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Customary International Law




The UN charter acknowledges the existence of customary international law (article 38(1)(b) of the Statute, incorporated into the Charter by article 92 thereof): "The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply... international custom, as evidence of a general practice accepted as law;".

Customary international law "... consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way." (Rosenne, ''Practice and Methods of International Law'', p. 55). It follows that customary international law can be discerned by a "widespread repetition by States of similar international acts over time (State practice); Acts must occur out of sense of obligation ( Opinio Juris ); Acts must be taken by a significant number of States and not be rejected by a significant number of States."

Even extreme conservatives have acknowledged the existence of customary international law. For example, Doug Bandow of the Cato Institute testifed to the US Senate 8 April 2004 {Link without Title} in regard to the United Nations Convention On The Law Of The Sea (UNCLOS) that:
:The treaty's authorization of 200-mile km exclusive economic zones (EEZs) merely reflects what has become customary international law.
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International law analyst Gary Knight even argues that "the difficulty of establishing our legal right to EEZ navigation and submerged straits passage would be no more difficult under an existing customary international law argument than under the convoluted text of the proposed UNCLOS." In short, there is only modest theoretical advantage in this area for which to trade away the mining provisions.

Amnesty International writes that:

: ''Customary international law results from a general and consistent practice of states followed out of a sense of legal obligation, so much so that it becomes custom. As such, it is not necessary for a country to sign a treaty for customary international law to apply.''

In other words, customary international law must be derived from a clear consensus among states, as exhibited both by widespread conduct and a discernible sense of obligation.

Customary international law can therefore not be declared by a majority of States for their own purposes; it can be discerned only through actual widespread practice. For example, laws of war were long a matter of customary law before they were codified in the Geneva Conventions and other treaties.

A particular category of customary international law, Jus Cogens refers to a principle of international law so fundamental that no state may opt out by way of treaty or passage of domestic law. Examples of this is the act of Genocide and Crimes Against Humanity .

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