知己知彼,百战不殆;
不知彼而知己,一胜一负;
不知彼,不知己,每战必殆。。。。Know yourself and know your enemy, and you will never be defeated in a hundred battles.
If you know yourself but do not know your enemy, you will win one battle and lose one battle.
If you know neither the enemy nor yourself, you will be defeated in every battle.