 " Disease of the mind " is not a medical term ; it instead means that the defendant must show he was suffering from a disease which affected the functioning of the mind , which does not necessarily have to be a disease of the brain . This was confirmed in the case of R v Kemp [ 1957 ] 1 QB 399 , where the defendant 's arteriosclerosis led to him assaulting his wife while unconscious . It must then be shown that this disease of the mind led to a " defect of reason " ; that the defendant 's ability to reason was impaired by the disease . Alternately , the defendant can try to show that he did not know " the nature or quality of his act or that it was wrong " . The first requires proof that the defendant did not know what he was doing ; that he had no awareness of what he was happening , that he was unaware of the consequences of his act or that he knew what he was doing , but was deluded as to the circumstances ; for the latter , Jonathan Herring gives the example of a man who " thought he was killing a monstrous person when he was in fact killing a person " . When arguing that the defendant was " not knowing the act was wrong " , " wrong " is taken to mean " illegal " , as set out in R v Windle [ 1952 ] 2 QB 826 . 
