The Use of Spies

The book ends not with battle but with intelligence, and Sun Tzu frames it as the logical conclusion of everything before. A great campaign costs the state and the people enormously — "a thousand ounces of silver a day" — and may drag on for years to decide victory in a single day. To grudge the comparatively trivial expense of paying spies, and so remain ignorant of the enemy's condition, is "the height of inhumanity," because it wastes those vast costs and those many lives. "What enables the wise sovereign and the good general to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge."

That foreknowledge, he insists, cannot be conjured. "Knowledge of the enemy's dispositions can only be obtained from other men." It cannot be elicited from spirits, nor obtained inductively from experience, nor arrived at by any deductive calculation. It must come from people who know the enemy's situation directly. This is one of the most clear-eyed statements of empiricism in the ancient world: real plans rest on real information about the actual adversary, not on theory or omen.

Sun Tzu enumerates five classes of spies, used together as "the divine manipulation of the threads." Local (native) spies are the enemy's own country-folk. Inward spies are the enemy's officials, turned to your service. Converted (double) spies are the enemy's spies, found out and won over. Doomed spies are your own agents, deliberately fed false information and allowed to be captured so they report the falsehood to the enemy. Surviving (living) spies are those who return with news. "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."

Among the five, the converted spy is supreme, because everything else depends on him: through him you recruit and direct the local and inward spies, through him the doomed spy can carry plausible falsehoods to the enemy, and through him your living spies can be used at the right season. "Hence it is essential that the converted spy be treated with the utmost liberality." Spies require subtlety, and only a ruler of great wisdom and benevolence can manage them; without intuitive sagacity one cannot be sure of the truth of their reports.

The chapter — and the book — close on the strategic primacy of intelligence: the rise of the Shang and Chou dynasties, Sun Tzu argues, came because their founders used men who knew the enemy's court intimately. "Spies are a most important element in war, because on them depends an army's ability to move." The enlightened ruler and the wise general use the highest intelligence for purposes of spying, "and thereby they achieve great results." The final lesson reframes the whole treatise: the deception, positioning, and timing of every earlier chapter are only as good as the knowledge they are built on — win the information war first, and the rest of strategy becomes executable.