 On 20 January 1843 , Daniel M 'Naghten attempted to assassinate Robert Peel , Prime Minister of the United Kingdom . Approaching a man he believed to be Peel , M 'Naghten fired into his back , in fact killing Edward Drummond , Peel 's secretary . Immediately arrested , he was charged with murder and tried on 3 March 1843 at the Old Bailey . He was assisted in his defence by two solicitors , four barristers including Alexander Cockburn and nine medical experts , along with eight lay witnesses . Both sides agreed that M 'Naghten was insane ; the question was what constituted a valid legal defence of insanity . The judges decided that " every man is 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 what he was doing was wrong " , which was boiled down to " did the defendant know what he was doing , and if so , that what he was doing was wrong ? " . This established the M 'Naghten Rules , which remain the principal method of deciding insanity in English law . 
