In every case, the defendant is presumed to be sane until the contrary is proved.
To establish a defense of insanity, it must be proved that at the time of committing the act, the party accused was laboring under a defect of reason from a disease of the mind that he did not know the nature and quality of the act he was doing, or if he did know it, he did not know he was doing what was wrong.
The Court took this case as an opportunity to set out the test to determine if a defendant is legally insane and thus cannot be found guilty of the crime he is accused of committing. The Court ruled that the key question is whether Defendant knew he was acting contrary to the law. If the defendant, as in this case, believed he was acting to defend himself and he is found not know the nature of his actions, he cannot be found guilty. If however, he is acting because he thinks his target inflicted some injury on his character, this is not excusable under the law. This rule is called the M’Naghten rule and still survives in some U.S. jurisdictions today.