On manual art...................

 

He made his living at first by teaching children in Van den Ende's school, and then by polishing lenses, as if he had an inclination for dealing with refractory material.  He had learned the optical trade while living in the Jewish community; in accord with the Hebrew canon that every student should acquire some manual art; not only because study and honest teaching can seldom make a livelihood, but as Gamaliel had said, work keeps one virtuous, whereas "every learned man who fails to acquire a trade will at last turn out a rogue."

-Will Durant, from the chapter on Spinoza in The Story of Philosophy