 The idea of insanity in English law dates from 1324 , when the Statute de Praerogativa Regis allowed the King to take the lands of idiots and lunatics . The early law used various words , including " idiot " , " fool " and " sot " to refer to those who had been insane since birth , and " lunatic " for those who had later become insane , or were insane with some lucid intervals . In the criminal law , insanity was used as a defence in a roughly identical way from this point until the late 16th century ; if an insane person commits a crime , he was not punished in the same way that a sane felon who committed the same crime would be . This was for several reasons ; firstly , the cruel punishment usually meted out to felons to set an example would not have the same effect on the insane . Secondly , as felonies required a mens rea , an insane person could not be guilty because they did not have the capacity to hold a mens rea . Thirdly , the phrase furiosus solo fitrere punitur was used ; " a lunatic was punished by his madness alone " . 
