I write for the unlearned about things in which I am unlearned myself… It often happens that two schoolboys can solve difficulties in their work for one another better than the master can. When you took the problem to a master, as we all remember, he was very likely to explain what you understood already, to add a great deal of information which you didn’t want, and say nothing at all about the thing that was puzzling you… The fellow pupil can help more than the master because he knows less. The difficulty we want him to explain is one he has recently met. The expert met it so long…