Daniel M'Naghten shot and killed Edmund Drummond the private secretary of British Prime Minister Sir Robert Peel; mistakenly thinking Drummond was Peel, his intended target.

In a landmark legal ruling M'Naghten was found not guilty of murder on the grounds that his delusional mental state rendered him incapable of knowing that what he was doing was wrong. This enduring legal precedent within the insanity defense became known as the 'The M'Naghten Rules.'

(Note: The correct spelling of M'Naghten's surname is the subject of much debate and is sometimes documented as McNaughton, McNaughten or McNaughtan.)

See following link to learn all about the insanity defense.

The Insanity Defense