A schizophrenic man kills believing the victim is a demon — McNaughten may exempt. McNaughten rule: at the offence, defect of reason from unsoundness — not knowing the nature/quality of the act, or not knowing it was wrong. Psychiatric diagnosis (schizophrenia) informs but doesn't decide — the legal test is cognitive at the moment. Lucid intervals count as sane. The psychiatrist advises, the court decides — medicine describes, law judges.