Confucius

The gem cannot be polished without friction, nor man perfected without trials.

He who learns but does not think is lost. He who thinks but does not learn is in great danger.

To see what is right and not to do it is want of courage.

By three methods we may learn wisdom: first, by reflection, which is noblest; second, by imitation, which is easiest; and third, by experience, which is the bitterest.