 "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.
