objected, however, to the practice may in theory not be bound by a subsequent customary international law.27 Opinio Iuris 6. Concurrency of practice is clearly not sufficient, in and of itself, to show the existence of customary international law. In addition, there must be evidence that the States concerned regarded themselves as acting out of a sense of legal obligation. Evidence of this subjective element can be shown through, for example, declarations and statements of States affirming the obligation. It can be difficult to justify. 7. Although there is not an authoritative and universally agreed upon list of customary international rules, it has been held to include the prohibition of genocide, freedom from slavery and racial discrimination.28 In addition, some of the principles in the UDHR are considered to have attained the status of customary international law. The Human Rights Committee has also included prohibition of torture, arbitrary killing, arbitrary arrest and detention, denial of the freedom of thought, conscience and religion, presumption of innocence, the right to a fair trial in general terms, prohibition on the execution of pregnant women and children, prohibition on national, racial or religious hatred, the right to marry, and the right of minorities to enjoy their own culture, profess their religion, use their language.29 Ius cogens 8. It is not infrequently the case that certain rights are spoken of, not only in terms of their being part of customary international law, but as having the status of ius cogens. The term ius cogens is primarily used to refer to those rules or principles of general international law which are regarded as being ‘peremptory’ in nature and as enjoying a non-derogable character.30 9. Erga omnes obligations are those that are owed to the international community as a whole, rather than other individual states. If a rule gives rise to an obligation erga omnes, it will allow any other state to institute a claim against it before an international court or tribunal irrespective of whether or not it (or its nationals) might have been palpably ‘injured’ by the action in question. It gives rise, in other words, to a general right of enforcement. Obligations erga omnes may well also justify the assertion of universal jurisdiction by national courts or tribunals. Other sources of international law 10. Article 38(1) of the ICJ Statute refers to other sources of international law including general principles and judicial decisions or writings of jurists. General 27 ICJ, Nicaragua (Merits), Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996. 28 ICJ, Reservations to the Genocide Convention, Advisory Opinion, ICJ Reports 1951, p.23. 29 Human Rights Committee, General Comment No.24, CCPR/C/21/Rev.1/Add.6 (1994). 30 Article 53 Vienna Convention on the Law of Treaties. 23