Chapter 3, Verse 18 - Sun Tzu

Hence the saying: 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.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.