M'Naghten's Case
House of Lords
8 Eng. Rep. 718 (1843)
M'Naghten (defendant) shot and killed Edward Drummond, secretary to the British prime minister, mistakenly believing Drummond was the prime minister himself, whom M'Naghten believed was persecuting him as part of a broader delusion involving political enemies. At trial, extensive expert and lay testimony established that M'Naghten suffered from acute delusions and insanity, and the trial judge instructed the jury that M'Naghten should not be convicted if he was not of sound mind at the time of the killing. The jury returned a verdict of not guilty by reason of insanity, prompting members of the English judiciary to convene and clarify the proper standard for the insanity defense going forward.
Whether a defendant will be found not guilty by reason of insanity if, due to a disease of the mind, he did not know or appreciate the nature or quality of his criminal act, or did not know the act was wrong.