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.
If an item of intelligence is heard before a spy reports it, then both the spy and the one who told about it die. Whenever you want to attack an army, besiege a city, or kill a person, first you must know the identities of their defending generals, their associates, their visitors, their gatekeepers, and their chamberlains, so you have your spies find out. You must seek out enemy agents who have come to spy on you, bribe them and induce them to stay with you, so you can use them as reverse spies. By intelligence thus obtained, you can find local spies and inside spies to employ. By intelligence thus obtained, you can cause the misinformation of dead spies to be conveyed to the enemy.