there should be any hilly country,
險阻 is 邱阜之地, according to Chang Yü.
ponds surrounded by aquatic grass, hollow basins filled with reeds,
The original text omits 蔣 and 生, so that 潢 and 井 join to make a pair: “ponds and basins.” This is plausible enough at first sight, but there are several objections to the reading: (1) 蔣 is unlikely to have got into the text as a gloss on 潢; (2) it is easy to suppose, on the other hand, that 蔣 and afterwards 生 (to restore the balance of the sentence) were omitted by a copyist who jumped to the conclusion that 潢 and 井 must go together; (3) the sense, when one comes to consider it, actually requires 蔣, for it is absurd to talk of pools and ponds as in themselves suitable places for an ambush; (4) Li Ching (571—649 A. D.) in his 兵法 “Art of War” has the words: 蔣潢蘙薈則必索其伏. This is evidently a reminiscence of Sun Tzŭ, so there can be little doubt that 蔣 stood in the text at this early date. It may be added that the T‘ung Tien and Yü Lan both have 蔣, and the latter also reads 并 for 井.
or woods with thick undergrowth,
I read 小林 with the Yü Lan in preference to 山林, given in the original text, which is accepted by the commentators without question. The text of the T‘u Shu up to this point runs as follows: 潢井蒹葭林木蘙薈者.
they must be carefully routed out and searched; for these are places where men in ambush or insidious spies are likely to be lurking.
The original text omits 藏, which has been restored from the T‘ung Tien and Yü Lan. The T‘u Shu omits 處 as well, making 所 a substantive. On 姦 Chang Yü has the note: 又慮姦細潛隱覘我虛實聽我號令伏姦當爲兩事 “We must also be on our guard against traitors who may lie in close covert, secretly spying out our weaknesses and overhearing our instructions. Fu and chien are to be taken separately.”
18. When the enemy is close at hand and remains quiet, he is relying on the natural strength of his position.