This short book explains the methods used by two of the greatest detectives in history to solve mysteries: Auguste Dupin, Edgar Allen Poe's detective who solved the problems of the 'Murders in the Rue Morgue' and 'The Purloined Letter', and Sherlock Holmes, who solved many cases including 'The Speckled Band' and 'The Hound of the Baskervilles'. Alan Macfarlane, a retired Professor of Anthropology, has found the method very useful in approaching the largest questions in anthropology and history, an equivalent in the arts and social sciences to the methodological rules laid down by the French philosopher and mathematician Descartes.