„Here Chopin has the conviction that he has lost his power of expression. With the determination to discover whether his brain can still originate ideas, he strikes his head with a hammer (here the sixteenths and thirty-seconds are to be carried out in exact time, indicating a double stroke of the hammer). In the third and fourth measures on can hear the blood trickle (trills in the left hand). He is desperate at finding no inspiration (fifth measure); he strikes again with the hammer and with greater force (thirty-second notes twice in succession during the crescendo). In the key of A flat he finds his powers again. Appeased, he seeks his former key and closes contentedly.“
— Hans von Bülow
On Chopin's E major Prelude Op.28 No.9, quoted in Harold C. Schonberg, The Great Pianists.