It is their business to watch the beginnings of this conspiracy, not merely in a criminal but in a controversial sense.

I am a democrat myself, and I am fully aware of the value of the ordinary man in matters of ordinary valour or virtue.

But it would obviously be undesirable to employ the common policeman in an investigation which is also a heresy hunt." Syme's eyes were bright with a sympathetic curiosity.
"What do you do, then ?" he said.
"The work of the philosophical policeman," replied the man in blue, "is at once bolder and more subtle than that of the ordinary detective.
The ordinary detective goes to pot-houses to arrest thieves; we go to artistic tea-parties to detect pessimists.

The ordinary detective discovers from a ledger or a diary that a crime has been committed.

We discover from a book of sonnets that a crime will be committed.