If the enemy leaves a door open, you must rush in.
This looks a very simple sentence, yet Ts‘ao Kung is the only
commentator who takes it as I have done. Mêng Shih, followed by
Mei Yao-ch‘ên and Chang Yü, defines 開闔 as 間者 “spies,” and
makes 入 an active verb: “If spies come from the enemy, we must
quickly let them in.” But I cannot find that the words 開闔 have
this meaning anywhere else. On the other hand, they may be taken
as two verbs, 或開或闔, expressing the enemy’s indecision whether
to advance or retreat, that being the best moment to attack him.
[Cf. _Tao Tê Ching_, chap. X: 天門開闔能爲雌乎; also _Li Chi_, 曲禮,
I. ii. 25.] It is not easy to choose between this and Ts‘ao Kung’s
explanation; the fact that 敵人開戶 occurs shortly afterwards, in
§ 68, might be adduced in support of either. 必 must be understood
in the sense of 宜 or 當. The only way to avoid this is to put 開闔
between commas and translate: “If we leave a door open, the enemy
is sure to rush in.”