As iron is eaten away by rust, so the envious are consumed by their own passion.
Not to unlearn what you have learned is the most necessary kind of learning.
Pay attention to your enemies, for they are the first to discover your mistakes.
The most useful piece of learning for the uses of life is to unlearn what is untrue.
There are only two people who can tell you the truth about yourself - an enemy who has lost his temper and a friend who loves you dearly.