Sun Tzu's Art of War:

故曰：知彼知己，百戰不殆；不知彼而知己，一勝一負；不知彼，不知己，每戰必敗。

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.