Opinio juris sive necessitatis

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 as a legal obligation. This is in contrast to an action resulting from cognitive reaction or behaviors habitual to an 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.

A situation where opinio juris would be feasible is a case concerning self-defense. A condition must be met where the usage of force is limited to the situation at hand. The act of striking an attacker may be done with legal justification; however, legal territory limits the acceptability of such a claim. Even in this case, the usage of force must be acceptable to the conditions of the environment, the attacker, and the physical conditions of the people involved, as well as any weapons or tools used.^{[citation needed]}