21. 必索敵人之間來間我者因而利之導而舍之故反間可得而用也
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