The general who wins a battle makes many calculations in his temple before the battle is fought. The general who loses makes but few calculations beforehand. Thus do many calculations lead to victory, and few calculations to defeat: how much more no calculation at all!

If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.

Invincibility lies in the defense; the possibility of victory lies in the attack. One defends when one's strength is inadequate; one attacks when one's strength is abundant.