Tu Mu’s note makes the meaning clear: 象, he says, is the same as 類 reasoning by analogy; 不可以他事比類而求 “[knowledge of the enemy] cannot be gained by reasoning from other analogous cases.”
nor by any deductive calculation.
Li Ch‘üan says: 夫長短闊狹遠近小大卽可驗之於度數人之情僞度不能知也 “Quantities like length, breadth, distance and magnitude, are susceptible of exact mathematical determination; human actions cannot be so calculated.”
6. 必取於人知敵之情者也
Knowledge of the enemy’s dispositions can only be obtained from other men.
Mei Yao-ch‘ên has rather an interesting note: 鬼神之情可以筮卜知形氣之物可以象類求天地之理可以度數驗唯敵之情必由間者而後知也 “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. 故用間有五有鄉間有內間有反間有死間有生間
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.
8. 五間俱起莫知其道是謂神紀人君之寳也
When these five kinds of spy are all at work, none can discover the secret system.