Be aware of yourself and know yourself. No matter how much you have learned and how much you know, if you don't know yourself you don't know anything. Indeed, if you don't know yourself you cannot know anything else. People who don't know themselves criticize others from the point of view of their own ignorance. They consider what agrees with them to be good, and hate whatever doesn't go their way. They become irritated about everything, causing themselves to suffer by themselves, bothering themselves solely because of their own prejudices. If you know that not everyone will be agreeable to you, know that you won't be agreeable to everyone either. Those who have no prejudices in themselves do not reject people, and therefore people do not reject them.
Source: The Japanese Art of War
Contributed by: Laurens