Daniel M'Naghten believed that Prime Minister Sir Robert Peel was conspiring against him. In 1843 in London, he shot and killed the secretary to the British prime minister -- but he meant to kill the prime minister. M'Naghten was tried and acquitted by reason of insanity, and sentenced to spend his life confined in Bedlam, the first mental asylum in Europe.
Due to public outcry over his acquittal, the M'Naghten rule was established by the House of Lords, which stated if a person charged with a crime was unable to distinguish right from wrong while in the act of committing that crime, he can plead the insanity defense and prove he was not sane at the time. The M'Naghten rule was the standard method of determining a person's sanity in the eyes of the law for more than 100 years after the M'Naghten case, until John Hinckley, Jr. attempted to assassinate now-former President Reagan to impress actress Jodie Foster -- we'll get in more detail about Hinckley later on.