The local spy, the inside spy, the reverse spy, the dead spy, and the living spy. When the five kinds of spies are all active, no one knows their routes - this is called organizational genius, and is valuable to the leadership. Local spies are hired from among the people of a locality. Inside spies are hired from among enemy officials. Reverse spies are hired from among enemy spies.
Dead spies transmit false intelligence to enemy spies.
Living spies come back to report. Therefore no one in the armed forces is treated as familiarly as are spies, no one is given rewards as rich as those given to spies, and no matter is more secret than espionage. One cannot use spies without sagacity and knowledge, one cannot use spies without humanity and justice, one cannot get the truth from spies without subtlety. This is a very delicate matter indeed. Spies are useful everywhere.