The devil’s agents may be of flesh and blood, may they not?

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.

There are no crimes and no criminals in these days. What is the use of having brains in our profession? I know well that I have it in me to make my name famous. No man lives or has ever lived who has brought the same amount of study and of natural talent to the detection of crime which I have done. And what is the result? There is no crime to detect, or, at most, some bungling villainy with a motive so transparent that even a Scotland Yard official can see through it.