My money’s on this line from a letter Scott Fitzgerald wrote to his daughter “Scottie”:
All good writing is swimming under water and holding your breath.
I don’t know why, but maybe because I am not a good swimmer, the image appeals deeply to me. It certainly has great explanatory power; I’ve used it many times, in classes and seminars, since at least the mid-1990s.
I found the line in a book of Fitzgerald’s letters, which I bought in 1988 or 1989, when good friend Gigi brought me to a wonderful second-hand bookstore somewhere in Greater L.A. Still have the book, although I found out today that the line is actually the first quotation listed in Fitzgerald’s Wikiquote page. Breathe deeply.