- 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.
- To establish a defense on the ground of insanity it must be clearly proved that, at the time of committing the act, the accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know he was doing what was wrong. If the accused was conscious that the act was one that he ought not to do, and if the act was at the same time contrary to the law of the land, he is punishable.
- A person under a partial delusion is to be considered as if the facts with respect to which the delusion exists were real.
The rules have been treated in England as if of statutory force, and were followed in many Commonwealth countries and parts of the US. They were criticized for excusing from criminal responsibility only those whose insanity resulted in lack of knowledge. The fact that an individual might know what they were doing and that it was wrong and yet because of their abnormal mental state might lack the capacity to control the action was not considered. It was argued the lack of capacity must be the key issue over responsibility. The English law was amended in 1957 to include irresistible impulse.
The rules were based on the 1843 trial Rex v. McNaghten. Daniel McNaghten was a Scottish woodworker who killed the Prime Minister's Secretary Edward Drummond by mistake for the Prime Minister Robert Peel, under an insane belief that the Government was plotting against him. He was tried and acquitted on the ground of insanity and committed to Bethlehem Hospital and then Broadmoor. Nine experts all found him insane