In the final chapter to The Art of War, Sun Tzu writes, "There are five classes of spies-Local spies, Internal spies, Turned spies, Expendable spies, and Surviving spies. When these five kinds of spy are all at work, none can discover their secret system. This system is called The Divine Threads, and it is the sovereign's most precious faculty."

Sun Tzu chose this metaphor to convey how a variety of spies, who deceptively appear to be unrelated, are in fact connected to the same general or ruler in a manner not easily discernible to the unsuspecting enemy. Centuries later, this and other teachings of the Chinese Strategist would serve as the basis for Ninjutsu-the Way of the Ninja-and the deadliest art to have evolved in Feudal Japan.