that: ‘There are … times when the intentions of the parties were that the treaty terms should be interpreted not in an evolutionary manner but rather contemporaneously. In such cases too, it is the common intention of the parties which is controlling’ (p. 139). Eirik Bjorge also admits that there are cases where neither evolutionary interpretation nor contemporaneous interpretation should be used. This is the case when treaty provisions are drafted in sufficiently generic terms. An illustration is the dispute in European Communities and its Member States — Tariff Treatment of Certain Information Technology Products (DS375, 376 and 377) in which the WTO Panel considered precisely that: ‘Generic terms had been used in the treaty, to cover a wide range of products and technologies' and ‘[t]he Panel did not consider it necessary to resort to any form of evolutionary interpretation’ (p. 141). For Bjorge, in such a situation, speaking of evolutionary interpretation is of little use, ‘as the mere wording comes so close to providing us with the answer’.

It is in this sense that ‘there is nothing exceptional about evolutionary interpretation’. Or, as Bjorge also puts it: ‘it comes to us not as an aberrant and freewheeling interpretive technique, but as nothing else than a result of the traditional canons of treaty interpretation’ (p. 140).

The reader could point to an apparent weakness in the argument. If the common intention of the parties is the decisive factor for deciding whether or not recourse to evolutionary interpretation is admissible, then can evolutionary interpretation also be the key to determining the common intention of the parties? In such a case, the common intention of the parties would be both the triggering factor and the result of evolutionary interpretation. But this apparent ‘egg or chicken’ dilemma can be overcome by pointing to the fact that treaty interpretation does not necessarily follow a logical progression. It is rather an inclusive intellectual process, or as the WTO Appellate Body described it, ‘an integrated operation, where interpretative rules and principles must be understood and applied as connected and mutually reinforcing components of a holistic exercise’ (US–Continued Zeroing, DS350, para 268).

Eirik Bjorge's book is a well-written and well-documented analysis, which convincingly achieves its intended goal. As modest as its stated goal may be, it is stimulating reading worth the reader's time and attention, and it covers, in a concise manner, some of the most important issues of treaty interpretation. The main achievement of this book/treatise is not to add its contribution to the vast amount of theories written about treaty interpretation, but rather to offer a unified (and as such, simplified) understanding of the various ways of giving meaning to a treaty over time. Such an effort is all the more welcome as ‘never have international tribunals produced so much treaty analysis as now ; the law of treaty interpretation is indeed, and despite having been codified more than 40 years ago, one of the most dynamic field of international law’ (p. 55).