sagacity.
[Mei Yao-ch’en says: "In order to use them, one must know fact from
falsehood, and be able to discriminate between honesty and
double-dealing." Wang Hsi in a different interpretation thinks more
along the lines of "intuitive perception" and "practical intelligence."
Tu Mu strangely refers these attributes to the spies themselves:
"Before using spies we must assure ourselves as to their integrity of
character and the extent of their experience and skill." But he
continues: "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."]