McNaghten Rule

Definition from Nolo’s Plain-English Law Dictionary

The earliest and most common test for criminal insanity, in which a defendant in a criminal trial is judged legally insane only if the defendant, at the time of the crime: 1) did not know what he or she was physically doing, or 2) did not know that what he or she was doing was wrong. For example, a delusional psychotic who believed that his assaultive acts were in response to the will of God would not be criminally responsible for his acts.

Definition provided by Nolo’s Plain-English Law Dictionary.