Chapter 13, Verses 7 and 8 - Sun Tzu

Hence the use of spies, of whom there are five classes: (1) Local spies; (2) inward spies; (3) converted spies; (4) doomed spies; (5) surviving spies. When these five kinds of spy are all at work, none can discover the secret system. This is called "divine manipulation of the threads." It is the sovereign's most precious faculty.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.