M’Naghten rule a rule in Anglo-American criminal law defining legal insanity for purposes of creating a defense to criminal liability: legal insanity is any defect of reason, due to disease of the mind, that causes an accused criminal either not to know the nature and quality of his act, or not to know that his act was morally or legally wrong. Adopted in the M’Naghten case in England in 1843, the rule harks back to the responsibility test for children, which was whether they were mature enough to know the difference between right and wrong. The rule is alternatively viewed today as being either a test of a human being’s general status as a moral agent or a test of when an admitted moral agent is nonetheless excused because of either factual or moral/legal mistakes. On the first (or status) interpretation of the rule, the insane are exempted from criminal liability because they, like young children, lack the rational agency essential to moral personhood. On the second (or mistake) interpretation of the rule, the insane are exempted from criminal liability because they instantiate the accepted moral excuses of mistake or ignorance. See also DIMINISHED CAPACITY, RESPONSIBILIT. M.S.M.