The enemy’s spies who have come to spy on us must be sought out, 必索 is omitted by the _T‘ung Tien_ and _Yü Lan_. Its recurrence is certainly suspicious, though the sense may seem to gain by it. The _T‘u Shu_ has this variation: ... 敵間之來間吾者, etc. tempted with bribes, led away and comfortably housed. 舍 is probably more than merely 居止 or 稽留 “detain.” Cf. § 25 _ad fin._, where Sun Tzŭ insists that these converted spies shall be treated well. Chang Yü’s paraphrase is 館舍. Thus they will become converted spies and available for our service. 22. 因是而知之故鄉間內間可得而使也 It is through the information brought by the converted spy that we are able to acquire and employ local and inward spies. Tu Yu expands 因是而知之 into 因反敵間而知敵情 “through conversion of the enemy’s spies we learn the enemy’s condition.” And Chang Yü says: 因是反間知彼鄉人之貪利者官人之有隙者誘而使之 “We must tempt the converted spy into our service, because it is he that knows which of the local inhabitants are greedy of gain, and which of the officials are open to corruption.” In the _T‘ung Tien_, 鄉 has been altered to 因, doubtless for the sake of uniformity with § 9. 23. 因是而知之故死間爲誑事可使告敵 It is owing to his information, again, that we can cause the doomed spy to carry false tidings to the enemy. “Because the converted spy knows how the enemy can best be deceived” (Chang Yü). The _T‘ung Tien_ text, followed by the _Yü Lan_, has here the obviously interpolated sentence 因是可得而攻也. 24. 因是而知之故生間有使如期 Lastly, it is by his information that the surviving spy can be used on appointed occasions. Capt. Calthrop omits this sentence. 25. 五間之事主必知之知之必在於反間故反間不可不厚也 The end and aim of spying in all its five varieties is knowledge of the enemy; I have ventured to differ in this place from those commentators—Tu Yu and Chang Yü—who understand 主 as 人主, and make 五間之事 the antecedent of 之 (the others ignoring the point altogether). It is plausible enough that Sun Tzŭ should require the ruler to be familiar with the methods of spying (though one would rather expect 將 “general” in place of 主). But this involves taking 知之 here in quite a different way from the 知之 immediately following, as also from those in the previous sentences. 之 there refers vaguely to the enemy or the enemy’s condition, and in order to retain the same meaning here, I make 主 a verb, governed by 五間之事. Cf. XI. § 19, where 主 is used in exactly the same manner. The sole objection that I can see in the way of this interpretation is the fact that the 死間, or fourth variety of spy, does not add to our knowledge of the enemy, but only misinforms the enemy about us. This would be, however, but a trivial oversight on Sun Tzŭ’s part, inasmuch as the “doomed spy” is in the strictest sense not to be reckoned as a spy at all. Capt. Calthrop, it is hardly necessary to remark, slurs over the whole difficulty. and this knowledge can only be derived, in the first instance, from the converted spy.