Observe your enemies, for they first find out your faults.
Pay attention to your enemies, for they are the first to discover your mistakes.
The advantages of philosophy? That I am able to hold converse with myself.
The most necessary learning is that which unlearns evil.
Not to unlearn what you have learned is the most necessary kind of learning.
We must not contradict, but instruct him that contradicts us; for a madman is not cured by another running mad also.