[18] And whenever there is an answer, there then must of necessity have been, in every case, a question. But whenever any one puts a question it is respecting something which he does not know, because he is desirous to learn; inasmuch as he is aware that there is nothing so useful with regard to acquiring knowledge as to ask, to inquire, to investigate, to appear to know nothing, and not to have an idea that one comprehends anything firmly.