The Art of War closes with a chapter on the use of spies. “What enables the wise ruler and the good general to move and conquer, and to achieve more than the many, is foreknowledge. Foreknowledge cannot be drawn from spirits, cannot be inferred from past events and cannot be tested by calculation. It must come from people who know the enemy’s situation.”
The chapter counts the cost of a war in thousands of gold a day. A general who grudges “ranks, salaries and a hundred pieces of gold” and so does not know the enemy “is the height of inhumanity.” “In the affairs of the army, none is closer than the spy, no reward is richer than the spy’s, and no matter is more secret than spying.”