More wisdom from Sun Tzu.

13. The Use Of Spies

  1. Now this foreknowledge cannot be elicited from spirits; it cannot be obtained inductively from experience, nor by any deductive calculation.
  2. Knowledge of the enemy’s dispositions can only be obtained from other men.
  3. Hence the use of spies, of whom there are five classes: (1) Local spies; (2) inward spies; (3) converted spies; (4) doomed spies; (5) surviving spies.