Art of War 13.25

I have ventured to differ in this place from those commentators — Tu Yu and Chang Yu — 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.

As explained in §§22—24. He not only brings information himself,

but makes it possible to use the other kinds of spy to advantage.