“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.”

I like it a lot - and, its accuracy is more often correct than not. Even mental, religious, political, etc. examples can fit within the scope of the quote.

Can't argue with that.

I don't want to battle, I want to be friends. If you really know yourself,  don't you know you yourself are the one that you are at opposition with?