If not you, who? If not now, when?
Moreover he saw a skull floating on the surface of the water and he said unto it: Because you drowned others they drowned you; and those that drowned you will eventually be drowned.
He who refuses to learn deserves extinction.
Do not unto others that which is hateful unto thee.
The shy man will not learn; the impatient man should not teach.
Who does not grow, declines.