The Scholar, the Ignorant, the Heedless and the Fool

It is reported that Al-Khalīl b. Aḥmad said:

Men are of four types: There is the man who knows and he knows that he knows. He is knowledgeable, so follow him and ask him. And there is the man who does not know, and he knows that he does not know. He is ignorant, so teach him. And there is the man who knows, but he does not know that he knows. He is heedless, so remind him. And then there is the man who does not know, but he does not know that he does not know (he thinks he knows). This is the fool (idiot), so beware of him.

Ibn ˁAbd Al-Barr, Jāmiˁ Bayān Al-ˁilm wa Faḍlihī article 1538.