Non compos mentis

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Non compos mentis (latin) is the legal term meaning "not of sound mind".[1] Non compos mentis derives from the Latin non meaning "not", compos meaning "having command" or "composed", and mentis (genitive singular of mens), meaning "of mind". It is the direct opposite of Compos mentis (of a sound mind).


Although typically used in law, this term can also be used metaphorically or figuratively; e.g. when one is in a confused state, intoxicated, or similarly mentally impaired.

Also applicable in health care, when a determination of competency needs to be made by a physician for purposes of obtaining informed consent for treatments and, if necessary, assigning a surrogate to make health care decisions. While the proper sphere for this determination is in a court of law, this is practically, and most frequently, made by physicians in the clinical setting.[2]

In English law, the rule of non compos mentis was most commonly used when the defendant invoked religious or magical explanations for behaviour.