Al-Khalil b. Ahmad remarked: There are four types of men:
(1) One who knows and knows he knows; he is learned, so follow him!
(2) One who knows and knows not that he knows; he is asleep, so wake him!
(3) One who knows not and knows he knows not; he seeks to learn, so teach him!
(4) One who knows not and knows not that he knows not; he is a fool, so shun him!