Like all rules of customary international law, those existing in the field of international investment law are binding on all states. According to the theory of the persistent objector, however, a state is not bound by a rule if it objected to it in the early stages of its formation and continued to do so consistently thereafter. This paper analyses the different grounds of criticism that have been raised against the concept. We found that there is only very weak judicial recognition of the concept, that there is no actual state practice supporting it, and that it is logically incoherent. Specifically, this paper argues that the concept should not be successfully used in investor–state arbitration proceedings to prevent the application of a custom rule by an arbitral tribunal. This is essentially because of the great importance of the few custom rules existing in that field and the fact that they represent universally recognized values.
