The most difficult crime to track is the one which is purposeless.
The larger crimes are apt to be the simpler, for the bigger the crime, the more obvious, as a rule, is the motive.
Ex-Professor Moriarty of mathematical celebrity... is the Napoleon of crime, Watson.
The stage lost a fine actor, even as science lost an acute reasoner, when [Holmes] became a specialist in crime.
The more featureless and commonplace a crime is, the more difficult it is to bring it home.
Crime is common. Logic is rare. Therefore it is upon the logic rather than upon the crime that you should dwell.