Antisthenes

"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."

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

"As iron is eaten by rust, so are the envious consumed by envy."

"Pay attention to your enemies, for they are the first to discover your mistakes." - quoted by Diogenes Laertius in Lives of the Philosophers