Legal Terms: NON COMPOS MENTIS

Not in your right mind

Saturdays are perfect days for legal surprises. Surprise—we’re talking about meaning of non compos mentis!

Anyone who has watched a criminal justice procedural has heard the defense “not guilty by reason of insanity” invoked. This defense is part of a broader condition termed non compos mentis, a Latin phrase meaning not of sound mind or not having mastery of one’s mind.

Criminal defendants enter the non compos mentis plea to assert that they were not of sound mind–either at the time of the offense or in general–and thus not responsible for the crime they are charged with committing. In civil cases, being deemed non compos mentis may affect a person’s ability to enter into contracts or manage their affairs.

The opposite and obviously more desirable condition (unless perhaps you are facing life in prison) is that of compos mentis, having control of one’s mind. The phrase “of sound mind and body” often heard during the reading of wills and other legal documents is a claim of compos mentis, asserting that a person is mentally competent and capable of managing their own affairs.

BREAKDOWN: In Latin, non compos mentis means not in command of one’s mind.

• The prefix NON- means not in words like nonfiction and nonviolent.

• The root POT- means power or control in words like impotent and potential.

• The root MENT- means to think in words like demented and mental.

“You can’t win an argument. You can’t because if you lose it, you lose it; and if you win it, you lose it. Why? Well, suppose you triumph over the other person and shoot their argument full of holes and prove that they are non compos mentis. Then what? You will feel fine. But what about them? You have made them feel inferior. You have hurt their pride. They will resent your triumph. A man convinced against his will is of the same opinion still.” —Dale Carnegie