Although the issue may have become obscured at some point after the drafting of the Vienna Convention on the Law of Treaties, it is increasingly acknowledged in international law that the goal of treaty interpretation is, as the International Law Commission’s first Special Rapporteur on the law of treaties JL Brierly put it, ‘to give effect to the intention of the parties as fully and fairly as possible’ (The Law of Nations (OUP, 1928) 168; A Clapham, Brierly’s Law of Nations (7th edn, OUP, 2013) 349).
The question of the intention of the parties in treaty interpretation might be thought to take on a particular interest in connection with the evolutionary interpretation of treaties. Though there is no standard definition of the term ‘evolutionary interpretation’, the upshot is that the meaning of treaty terms may be liable to change over time, without the specific intervention of the parties to amend or modify the treaty terms.
It is noteworthy that international courts and tribunals refer, at an increasing rate, to the exercise of treaty interpretation with specific reference to the intention of the parties. Thus, in late 2014, the Tribunal in the Railway Land Arbitration observed that, in applying the principles set out in Articles 31–32 of the Vienna Convention, ‘it is important not to lose sight of the object of the exercise. This is to identify the common intention of the Parties at the time that the treaty was concluded as to its meaning and effect’ (para 43). Similarly in Conclusion 3 in the recently published 2013 Report of the International Law Commission the intention of the parties takes centre stage in relation to the evolutionary interpretation of treaties:
Interpretation of treaty terms as capable of evolving over time
Subsequent agreements and subsequent practice under articles 31 and 32 may assist in determining whether or not the presumed intention of the parties upon the conclusion of the treaty was to give a term used a meaning which is capable of evolving over time.
Though both of these were published subsequent to my book, The Evolutionary Interpretation of Treaties (OUP, 2014), it would, to paraphrase Evelyn Waugh, be wholly inappropriate to attempt in this connection ‘to disentangle the post hoc from the propter hoc’. In any event, this development is to my mind a salutary one.
The thesis of The Evolutionary Interpretation of Treaties is that the evolutionary interpretation of treaties can be explained by a proper understanding of the intention of the parties. I try in other words to link evolution with intention, as I believe that is the best way to explain what is the evolutionary interpretation of treaties. Being in my view closely linked to the the common will of the parties, the evolutionary interpretation of treaties is not a separate method of interpretation; it is rather the result of a proper application of the usual means of interpretation, as means by which to establish the intention of the parties. Two misgivings might be raised in this connection.
First, it could legitimately be interjected, except for the oblique mention in Article 31(4), the general rule of interpretation does not mention ‘intention’ at all!
This is true. But the general rule of interpretation fails to mention, amongst the means of interpretation, ‘the intention of the parties’ for the same reason that a cake recipe would normally fail to mention, amongst the ingredients, ‘cake’. Like the various ingredients that go into a cake, ‘all the elements of the general rule of interpretation’, as the Tribunal in Rhine Chlorides put it, ‘provide the basis for establishing the common will and intention of the parties’ (para 62).
Secondly, saying that, properly understood, treaty interpretation is concerned with the establishment of the intention of the parties might be criticized for begging the question. After all, if one posits that treaty interpretation is about establishing the intention of the parties, the question becomes, how does one go about establishing that intention? Getting the starting coordinates right is, nevertheless, important.
It is worth mentioning in this regard that leading public international lawyers differ as to how to conceptualize the issue of the intention of the parties in treaty interpretation. Some have seen the intention of the parties as being no more than a means of interpretation, similar to the way that the wording or the object and purpose are means by which to construct the treaty at issue. Thus the distinguished Tribunal in Iron Rhine observed, wrongly in my view, that ‘[t]he object and purpose of a treaty, taken together with the intentions of the parties, are the prevailing elements for interpretation’ (para 53). Similarly, Professor Dapo Akande has posed the question, should the ordinary meaning of a treaty text trump the intention of the parties with regard to that treaty? His answer to that question is yes, and he makes a strong case for this. In my view, however, even asking this question is to stand on its head the whole process of treaty interpretation.
The intention of the parties, in my understanding, is something quite different from this. On the correct reading of the approach set out in Articles 31–33 of the Vienna Convention, the treaty interpreter reconstructs the meaning of an intention of the parties; the means of interpretation are the objective means which guide the treaty interpreter to the establishment of what the law of treaties calls the intention of the parties.
Thus, as the 2013 Report of the International Law Commission stated, in its commentary to the conclusion cited above, relying on slightly different terminology:
the phrase ‘presumed intention’, refers to the intention of the parties as determined through the application of the various means of interpretation which are recognized in articles 31 and 32. The ‘presumed intention’ is thus not a separately identifiable original will, and the travaux préparatoires are not the primary basis for determining the presumed intention of the parties (p 27).
The Objectivized Intention of the Parties
I call this in my book the objectivized intention of the parties. That it was meant to be objective is clear from the fact that, in drafting the Vienna rules, the ILC set out by its own admission to codify, in the general rule of interpretation, ‘the means of interpretation admissible for ascertaining the intention of the parties’ (p 218–19). This objectivized nature of the concept of ‘the intentions of the parties’ is clear also from what the International Court observed in Navigational and Related Rights ICJ Rep 2009 p 213, 237 where the Court stated that the aim of treaty interpretation is to establish ‘the intentions of the parties as reflected by the text of the treaty and the other relevant factors in terms of interpretation’.
It is perhaps surprising to note that every time the International Court has made an evolutionary interpretation it has done so with explicit reference to the intention of the parties. Thus in Namibia  ICJ Rep 1971 16, 31 the Court referred to ‘the primary necessity of interpreting an instrument in accordance with the intentions of the parties’, and used this as a basis on which to make an evolutionary interpretation of the concepts contained in Article 22 of the Covenant of the League of Nations. The same was the case in Aegean Sea  ICJ Rep 3, 32; Gabcikovo–Nagymaros  ICJ Rep 7, 79–9; Navigational and Related Rights  ICJ Rep 213, 242; and Pulp Mills  ICJ Rep 14, 82–3. In all of these cases the Court arrived at an evolutionary interpretation of the instrument at issue specifically by stressing the importance in treaty interpretation of the intention of the parties. On the basis of the admissible means of interpretation, the evolution is, in other words, taken to have been an intended one.
This shows that the evolutionary interpretation of treaties is not a separate method of interpretation; it is rather the result of a proper application of the usual means of interpretation, as means by which to establish the intention of the parties. The intention of the parties is a construct to be derived from the articulation of the means of interpretation admissible in the process of interpretation. There is nothing special about it, and there is no need to fetishize it or single it out for the type of criticism to which it has been subject.