 In many cases , the insane defendant was released and allowed to go home ; in some , he was held in prison until the King decided to grant him a pardon . A lunatic who became insane prior to the trial could not be executed , nor , after 1542 , trialled for felonies up to and including high treason . It was then established that somebody found not guilty due to insanity should be immediately released ; up until the beginning of the 19th century , this was almost all that could be done , although the Vagrancy Act 1714 allowed two Justices of the Peace to confine a dangerous lunatic . The test of insanity was extremely narrow ; defendants had to prove that they were incapable of distinguishing between good and evil , and , following the trial of John Firth in 1790 , that they suffered from a mental disease which made them incapable of " forming a judgment upon the consequences of [ their ] actions " . 
