This is the _nuance_ of Tu Yu’s paraphrase 不能得間人之用.
without a certain intuitive sagacity.
Mei Yao-ch‘ên says: 知其情僞辨其邪正則能用 “In order to use them, one
must know fact from falsehood, and be able to discriminate between
honesty and double-dealing.” Wang Hsi takes 聖 and 智 separately,
defining the former as 通而先識 “intuitive perception” and the
latter as 明於事 “practical intelligence.” Tu Mu strangely refers
these attributes to the spies themselves: 先量間者之性誠實多智然後可用之
“Before using spies we must assure ourselves as to their integrity
of character and the extent of their experience and skill.” But he
continues: 厚貌深情險於山川非聖人莫能知 “A brazen face and a crafty
disposition are more dangerous than mountains or rivers; it takes
a man of genius to penetrate such.” So that we are left in some
doubt as to his real opinion on the passage.