We weed out the darnel from the corn and the unfit in war, but do not excuse evil men from the service of the state.
When brothers agree, no fortress is so strong as their common life.
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.
The most necessary learning is that which unlearns evil.
Observe your enemies, for they first find out your faults.