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5. Now this foreknowledge cannot be elicited from spirits; it

cannot be obtained inductively from experience,

[Tu Mu's note is: "[knowledge of the enemy] cannot be gained by

reasoning from other analogous cases."]

nor by any deductive calculation.

[Li Ch'uan says: "Quantities like length, breadth, distance and

magnitude, are susceptible of exact mathematical determination;

human actions cannot be so calculated."]

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."]

7. 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.