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The Latin phrase “non compos mentis” translates as “not of sound mind.” This term is used to refer to someone who cannot be held legally responsible for something because he or she is not mentally competent. There are a number of things which may call mental competency into question.
One reason someone may be determined non compos mentis is severe developmental or intellectual disability which inhibits a person's ability to understand a situation. Some forms of mental illness may also limit mental competency, as can temporary situations such as intoxication or illness which leads to an altered level of consciousness. In all cases, it is clear that the person cannot make decisions because of an inability to understand.
In law, someone who is non compos mentis cannot be held liable for contracts and other legal situations. Even if the person signed a contract willingly, lack of mental capacity would have meant that the individual did not understand the contract and should not be held to it. Mental capacity is also a concern when testing a will; if the author of a will is deemed non compos mentis, the will cannot stand up in court because it was written by someone who did not know what she or he was doing.
This issue can also come up in medical care. Doctors must sometimes appoint surrogates for patients who are clearly unable to make their own decisions and in some cases patients may make advance arrangements for someone to represent their wishes in the event that they are incapacitated. A patient who is non compos mentis may be compelled to accept treatment under the argument that the patient does not comprehend the situation or realize the potential consequences of refusing treatment.
To determine whether or not someone is of sound mind, a psychiatrist or psychologist may be called in for an examination. An expert in a medical condition may also be consulted. For example, if a patient has a brain tumor which appears to be impairing judgment, an neurologist could be asked to testify on whether or not the patient has the capacity to understand and make decisions.
The state of being non compos mentis can be temporary. If it is temporary, the person will be held responsible for actions taken during periods of time in which the individual was of sound mind. For example, if someone was extremely ill and could not make medical decisions but later recovered and made a will during the period of recovery, the will would be deemed valid because the patient was of sound mind when it was written.