CYIL vol. 8 (2017)

CYIL 8 ȍ2017Ȏ THE IMPORTANCE OF CUSTOMARY LAW FOR THE CODIFICATION OF THE LAW … of individuals which are realized in the municipal law sphere and subject to the standards of rule of law provided for by the constitutions of particular countries. The fundamental essence of custom is an established practice supported by shared common perception of a binding legal rule that is to be observed. This does not change over time. However, the ways to learn about the content of customary rules do change over time. Traditional custom was expressed as a general rule extracted from single repeating practical situations, while modern custom is built upon declarations concerning the legal perception of states. In other words, the former inductive method was replaced by a more flexible deductive one. 5 It is to be emphasized that state practice is an important component of custom but it is not sufficient to constitute a customary rule. It is the opinio iuris that is decisive. 6 The mere practice is a useful instrument to interpret the rules, above all those expressed in treaties, because it expresses how the treaty parties understand the meaning of the rule, as well as the intentions of the state parties at the conclusion of the treaty. In the case of the North Sea Continental Shelf , 7 the International Court of Justice stressed that the practice of states performing treaty obligations cannot be understood as a proof of the existence of the custom. Also in the case when the military and paramilitary activities in Nicaragua were in question, 8 the Court examined the opinio iuris as a key factor, and not the sole practice only, when searching for the customary nature of the principles of prohibition on the use of force and non-interference with internal affairs of foreign states enshrined in UN Charter. Further, it was concluded by the Court that practice must be completely consistent with the customary rule, without significant derogations, which are to be understood as a breach of the legal rule. The Court examines the opinio iuris , according to the case-law, through various formal sources of knowledge, such as multilateral treaties and their preparatory work, UN resolutions and other non-binding instruments, factual circumstances in which the rule is applied, and analogy. 9 The UN General Assembly resolution itself cannot be treated as a sufficient proof of custom, it should be searched whether its content expresses the fact that states together are convinced about any binding legal rule inside it. 10 2. The Codification Convention – The Vienna Convention on the Law of Treaties – and its relation to custom The Vienna Convention on the Law of Treaties (VCLT) was adopted on 23 May 1969 and has become one of the most successful codifications in the field of international law ever since. It is the first treaty dealing with the system of sources of international law. The VCLT enshrines the rules of treaty law stemming from centuries of development in real state practice, while these rules are vitally needed and used to reach states’ aims in foreign policy through establishing and conducting mutual treaty relations. 5 See e.g. ALVÁREZ-JIMÉNEZ, Alberto, Methods for the Identification of Customary International Law in the International Court of Justice’s New Millennium Jurisprudence, International & Comparative Law Quarterly , 2011, pp. 686-689. 6 Legality of the Threat or Use of Nuclear Weapons , Advisory Opinion, July 8, 1996, ICJ Reports 1996, p. 226. 7 North Sea Continental Shelf, Judgment , ICJ Reports 1969, p. 3. 8 Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America). Merits, Judgment. I.C.J. Reports 1986, p. 14.

9 TOMKA, Peter, op. cit. , p. 47. 10 TOMKA, Peter, op. cit. , p. 43.

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