Here, then, is the field of the secret agent or private detective, and here, forsooth, is where the detective of genuine deductive powers and the polished address of the so-called "man of the world" is required.
There are two classes of cases where a private detective must needs be used, if indeed any professional assistance is to be called in: first, where the person whose identity is sought to be discovered or whose activities are sought to be terminated is not a criminal or has committed no crime, and second, where, though a crime has been committed, the injured parties cannot afford to undertake a public prosecution.
For example, if you are receiving anonymous letters, the writer of which accuses you of all sorts of unpleasant things, you would, of course, much prefer to find out who it is and stop him quietly than to turn over the correspondence to the police and let the writer's attorneys publicly cross-examine you at his trial as to your past career.

Even if a diamond necklace is stolen from a family living on Fifth Avenue, there is more than an even chance that the owner will prefer to conceal her loss rather than to have her picture in the morning paper.

Yet she will wish to find the necklace if she can.
When the matter has no criminal side at all, the police cannot be availed of, although we sometimes read that the officers of the local precinct have spent many hours in trying to locate Mrs.So-and-So's lost Pomeranian, or in performing other functions of an essentially private nature--most generously.

But if, for example, your daughter is made the recipient, almost daily, of anonymous gifts of jewelry which arrive by mail, express, or messenger, and you are anxious to discover the identity of her admirer and return them, you will probably wish to engage outside assistance.
Where will you seek it?
You can do one of two things: go to a big agency and secure the services of the right man, or engage such a man outside who may or may not be a professional detective.

I have frequently utilized with success in peculiar and difficult cases the services of men whom I knew to be common-sense persons, with a natural taste for ferreting out mysteries, but who were not detectives at all.