From the Wikipedia entry:
The M'Naghten Rule currently is the standard for legal insanity in nearly half the states.
- Persons acting under the influence of an insane delusion are punishable if they knew at the time of committing the crime that they were acting contrary to law.
- Every man is presumed sane and to have sufficient reason to be held responsible for his crimes.
- A person under a partial delusion is to be considered as if the facts with respect to which the delusion exists were real.
- To establish a defence on the ground of insanity each element of it must be clearly proved that:
- (a) at the time of committing the act,
- (b) the accused was labouring under such a defect of reason, from disease of the mind, (c) as not to know the nature and quality of the act he was doing
- (d) if he did know the nature and quality of the act, the accused was labouring under such a defect of reason, from disease of the mind (e) that he did not know he was doing what was wrong.
- IF (1) the accused was conscious that the act was one that he ought not to do AND (2) if the act was at the same time contrary to the law of the land,
- THEN the accused is punishable.