"The work of the philosophical policemen," 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. We have to trace the origin of those dreadful thoughts that drive men on at last to intellectual fanaticism and intellectual crime. We were only just in time to prevent the assassination at Hartlepool, and that was entirely due to the fact that our Mr Wilks (a smart young fellow) thoroughly understood a triolet."
G. K. Chesterton - The Man who was Thursday