P was mental and stabbed someone while being mental, after D, the health authority, failed to realise how dangerous he was and failed to treat him. He had been convicted of manslaughter by diminished responsibility and sued D for damages for his consequential imprisonment etc. CA struck out his claim, saying that although the rule of policy that the court should not compensate a person relying on his own criminal activity was restricted to people who were presumed to have understood their activities to be unlawful, a case of diminished responsibility does not rebut this presumption.
Bedlam LJ: P could rebut the presumption if “he did not know the nature and quality of his act or that what he was doing was wrong,” i.e. insanity