M'Naghten Rules - The M'Naghten Rules

The M'Naghten Rules

The House of Lords, having deliberated, delivered the following exposition of the Rules:

the jurors ought to be told in all cases that every man is to be presumed to be sane, and to possess a sufficient degree of reason to be responsible for his crimes, until the contrary be proved to their satisfaction; and that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party 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.

The central issue of this definition may be stated as "did the defendant know what he was doing, or, if so, that it was wrong?", and the issues raised have been analysed in subsequent appellate decisions: