The Joy of Reading:

The Things You Pick Up from Books:
I am your enemy, the first one you've ever had who was smarter than you. There is no teacher but the enemy. No one but the enemy will ever tell you what the enemy is going to do. No one but the enemy will ever teach you how to destroy and conquer. Only the enemy shows you where you are weak. Only the enemy tells you where he is strong. And the rules of the game are what you can do to him and what you can stop him from doing to you. I am your enemy from now on. From now on, I am your teacher.
-- Orson Scott Card, Ender's Game