The law is, simply, that any person is to be held criminally responsible for a deed unless he was at the time laboring under such a defect of reason as not to know the nature and quality of his act and that it was wrong.

This doctrine first took concrete form in 1843, when, after a person named McNaughten, who had shot and killed a certain Mr. Drummond under an insane delusion that the latter was Sir Robert Peel, had been acquitted, there was such popular uneasiness over the question of what constituted criminal responsibility that the House of Lords submitted four questions to the fifteen judges of England asking for an opinion on the law governing responsibility for offences committed by persons afflicted with certain forms of insanity.  It is unnecessary to set forth at length these questions, but it is enough to say that the judges formulated the foregoing rule as containing the issue which should be submitted to the jury in such cases.* ___________________
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* The questions propounded to the judges and their answers are here given: 

Question 1.—­“What is the law respecting alleged crimes committed by persons afflicted with insane delusion in respect of one or more particular subjects or persons, as, for instance, where, at the time of the commission of the alleged crime, the accused knew he was acting contrary to law, but did the act complained of with a view, under the influence of insane delusion, of redressing or revenging some supposed grievance or injury, or of producing some supposed public benefit?

Answer 1.-"Assuming that your lordships’ inquiries are confined to those persons who labor under such partial delusions only, and are not in other respects insane, we are of opinion that, notwithstanding the accused did the act complained of with a view, under the influence of insane delusion, of redressing or revenging some supposed grievance or injury, or of producing some public benefit, he is, nevertheless, punishable, according to the nature of the crime committed, if he knew at the time of committing such crime that he was acting contrary to law, by which expression we understand your lordships to mean the law of the land.

Question 4:—­“If a person under an insane delusion as to existing facts commits an offence in consequence thereof, is he thereby excused?

Answer 4.—­“The answer must of course depend on the nature of the delusion; but, making the same assumption as we did before, namely, that he labors under such partial delusion only, and is not in other respects insane, we think he must be considered in the same situation as to responsibility as if the facts with respect to which the delusions exist were real.  For example, if under the influence of his delusion he supposes another man to be in the act of attempting to take away his life, and kills the man, as he supposes in self-defence, he would be exempt from punishment.  If his delusion was that the deceased had inflicted a serious injury to his character and fortune, and he killed him in revenge for such supposed injury, be would be liable to punishment.