“If you know the enemy and know yourself, your victory will not stand in doubt; if you know Heaven and know Earth, you may make your victory complete.”
Two levels of knowledge: knowing yourself and the other side removes doubt; knowing the environment — timing, conditions, context — completes the picture. The fullest strategy accounts for the whole situation, not just the two parties.
Chapter 10’s closing reward: the commander who knows the enemy and himself has a victory that "will not stand in doubt" — but only knowing Heaven and Earth makes victory complete, 胜乃不穷, inexhaustible. Heaven and Earth are Sun Tzu’s shorthand for the whole surrounding condition: seasons, weather, terrain, timing. The chapter about terrain thus ends by promoting the environment from scenery to variable.
Two funds hold the same stock and the same thesis about the company. One models only the company; the other models the company plus the rate cycle, the currency, and the regulatory calendar it sits inside. The thesis plays out identically for both — and the second fund makes materially more, because position sizing and exit timing were set by the weather around the trade, not just the trade.
"Know everything about everything" is not the instruction — that standard would excuse paralysis. Sun Tzu grades knowledge: parties first, environment second, and only the layers that touch the outcome. The exercise is naming which environmental forces actually carry your decision — and adding exactly those to the board, not all of them.