 The consent-based account runs into even greater difficulties when it comes to custom. There is, first, the notion of jus cogens, i.e., rules of customary international law (such as the prohibition on torture, genocide and apartheid) which have acquired peremptory status and from which no state is permitted to derogate. Dworkin maintained that jus cogens too comes “under the umbrella of consent” in contemporary international legal doctrine (Dworkin 2013: 6), but there is nothing in the definition of jus cogens in the Vienna Convention that suggests  this.[1]