He had once said that there was a certain affinity between solving chess
problems and the detection of crime mystery: once the key-move was found,
the rest was comparatively easy. But he added with a sigh that a really
perfect crime mystery was as rare as a perfect chess problem: human
ingenuity was not sufficiently skilful, as a rule, to commit a crime or
construct a chess problem with completely artistic concealment of the
key-move, and for that reason most problems and crimes were far too easy
of detection to absorb one's intellectual interests and attention.

It was the morning after Crewe's visit to Riversbrook, and the detective
sat in his private office glancing through a note-book which contained a
summary of the Hampstead mystery. Crewe was a painstaking detective as
well as a brilliant one, and it was his custom to prepare several
critical summaries of any important case on which he was engaged, writing
and rewriting the facts and his comments, until he was satisfied that he
had a perfect outline to work upon, with the details and clues of the
crime in consecutive order and relation to one another. Experience had
taught him that the time and labour this task involved were well-spent.
If an unexpected development of the case altered the facts of the
original summary Crewe prepared another one in the same painstaking way.
The summaries, when done with, were methodically filed and indexed and
stored in a strong room at the office for future reference, where he also
kept full records of all the cases upon which he had been engaged,
together with the weapons and articles that had figured in them: huge
volumes of newspaper reports and clippings; photographs of criminals with
their careers appended; and a host of other odds and ends of his
detective investigations--the whole forming an interesting museum of
crime and mystery which would have furnished a store of rich material
for a fresh Newgate Calendar. It was an axiom of Crewe's that a detective
never knew when some old scrap of information or some trifling article of