The M'Naughten Rule is a test for the legal sanity of the defendant in a criminal case. Under the M'Naughten rule, a defendant is legally insane if he/she cannot distinguish between right and wrong in regard to the crime with which he/she is charged. If it is found that the accused can't understand the wrongfulness of the act of which they are accused, there can be no criminal intent, which is necessary to establish guilt of a crime.
M'Naughten was a Scottish woodcutter who murdered the secretary to the prime minister, Sir Robert Peel, in a botched attempt to assassinate the prime minister himself. M'Naughten apparently believed that the prime minister was the architect of the myriad of personal and financial misfortunes that had befallen him. During his trial, nine witnesses testified to the fact that he was insane, and the jury acquitted him, finding him "not guilty by reason of insanity."