"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… 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 ago that he has forgotten… I write as one amateur to another, talking about difficulties I have met, or lights I have gained…"