One wonders whether it was right for the Tribunal to proceed in this way. The International Court of Justice has emphasized in several cases that it must protect the integrity of its judicial function. In the same way, it is legitimate to ask whether the Tribunal can be said to be acting with due judicial integrity when it seeks to exercise jurisdiction on the basis that it can rule on one element of a case, but not on two prior and indispensable elements of that case.

China has always maintained that any disputes concerning the South China Sea should be settled by negotiation between the parties, rather than through recourse to judicial procedures, and in this respect it points to the Declaration on the Conduct of Parties in the South China Sea, signed by China and ASEAN (including the Philippines) in 2002, which states this explicitly. A document like the Declaration may not be formally legally binding in itself, but to international lawyers it would normally be regarded as giving rise to what is called an estoppel. This is where State A makes a representation to State B, which State B relies upon to its detriment; in such circumstances, State A cannot go back on its representation. But in this case the Tribunal held that the Declaration did not amount to a representation by the Philippines. This is very difficult to understand: the Declaration was a jointly negotiated document, which was signed at a high level, so to say that it does not constitute a representation seems odd. But the result was that the Tribunal allowed the Philippines to resile from what had been said in the Declaration and to proceed with the arbitration. This may be an unfortunate precedent: there are many tens of thousands of similar documents negotiated between States, which may not be legally binding, but which States feel they ought to abide by; the Tribunal's decision is therefore potentially destabilizing in international relations generally.

Finally, under UNCLOS, the Tribunal is obliged to ensure that a case is "well founded" before proceeding. One of the arbitrators in the Philippines case, speaking in an earlier case, likened this to the standard "beyond reasonable doubt" applied in criminal cases in common law countries－which is of course an exacting standard. The question here is ultimately whether the Tribunal applied this high standard in deciding that it had jurisdiction to hear the Philippines' claims.

The author is former Deputy Legal Adviser of the United Kingdom's Foreign and Commonwealth Office.