As iron is eaten away by rust, so the envious are consumed by their own passion.
Antisthenes (444 - 371 BC)
Contributed by: Zaady
It is better to fall among crows than flatterers; for those devour only the dead - these the living.
Not to unlearn what you have learned is the most necessary kind of learning.
Source: The Lives and Opinions of Eminent Philosophers, by Diogenes Laertius, translated by C.D. Yonge
On being praised by some wicked men, I am sadly afraid that I must have done some wicked thing.
The most useful piece of learning for the uses of life is to unlearn what is untrue.
Pay attention to your enemies, for they are the first to discover your mistakes.