Opinio juris sive necessitatis ("an opinion of law or necessity") or simply opinio juris ("an opinion of law") is the belief that an action was carried out because it was a legal obligation. This is in contrast to an action being the result of different cognitive reaction, or behaviors that were habitual to the individual. This term is frequently used in legal proceedings such as a defense for a case.
Opinio juris is the subjective element of custom as a source of law, both domestic and international, as it refers to beliefs. The other element is state practice, which is more objective as it is readily discernible. To qualify as state practice, the acts must be consistent and general international practice.
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