For some reason not wholly obscure the American people generally have been kept in such ignorance of the facts of this commerce that few even dream that it exists. And I am fully conscious of the need for proof in support of what to many must appear to be unwarranted assertions. Indeed, it is rare to find anyone who suspects the character of the private detective. The general impression seems to be that he performs a very useful and necessary service, that the profession is an honorable one, and that the mass of detectives have only one ambition in life, and that is to ferret out the criminal and to bring him to justice. To denounce detectives as a class appears to most persons as absurdly unreasonable. To speak of them with contempt is to convey the impression that detectives stand in the way of some evil schemes of their detractor. Fiction of a peculiarly American sort has built up among the people an exalted conception of the sleuth. And it must appear with rather a shock to those persons who have thus idealized the detective to learn that thousands of men who have been in the penitentiaries are constantly in the employ of the detective agencies. In a society which makes it almost impossible for an ex-convict to earn an honorable living it is no wonder that many of them grasp eagerly at positions offered them as "strike-breakers" and as "special officers." The first and most important thing, then, in this chapter is to prove, with perhaps undue detail, the ancient saying that "you must be a thief to catch a thief," and that possibly for that proverbial reason many private detectives are schooled and practiced in crime.