Before turning to those moral and mental aspects of the matter which present the greatest difficulties, let the inquirer begin by mastering more elementary problems. Let him, on meeting a fellow-mortal, learn at a glance to distinguish the history of man, and the trade or profession to which he belongs. Puerile as such an exercise may seem, it sharpens the faculties of observation, and teaches one where to look and what to look for. You say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or my work.
All knowledge comes useful to the detective.
Breadth of view is one of the essentials of our profession. The interplay of ideas and the oblique uses of knowledge are often of extraordinary interest. In my profession all sorts of odd knowledge comes useful, and this room of yours is a storehouse of it.