 On 15 May 1800 , James Hadfield attempted to assassinate George III ; he had come to believe that the second coming of Christ would be brought about by his own death , and therefore attempted to be judicially executed . Hadfield approached the King in the royal box at the Theatre Royal , Drury Lane , firing a pistol at him ; however , the King was bowing to the audience at the time , and the shot passed over his head . He was tried on 26 June 1800 at the Court of King 's Bench , and his counsel , Thomas Erskine , argued that although Hadfield 's planning of the attack meant that the normal defence of insanity would not have been sufficient , the true test of insanity is delusions and " frenzy or raving madness " , which Hadfield suffered from . Several medical experts testified that Hadfield 's injuries at the Battle of Tourcoing , where he was repeatedly struck in the head by a sabre , had caused insanity , and Lord Kenyon immediately sent the jury away to reach a decision . Their verdict was " not guilty ; he being under the influence of insanity at the time the act was committed " , the first time a jury had been asked to give a reason for their decision and the origins of the phrase " not guilty by reason of insanity " . 
