Sun Tzu closes the entire book with what he considers the foundation on which everything else rests: intelligence. Not tactics. Not terrain. Not morale. Knowledge.
He opens with a stark calculation. Raising and marching a hundred thousand men costs enormous sums and exhausts the state for years. All of that preparation comes down to a single day of battle. To then remain ignorant of the enemy's condition , because you refuse to pay for intelligence , is, in Sun Tzu's words, 'the height of inhumanity.' The lives lost from acting in ignorance cost infinitely more than the knowledge that would have prevented it.
Sun Tzu identifies five types of spies, each serving a distinct purpose:
1. Local spies , enemy subjects recruited from the local population. They know the ground, the people, and the mood inside enemy territory.
2. Inward spies , enemy officials recruited to share information from within the enemy's own court or command. The most dangerous and valuable source.
3. Converted spies , enemy agents who have been captured or turned. They now work for you, and because they know how the enemy's intelligence system operates, they are invaluable.
4. Doomed spies , your own agents deliberately fed false information, who then pass it to the enemy. They are 'doomed' because when the deception is discovered, they will be killed. Used to mislead the enemy at critical moments.
5. Surviving spies , agents who penetrate enemy territory, gather real intelligence, and return to report.
Sun Tzu gives the converted spy special weight. All five types depend on the converted spy to function , because it is the converted spy who tells you who can be recruited locally, which officials can be bribed, what false information will be believed, and which of your own agents have been compromised.
The chapter closes with the principle that runs under the entire book: 'foreknowledge.' It cannot be obtained from spirits. It cannot be inferred from experience alone. It cannot be calculated from abstract theory. It can only come from people , from human sources who actually know the enemy's situation.
'What enables the wise sovereign and the good general to strike and conquer, achieving things beyond the reach of ordinary men, is foreknowledge.'
This is Sun Tzu's final word: all the plans, all the terrain analysis, all the psychology of war , none of it matters without intelligence. Know before you act. Invest in knowing. The cost of ignorance is always higher than the cost of knowledge.
