 Next to capacity and effect of consent, the last reason for invalidating a treaty is that its very object is illegal because one of its obligations conflicts with a fundamental rule of international law. Under Article 53 of the Vienna Convention that I quote, a treaty is void if at the time of its conclusion, it conflicts with a parametry norm of general international law. For the purposes of the present Convention, a parametry norm of general international law is a norm accepted and recognized by the international community of states as a whole. As a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character, end of quote. In the title of Article 53, parametry norms of general international law are called in Latin Yuskogens norms. Let us first try to understand the very notion of Yuskogens before turning to its normative status, the ways it is formed and finally give some concrete examples of parametry norms. As such Yuskogens is opposed to Yus dispositivum. What does that mean? Well simply that some norms can be derogated from by agreement between contracting parties and those norms are called a Yus dispositivum they can be freely disposed of. While other norms cannot be set aside by mutual agreement and those norms are parametry they are Yuskogens. This dichotomy exists in every domestic legal system. Some legal provisions apply in the absence of any particular contract while other legal provisions apply despite any contract and contracts that do not conform with those parametry provisions cannot be upheld in a court of law they are invalid. Parametry norms embody the notion of ordre publique and if the notion of Yuskogens is not difficult to understand it is nevertheless quite troubling in a legal order based on the sovereignty of states and resulting from their consent and indeed Yuskogens means that despite their sovereignty states are not entitled to validly consent and make treaties about whatever they wish. Yuskogens is out of the reach of states it is above them and they must always comply with it since they cannot escape it by concluding treaties which would pretend to dispose of it. Any such treaty would be null and void from the start or as stated under article 64 of the Vienna Convention it would become void in case a new parametry norm arises after the treaty has been concluded. The conceptual novelty introduced by Yuskogens and the perceived limitation to state sovereignty it entails were not easily accepted and for many years the issue of Yuskogens has been very divisive. However international courts and tribunals the European and the inter American courts of human rights the international criminal court the international criminal tribunal for the former Yugoslavia and finally the international court of justice those courts and tribunals have gradually referred to the concept of Yuskogens so that its very notion is undoubtedly part of today's international law. But what is the nature of Yuskogens and how do Yuskogens norms come to existence? Well there have been endless debates about the nature of Yuskogens. Is Yuskogens part of Kashmir international law or is it something completely different? Does it belong to the category of general principles or to another specific category of sources? In a case between Belgium and Senegal about the failure by Senegal to prosecute the former head of state of Chad accused of acts of torture the international court of justice briefly said this and I quote in the court's opinion the prohibition of torture is part of Kashmir international law and it has become a peremptory norm Yuskogens end of quote. The court went on to refer to practice and to opinion Yuris so from the ICJ's point of view it seems undisputable that Yuskogens norms are part of customary international law. They come to existence through the usual customary process. However and because each and every rule of Kashmir international law is not peremptory Yuskogens norms are a special category of norms within Kashmir international law and this is because of the specific opinion Yuris that is required for their formation. Article 53 of the Vienna Convention makes this very clear by saying this and I quote again a peremptory norm of general international law is a norm accepted and recognized by the international community of states as a whole as a norm from which no degradation is permitted and which can be modified only by a subsequent norm of general international law having the same character end of quote. This is the specific opinion Yuris. This is all good and well but which concrete norms can be said to a pertinent to Yuskogens if one takes the threshold expressed by article 53 seriously not many rules of Kashmir international law can be said to be peremptory and it is of course very tempting to resort to the language of Yuskogens in order to affirm that a rule is very important but this should not be done lightly. So for the time being Yuskogens is made of only a few new norms and the collection of norms is not extensive. There is no official exhaustive list of them but from the case law the following Yuskogens norms can be gathered the prohibition of wars of aggression the prohibition of genocide of crimes against humanity and of war crimes the prohibition of slavery the prohibition of apartheid and probably also of racial discrimination or ethnic cleansing the prohibition of torture as we called above from the Belgian Senegal case. There may be other rules a pertaining to Yuskogens and I do not pretend that this short list is exhaustive but the prohibition I mentioned certainly belong to the felt necessities of contemporary international law and it is good to take stock of the progress they represent in the common conscience of humanity. Let me make two final remarks about Yuskogens and the first remark is as follows the concept of Yuskogens was introduced in the Vienna Convention as a ground for contesting the validity of treaties but so far states have hopefully refrained from concluding treaties that would be contrary to parametry norms. So invalidity of treaties for breach of Yuskogens is rather hypothetical. However this does not mean that Yuskogens is not important as a legal concept on the contrary having parametry norms within international law profoundly changes that legal order as it conveys the idea that certain core values are common to mankind and must be protected by norms that cannot be transgressed. Moreover and as we shall see later when discussing international responsibility serious breaches of parametry norms entail specific consequences. My second remark under the Vienna Convention Yuskogens is the only content oriented ground for treaty invalidity in other words the fact that the treaty is substantially unequal that one party takes all the benefit from the treaty while the other bears all the cost of it this is not a ground for invalidating the treaty and this may sound unfair but the substantial inequality of the respective party's obligation under the treaty is not a legal ground for considering that it is null and void and this of course is without prejudice to a possible defect of consent as we have studied earlier.