A student of Beethoven’s, Ferdinand Ries, went on a walk with his teacher in the country:
Beethoven muttered and howled the whole time, without emitting any definite notes. When I asked him what he was doing he answered, “A theme for the last allegro of the sonata [the Appassionata] has occurred to me.” When we reached the house he ran, without stopping to take off his hat, to the piano. I sat in the corner, and he soon forgot all about me. At last he got up; he was astonished to find me still there, and said, “I cannot give you a lesson today: I must go on working.”
Wegeler, Franz and Ries, Ferdinand (1838) Biographische Notizen über Ludwig van Beethoven, Koblenz. Cited in: Marek, George (1969) Beethoven: Biography of a Genius. London: William Kimber, p.160.