The M'Naghten Rule was the first legal standard for determining the sanity or insanity of a defendant who was attempting to argue that he or she did not have the requisite level of sanity to be charged with a crime. The M'Naghten Rule shares a name with Daniel M'Naghten, who attempted to assassinate the British Prime Minister Robert Peel in 1843. The M'Naghten Rule was devised by a panel of judges whom the House of Lords asked to provide an opinion on the proper way to handle a defendant claiming to not be responsible for his actions.
The judges ruled that a person should be presumed to be sane unless proven otherwise, and that such sanity meant the defendant knew and was aware of the nature of the act being committed. In the event that the person was aware of his or her actions, the person could still be considered insane if he or she did not know the act was legally wrong, although ignorance of the law was not a permitted argument.
The reason the defendant undertook the insane action must be determined to be the result of a disease of the mind which could not have been foreseen, or if it could have been foreseen, was not preventable. A person suffering from an epileptic fit is not considered liable for the actions he or she commits while in the middle of that fit, but maintains standard levels of responsibility when not suffering the fit.
In contrast, a diabetic who took insulin, consumed alcohol, did not eat, and caused damage while suffering from a hypoglycemic incident would be liable for any damage he or she caused because he or she took actions which would not have occurred had he or she behaved in a way conducive to the treatment of his or her disease.
In order to determine if a defendant's insanity claim has merit, the M'Naghten Rule asks a simple two-part question: was the accused aware of his or her actions, and if so, did he or she know those actions were wrong?