As long as a book would write itself I was a faithful and interested amanuensis and my industry did not flag; but the minute the book tried to shift to my head the labour of contriving its situations, inventing its adventures, and conducting its conversations I put it away and dropped it out of my mind.... It was by accident that I found out that a book is pretty sure to get tired along about the middle and refuse to go on with its work until its powers and its interest should have been refreshed by a rest and its depleted stock of raw material reinforced by lapse of time.
It was when I had reached the middle of Tom Sawyer that I made this invaluable find. At page 400 of my manuscript the story made a sudden and determined halt and refused to proceed another step. Day after day it still refused. I was disappointed, distressed and immeasurably astonished, for I knew quite well that the tale was not finished and I could not understand why I was not able to go on with it. The reason was very simple -- my tank had run dry; it was empty; the stock of materials in it was exhausted; the story could not go on without material; it could not be wrought out of nothing. When the manuscript had lain in the pigeon hole two years I took it out one day and read the last chapter that I had written. It was then that I made the great discovery that when the tank runs dry you've only to leave it alone and it will fill up again in time, while you are asleep--also while you are at work on other things and are quite unaware that this unconscious and profitable cerebration is going on. There was plenty of material now, and the book went on and finished itself without any trouble.