29. 夫兵形象水水之行避高而趨下
Military tactics are like unto water; for water in its natural course runs away from high places and hastens downwards.
行 is 劉晝子 Liu Chou-tzŭ’s reading for 形 in the original text.
30. 兵之形避實而擊虛
So in war, the way is to avoid what is strong and to strike at what is weak.
Like water, taking the line of least resistance.
31. 水因地而制流兵因敵而制勝
Water shapes its course according to the nature of the ground over which it flows;
The T‘ung Tien and Yü Lan read 制形,—the latter also 制行. The present text is derived from Chêng Yu-hsien.
the soldier works out his victory in relation to the foe whom he is facing.