The last lesson of the chapter on empty and full is a strange one for a manual of war. Sun Tzu suddenly starts talking about water.
Water has no fixed shape. Poured into a vessel it becomes the shape of the vessel, and where the ground slopes it runs that way. Nobody asks water what its shape is. Water is different every time.
Sun Tzu said a winning army has to be the same.
As Water Runs to the Low Place
夫兵形象水，水之形，避高而趨下，兵之形，避實而擊虛
fu bing xing xiang shui, shui zhi xing, bi gao er qu xia, bing zhi xing, bi shi er ji xu
The shape of an army resembles water. The nature of water is to avoid the heights and run to the low place. The shape of an army is to avoid the solid and strike the empty. (The Art of War, Empty and Full, translated directly)
Water does not force its way through a mountain. It finds the low road and goes around. An army is the same. There is no need to collide with their strong place. You only have to find the empty one.
The Ground Decides the Water’s Path
Water does not set its own course. The lie of the land sets it.
水因地而制流，兵因敵而制勝
shui yin di er zhi liu, bing yin di er zhi sheng
Water shapes its flow according to the ground. An army shapes its victory according to the enemy.
Same principle. A strategy is not set in advance. It is made on the spot from looking at the other side. As there is no watercourse that ignores the ground, there is no strategy that ignores the enemy.
There Is No Fixed Configuration
The conclusion of this chapter fits in one line.
故兵無常勢，水無常形
gu bing wu chang shi, shui wu chang xing
So an army has no constant configuration, and water has no constant shape.
Nothing guarantees that last year’s method works this year. When the ground has changed and only the channel stays, water stops flowing and turns stagnant.
The Name for Noticing Change
Sun Tzu gave this ability a special name. Shen (神), the uncanny.
The one who notices the enemy’s changes and changes accordingly. Only that one wins by a different method every time.
The principle is single, which is to win. The method is different every time. Knowing that difference is what he called uncanny.
Today, What Are You Still Using Unchanged
Look at whether you are reusing, unchanged, the method that worked last time.
If the situation has changed and only the method stays, that is not holding to a principle. It is sitting in momentum. Leave the goal where it is, and let only the method flow again like water.