Sun Tzu's Art of War:

故兵知彼知己,百戰不殆,不知彼而知己,一勝一負;不知彼不知己,每戰必殆.

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.