The thirteenth and final chapter addresses intelligence — the acquisition of foreknowledge through human sources rather than divination, spirits, or analogy. Sūnzǐ declares that foreknowledge is the essence of warfare, the foundation upon which all strategic decisions rest. The chapter classifies five categories of espionage and argues that the intelligence service is the most valuable instrument available to the ruler.

The title 用间 (Yòngjiān) combines 用 (Yòng, to use) with 间 (Jiān, spy or agent — literally "one who operates in the space between").

Foreknowledge as Foundation

The chapter opens with a reiteration of cost: raising an army of 100,000 and campaigning for years drains the state and disrupts hundreds of thousands of lives. All this — and the outcome may hinge on information that could have been obtained for a fraction of the cost. The ruler who begrudges the expense of intelligence operations while spending freely on armies is "the height of inhumanity" (不仁之至也 Bùrén zhī zhì yě).

Sūnzǐ is explicit: foreknowledge cannot be obtained from spirits, from analogy with past events, or from astronomical calculations. It can only come from people — from those who know the enemy's condition directly. This insistence on empirical, human-sourced intelligence over divination and supernatural guidance aligns with the rationalist strand in Daoist thought. The Dàodéjīng does not rely on omens; it teaches direct perception of reality as it is.

Five Categories of Agents

Sūnzǐ classifies five types of intelligence operatives:

Local agents (因间 Yīnjiān) — inhabitants of the enemy's territory who provide information about local conditions.

Internal agents (内间 Nèijiān) — officials or servants within the enemy's government who have been recruited.

Converted agents (反间 Fǎnjiān) — enemy spies who have been identified, recruited, and turned to work for you. The most valuable category: through them, the other four types can be managed.

Expendable agents (死间 Sǐjiān) — agents deliberately fed false information, which they then carry to the enemy. These agents may be sacrificed when the deception is discovered.

Surviving agents (生间 Shēngjiān) — agents who penetrate the enemy, gather intelligence, and return to report.

Connection to Martial Practice

The chapter's emphasis on foreknowledge — knowing the enemy's plans, dispositions, and intentions before engagement — translates directly to the highest skill in Wudang combat: reading intention before it manifests as action.

Tīngjìn (听劲, listening energy) is the martial equivalent of espionage. Through physical contact in Tuīshǒu, the practitioner gathers intelligence about the opponent's structure, weight distribution, tension patterns, and incipient movement. This information — obtained through sensitivity rather than force — provides the foreknowledge that enables a precise, economical response.

The concept of the "converted agent" also has a combat parallel: in Wudang practice, the opponent's force is not destroyed but redirected — "turned" to serve the practitioner's purpose, just as the captured spy is turned to serve the captor's intelligence network. The principle 借力打力 (Jiè lì dǎ lì — borrow the opponent's force to strike) is espionage logic applied to mechanics.

The Chapter's Final Teaching

The Art of War closes by declaring that the entire army's movement depends on the intelligence service. Foreknowledge is the foundation of all strategy. Without it, the general fights blind — and the state pays the price. The text ends as it began: with the insistence that warfare is too consequential for improvisation. Everything depends on knowing.