 The defence comes in two forms ; where the defendant claims he was insane at the time of the crime , and where the defendant asserts he is insane at the time of trial . In the first situation , the defendant must show that he was either suffering from a disease which damaged the functioning of the mind and led to a defect of reason that prevented him from understanding what he was doing , or that he could not tell that what he was doing was wrong . In the second situation , the test is whether or not the defendant can differentiate between " guilty " and " not guilty " verdicts , instruct counsel and recognise the charges he is facing . If successful , he is likely to be detained under the Criminal Procedure ( Insanity ) Act 1964 , although judges have a wide discretion as to what to do . 
