““Umar b. al-Khattâb contributes the rather resigned insight that men are never as jealous of knowledge as they are of their wives. Salmân indicates the need for teaching and publication in the words, “Knowledge which remains unexpressed is like treasure unused.” The hadîth about the two kinds of knowledge, discussed above, p. 243. Umar on the desirability of the combination of ilm and hilm. Abû d-Dardâ expresses himself on the burden scholars have to bear in these words: “An increase in knowledge means an increase in pain.” Statements by Plato and another sage to the effect that consciousness of not knowing indicates knowledge. An elaboration of this idea is attributed to al-Khalîl: “There are four kinds of men. There are men who know and know that they know. Put questions to them! There are men who know and do not know that they know. They are forgetful. Remind them! There are men who do not know and know that they do not know. They require guidance. Teach them! And there are men who do not know and do not know that they do not know. They are ignorant. Shun them!””
