The most difficult crime to track is the one which is purposeless.

When a doctor does go wrong he is the first of criminals.

The larger crimes are apt to be the simpler, for the bigger the crime, the more obvious, as a rule, is the motive.

It is a mistake to confound strangeness with mystery. The most commonplace crime is often the most mysterious because it presents no new or special features from which deductions may be drawn. This murder would have been infinitely more difficult to unravel had the body of the victim been simply found lying in the roadway without any of those outré and sensational accompaniments which have rendered it remarkable. These strange details, far from making the case more difficult, have really had the effect of making it less so.

It is a capital mistake to theorise before one has data.

As a rule, said Holmes, the more bizarre a thing is the less mysterious it proves to be. It is your commonplace, featureless crimes which are really puzzling, just as a commonplace face is the most difficult to identify.