Without subtle ingenuity of mind, one cannot make certain of the truth of their reports.
Mei Yao-ch‘ên says: “Be on your guard against the possibility of spies going over to the service of the enemy.” The T‘ung Tien and Yü Lan read 密 for 妙.
18. 微哉微哉無所不用間也
Be subtle! be subtle!
Cf. [VI. § 9]: 微乎微乎. Capt. Calthrop translates: “Wonderful indeed is the power of spies.”
and use your spies for every kind of business.
19. 間事未發而先聞者間與所告者皆死
If a secret piece of news is divulged by a spy before the time is ripe, he must be put to death together with the man to whom the secret was told.
The Chinese here is so concise and elliptical that some expansion is necessary for the proper understanding of it. 間事 denotes important information about the enemy obtained from a surviving spy. The subject of 未發, however, is not this information itself, but the secret stratagem built up on the strength of it. 聞者 means “is heard”—by anybody else. Thus, word for word, we get: “If spy matters are heard before [our plans] are carried out,” etc. Capt. Calthrop, in translating 間與所告者 “the spy who told the matter, and the man who repeated the same,” may appeal to the authority of the commentators; but he surely misses the main point of Sun Tzŭ’s injunction. For, whereas you kill the spy himself 惡其泄 “as a punishment for letting out the secret,” the object of killing the other man is only, as Ch‘ên Hao puts it, 以滅口 “to stop his mouth” and prevent the news leaking any further. If it had already been repeated to others, this object would not be gained. Either way, Sun Tzŭ lays himself open to the charge of inhumanity, though Tu Mu tries to defend him by saying that the man deserves to be put to death, for the spy would certainly not have told the secret unless the other had been at pains to worm it out of him. The T‘ung Tien and Yü Lan have the reading ... 先聞其間者與, etc., which, while not affecting the sense, strikes me as being better than that of the standard text. The T‘u Shu has ... 聞與所告者, which I suppose would mean: “the man who heard the secret and the man who told it to him.”
20. 凡軍之所欲擊城之所欲攻人之所欲殺必先知其守將左右謁者門者舍人之姓名令吾間必索知之