“Serve your enemies for they first find out your faults”
“Observe your enemies, for they first find out your faults.”
“Pay attention to your enemies for they are the first to discover your mistakes.”
“As iron is eaten away by rust, so the envious are consumed by their own passion.”
“We must not contradict, but instruct him that contradicts us; for a madman is not cured by another running mad also.”