As rust corrupts iron, so envy corrupts man.
The investigation of the meaning of words is the beginning of education.
Not to unlearn what you have learned is the most necessary kind of learning.
We must not contradict, but instruct him that contradicts us; for a madman is not cured by another running mad also.
Pay attention to your enemies, for they are the first to discover your mistakes.
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.