Antisthenes

As iron is eaten away by rust, so the envious are consumed by their own passion.

Pay attention to your enemies, for they are the first to discover your mistakes.

Observe your enemies, for they first find out your faults.

I am sadly afraid that I must have done some wicked thing.

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.

Quarrels often arise in marriages when the bridal gifts are excessive.

Not to unlearn what you have learned is the most necessary kind of learning.

The most useful piece of learning for the uses of life is to unlearn what is untrue.