M'Naghten rule refers to legal principle stating that, in order to claim a defense of insanity, accused persons must have been burdened by such a defect of reason, from disease of the mind, as not to know the nature and quality of the act they were doing or, if they did know it, that they did not know what they were doing what was wrong.
M'Naghten rule is the oldest standard for determining the insanity of a defendant. This rule requires the defense to prove that at the time of the unlawful act, the defendant's reasoning was so impaired by "a disease of the mind" that he or she either did not appreciate what he or she was doing or did not comprehend that it was wrong.