[Defined as "natural pens or prisons" or "places surrounded by precipices on three sides—easy to get into, but hard to get out of."] tangled thickets, [Defined as "places covered with such dense undergrowth that spears cannot be used."] quagmires [Defined as "low-lying places, so heavy with mud as to be impassable for chariots and horsemen."] and crevasses, [Defined by Mei Yao-ch’en as "a narrow difficult way between beetling cliffs." Tu Mu’s note is "ground covered with trees and rocks, and intersected by numerous ravines and pitfalls." This is very vague, but Chia Lin explains it clearly enough as a defile or narrow pass, and Chang Yu takes much the same view. On the whole, the weight of the commentators certainly inclines to the rendering "defile." But the ordinary meaning of the Chinese in one place is "a crack or fissure" and the fact that the meaning of the Chinese elsewhere in the sentence indicates something in the nature of a defile, make me think that Sun Tzŭ is here speaking of crevasses.] should be left with all possible speed and not approached. 16. While we keep away from such places, we should get the enemy to approach them; while we face them, we should let the enemy have them on his rear. 17. If in the neighbourhood of your camp there should be any hilly country, ponds surrounded by aquatic grass, hollow basins filled with reeds, or woods with thick undergrowth, they must be carefully routed out and searched; for these are places where men in ambush or insidious spies are likely to be lurking. [Chang Yu has the note: "We must also be on our guard against traitors who may lie in close covert, secretly spying out our weaknesses and overhearing our instructions."] 18. When the enemy is close at hand and remains quiet, he is relying on the natural strength of his position. [Here begin Sun Tzŭ’s remarks on the reading of signs, much of which is so good that it could almost be included in a modern manual like Gen. Baden-Powell’s "Aids to Scouting."] 19. When he keeps aloof and tries to provoke a battle, he is anxious for the other side to advance. [Probably because we are in a strong position from which he wishes to dislodge us. "If he came close up to us, says Tu Mu, "and tried to force a battle, he would seem to despise us, and there would be less probability of our responding to the challenge."] 20. If his place of encampment is easy of access, he is tendering a bait. 21. Movement amongst the trees of a forest shows that the enemy is advancing. [Ts’ao Kung explains this as "felling trees to clear a passage," and Chang Yu says: "Every man sends out scouts to climb high places and observe the enemy. If a scout sees that the trees of a forest are moving and shaking, he may know that they are being cut down to clear a passage for the enemy’s march."] The appearance of a number of screens in the midst of thick grass means that the enemy wants to make us suspicious.