Learn from friends, learn from enemies.

Knowledge can be gained from everywhere. Books, articles, radio, television, newspaper, and even friends.

From books to newspaper, they are dead things. They can't communicate. They can only give you that much information, until somebody write new things, the book is still the book 10 years ago.

Communicating with friends is easy, but communicating with enemies at the same time?

It is from their foes, not their friends, that cities learn the lesson of building high walls