 The various sources of international law that were surveyed in the preceding weeks do not live in isolation from each other, but they very often interact with each other. And for instance, a state may accept, by a unilateral undertaking, an obligation which is contained in a treaty to which it is not a party, or a customary rules, as you will see assist in the interpretation of treaties. Or again, customary international law may also arise out of the practice stemming from identical treaties or from multilateral treaties. But what the sources of international law have in common is that they are all formal processes by which international obligations are created. In other words, the origin of any international obligation and of any corresponding right is to be found in one or several of those processes. But things are a little bit complicated because the interaction between the sources is such that the same obligation may exist for a state or an international organization under a treaty and under a rule of customary international law or under a treaty and a unilateral act of that state or organization or a general principle and a treaty. Sources are grounds for international obligations and several different sources can each be ground of an identical obligation which will simply have different origins. Origins that remain legally distinct and independent from each other in terms of validity and in terms of scope. And conversely, and because the production of international law is, as you remember, decentralized, the same state or the same international organization will sometimes not be bound by the same international obligation through different sources but will be facing obligations that contradict each other. Those obligations will contradict each other because it is impossible to reconcile them and to perform each of them at the same time. And those irreconcilable obligations can be found in sources of the same nature, for instance in two treaties, or in sources having different nature, for instance a treaty and a unilateral act of an international organization. In light of this, when it comes to the application rather than to the making of international obligations, a few elementary questions arise. Who is bound by the obligation at stake and is it always owed by those who are bound? What does the obligation actually mean and what must be done in order to comply with the obligation? Which obligation should be preferred in case several obligations conflict with each other? Those are the questions that we will address this week and that we shall try to answer. But let me turn first to the first question that is the personal scope of what can also be called the rationer personer scope of the various sources of international law we have surveyed. Who is bound by the obligation at stake? That question is indeed a question that must be answered in light of the sources where the obligation is grounded. And let me recall some of the answers that you know already. On customary international law you may recall two important things. First, the fact that customary international law provides for rules that are binding on all states. And also all international organizations which are by their very object concerned by the rule at stake. That is customary international law is general international law. The exceptions to such generality in terms of the rationer personer scope of customary international law, the exceptions is the situation of the persistent objector you may remember and also the possibility for certain customs to be regional or local and therefore to be only binding on a few states. Secondly you may recall also that customary international law is use disposable and that it can be displaced derogated from by treaty. In such a case and as long as the treaty is binding on the parties to it, the parties to the treaty will have to respect between them the obligations contained in the treaty rather than the obligations existing under the customary rule at stake. But of course that customary rule continues to be binding on the parties to the treaty in their relations with states and organizations that are not party to that treaty. You also remember that when the customary obligation is not of a use dispositive nature but of a use co-gain nature it must be respected by all, irrespective of any contrary obligation contained in a treaty and such treaty is void. Let me turn now to the same question but in relation to general principles of law within the specific meaning of article 38 paragraph 1c of the ICJ statute and not general principles as synonymous to customary rules of international law. And as their name indicates and as the comparative law process forming them requires, general principles of law are general in personal scope. They are binding on all states but because general principles within the meaning of article 38 are filling the gaps as it were in the absence of a customary rule or an applicable treaty they are also of a use dispositive nature and they can be derogated from by custom or by treaty. As far as unilateral acts of states are concerned the answer to their rationer-personer scope is quite straightforward. Unilateral undertakings are binding on their authors only. The same is true as far as unilateral acts of international organizations are concerned. Being part of the rules of the organization, unilateral acts of international organizations are binding on the organization and on its members according to the conditions set forth in the basic instrument establishing the organization and as long as they have as they have not been changed or repaired by a subsequent act of the organization adopted under the same procedure. Unilateral acts of organizations may also be binding on individuals and on corporations again according to the conditions set forth in the basic instrument and this can notably be the case of some EU secondary legislation or some United Nations Security Council resolutions as we shall see at the end of the course. Treaties deserve a separate and more elaborate answer and this is what the next video will be about.