But their work does not involve any particular superiority or quickness of intellect--the quality which we are wont to associate with the detection of crime.
Now, if the ordinary householder finds that his wife's necklace has mysteriously disappeared, his first impulse is to send for a detective of some sort or other.

In general, he might just as well send for his mother-in-law.

Of course, the police can and will watch the pawnshops for the missing baubles, but no crook who is not a fool is going to pawn a whole necklace on the Bowery the very next day after it has been "lifted." Or he can enlist a private detective who will question the servants and perhaps go through their trunks, if they will let him.
Either sort will probably line up the inmates of the house for general scrutiny and try to bully them separately into a confession.

This may save the master a disagreeable experience, but it is the simplest sort of police work and is done vicariously for the taxpayer, just as the public garbage man relieves you from the burden of taking out the ashes yourself, because he is paid for it, not on account of your own incapacity or his superiority.
The real detective is the one who, taking up the solution of a crime or other mystery, brings to bear upon it unusual powers of observation and deduction and an exceptional resourcefulness in acting upon his conclusions.

Frankly, I have known very few such, although for some ten years I have made use of a large number of so-called detectives in both public and private matters.