The M'Naghten rule reads as follows:
A defendant may be excused from criminal responsibility if at the time of the commission of the act the party accused was laboring under such a defect of reason, from a disease of the mind, as not to know the nature and the quality of the act he was doing, or if he did know it, that he did not know that he was doing what was wrong.
It is the law still used in many states in America today to determine criminal insanity: did the defendant know the difference between right and wrong when they committed their illegal act?
Daniel M’Naghten had had just about enough.
The British government had been harassing him for too long. They had seized some of his land, turned his friends against him, and were planning to arrest him. Something had to be done.
M'Naghten, a poor Scottish lumberyard worker, traveled to London in early 1843 and arrived near the offices of the Prime Minister at the time, Robert Peel. He entered the office and promptly accosted Peel's secretary, Edward Drummond. When Drummond insisted M'Naghten leave the premises, M'Naghten shot Drummond. Later he would claim he thought he was shooting Peel himself, thereby ending his troubles with the government. There was only one problem with this defense.
The British government had no idea who M'Naghten was.
There was no plot: no seizure of land, no imminent arrest, no conspiracy whatsoever. M'Naghten had shot an innocent man for no reason at all. Cold blooded murder indeed.
At M'Naghten's trial, his attorney begged for clemency: M'Naghten was clearly insane. He publicly accused the entire courtroom of turning against him, and had to be restrained (violently at times) throughout the proceedings. Eventually, the judge agreed that M'Naghten simply did not deserve to be outright punished for his crime - he instead sentenced him to an indefinite institutionalization at the mental hospital in Bethlehem. Unfortunately for M'Naghten, the word "bedlam" comes from this hospital - the conditions there were so terrible, it might have proved healthier to simply sentence him to death and to be done with it. But that is another node altogether.
After M'Naghten's trial, the British House of Lords was quite upset with the verdict: they wrote off to the society of mental health professionals demanding a more stringent and objective set of rules be designed to determine insanity in a criminal case. When the physicians replied, they formed the basis for the M'Naghten Rule.
The M'Naghten rule, in effect, made the case for insanity very hard to prove, but very easy to escape punishment for those it applied to. The defendant only had to appreciate the moral consequences of their behavior - that is, to admit that murder or robbery or rape caused harm to someone else. Whether or not they suffered from a real mental illness was of little concern.
By 1851, the rule had spread throughout the world, and was adopted by both the United States federal court system and virtually all of the state courts. This rule also brought about another change, now common in most courts: the testimony of psychologists and psychiatrists became a very relevant issue in cases involving supposed mental illness.
The rule remained the primary litmus test for criminal insanity in both Great Britain and America until around 1904, when it was supplemented by the Irresistible Impulse corollary. Further knowledge about the basis of mental illness and its manifestations in society led to subsequent rewriting of the rules governing criminal insanity, although the M'Naghten rule is still used in 16 states with minor clarifications and minutia.