The substantive rules governing the insanity or mental disorder defence are derived from the 1843 decision of the House of Lords in M’Naghten’s Case.31In that case, the accused was found by a jury to be not guilty by reason of insanity of murdering the prime minister’s secretary. He suffered from delusions of persecution from the government. Subsequent controversy led to a reference to the House of Lords, which affirmed the availability of the insanity defence if it was "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 did know it, that he did not know he was doing what was wrong."32The House of Lords added that an accused suffering from delusions should have his responsibility determined on the basis that the delusions were real. Thus, an accused who killed another, supposing himself under deadly attack, may not be responsible, but one imagining only an injury to his character would be guilty of murder.33
In 1892 the M’Naghten rules were embodied in the Criminal Code of Canada with some variations. A person was defined as legally insane if he or she laboured "in a state of natural imbecility or disease of the mind, to such an extent as to render him incapable of appreciating the nature and quality of an act or omission, and of knowing that such an act or omission was wrong." This introduced the idea that an accused would have an insanity defence if he or she did not "appreciate," as opposed to "know," the nature and quality of the act. The 1892 Code also required the accused to be incapable of both appreciating the nature and quality of the act and of knowing that it was wrong. This was corrected as a misreading of the disjunctive M’Naghten rules.34Thus, either a failure to appreciate the nature and quality of the criminal act or to know that it was wrong will suffice to ground the mental disorder defence.
In 1992 the insanity defence was renamed the mental disorder de-fence, and the verdict of not guilty by reason of insanity was renamed the verdict of not criminally responsible by reason of mental disorder. The mental disorder defence now provides:
No person is criminally responsible for an act committed or an omission made while suffering from a mental disorder that rendered the person incapable of appreciating the nature and quality of the act...