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.