As I recall the long line of cases where these men have rendered service of great value, almost every one resolves itself into a successful piece of mere spying or trailing.
Little ingenuity or powers of reason were required.

Of course, there are a thousand tricks that an experienced man acquires as a matter of course, but which at first sight seem almost like inspiration.

I shall not forget my delight when Jesse Blocher, who had been trailing Charles Foster Dodge through the South (when the latter was wanted as the chief witness against Abe Hummel on the charge of subornation of perjury of which he was finally convicted), told me how he instantly located his man, without disclosing his own identity, by unostentatiously leaving a note addressed to Dodge in a bright-red envelope upon the office counter of the Hotel St.Charles in New Orleans, where he knew his quarry to be staying.

A few moments later the clerk saw it, picked it up, and, as a matter of course, thrust it promptly into box No.

420, thus involuntarily hanging, as it were, a red lantern on Dodge's door.
There is no more reason to look for superiority of intelligence or mental alertness among detectives of the ordinary class than there is to expect it from clerks, stationary engineers, plumbers, or firemen.