So it is said that if you know others and know yourself, you will not be imperiled in a hundred battles; if you do not know others but know yourself, you win one and lose one; if you do not know others and do not know yourself, you will be imperiled in every single battle.
-Sun Tzu, The Art of War 
‘Know Thyself’ has been a principle going back all the way to Ancient Greece, and possibly, even later. Sun Tzu is suggesting that even that is not enough–one would do well to also know others. Sun Tzu was clearly talking about battle, but his work has been co-opted into general wisdom from everything to self-help and business acumen today. From the sense that this advice is helpful exclusively in war, Chang Yu (a commentator in The Art of War and contemporary scholar of the time) expounds: “Knowing the enemy enables you to take the offensive, knowing yourself enables you to stand on the defensive.” And though most of us aren’t in the middle of a torrid war among several small kingdoms, the advice could still be applicable to every day life. Knowing your strengths and weaknesses allows you the self-awareness to make a good decision, but without also evaluating either the enemy, the situation, another person, etc, how can you truly make the best decision? One must also be aware that they are not the only actor on the stage of life, you can certainly do right by yourself in singular thought and will probably live fruitfully, but a true strategist evaluates all angles and possibilities, and part of that is anticipating and attempting to know beyond oneself and the actions and needs of others. Ignorance in both, surely, will get you nowhere.
Fact Check It, Yo!
 Tzu, Sun. The Art of War. Translated by Thomas F. Cleary, Shambhala, 2005.