Know the enemy and know yourself

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.

– Sun Tzu, The Art of War
http://www.goodreads.com/quotes/17976-if-you-know-the-enemy-and-know-yourself-you-need