The Art of War by active 6th century B.C. Sunzi

6. Knowledge of the enemy’s dispositions can only be obtained from

other men. [Mei Yao-ch’en has rather an interesting note: "Knowledge of the spirit-world is to be obtained by divination; information in natural science may be sought by inductive reasoning; the laws of the universe can be verified by mathematical calculation: but the dispositions of an enemy are ascertainable through spies and spies alone."]