Antisthenes — Greek Philosopher

Antisthenes was a Greek philosopher and a pupil of Socrates. Antisthenes first learned rhetoric under Gorgias before becoming an ardent disciple of Socrates. He adopted and developed the ethical side of Socrates' teachings, advocating an ascetic life lived in accordance with virtue. Later writers regarded him as the founder of Cynic philosophy... (wikipedia)

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