In Sun Wu's "The Art of War", there is a special chapter that tells the matter of espionage in detail, and there are many ways. The importance, content, characteristics, methods, and other aspects of espionage have been mentioned. Especially for the use of spies, he proposed five methods, which is very systematic.
The first is "Inma" (also known as the countryside), which is to use enemy villagers as spies; the second is the "inner room", that is, to buy the enemy's officials to be spies; The third is "anti-intermediary", that is, buying or using enemy spies to serve oneself and oneself; Fourth, "death", that is, deliberately spreading false information to let one's own spies know, and then passing it on to the enemy, so that the enemy is deceived, and one's own spies are often executed by them; Fifth, "Ikuma" refers to a person who can come back alive to report the enemy's situation after a spy is sent to the enemy to reconnoitre. Regarding these "five rooms", Sun Wu also said: If the five kinds of spies can be used at the same time, they can make the enemy unable to touch the law and be unable to deal with it, which is an unfathomable way and a magic weapon.
