Jacques Rancière said this quote

“ The master always keeps a piece of learning--that is to say, a piece of the student's ignorance--up his sleeve. I understood that, says the satisfied student. You think so, corrects the master. in fact, there's a difficulty here that I've been sparing you until now. We will explain it when we get to the corresponding lesson. What does this mean? asks the curious student. I could tell you, responds the master, but it would be premature: you wouldn't understand at all. It will be explained to you next year. The master is always a length ahead of the student, who always feels that in order to go farther he must have another master, supplementary explications. Thus does the triumphant Achilles drag Hector's corpse, attached to his chariot, around the city of Troy. ”