M'Naughten Rules
Daniel M'Naghten attempted to shoot the prime minister of England but instead shot tis private secretary. He was acquitted of murder because of his mental condition. There was general disquiet about this result, and in 1843 the judges of the House of Lords formulated a set of rules that would apply in such situations in the future. Based on the case, they noted that the accused had suffered a 'disease of the mind' which rendered him incapable of understanding the nature and quality of the unlawful act or of appreciating the difference between right and wrong. In such a case, he, or others similarly afflicted, shoud not be convicted of the offence. This still tends to remain the view of the law. It does mean that the legal definition is different from the modern medical and psychiatric opinion as mental illness. I hope this helps. This case established a set of rules which became the basis of the insanity plea as a result as it is believed that someone in such a condition cannot have a clear judgement of their actions and thus, the insanity plea became a complete defence in the eyes of the law.