Beethoven's way of mental composing appears at first sight to differ widely from Mozart's. But if we had as many specimens of Mozart's preliminary sketches as we have of Beethoven's, the difference would perhaps appear less pronounced, and would to a large extent resolve itself into the fact that Beethoven did not trust his memory so much as Mozart did, and therefore put more of his _tentative_, or rough sketches, on paper. He always carried in his pockets a few loose sheets of music paper, or a number of sheets bound together in a note-book. If his supply gave out accidentally, he would seize upon any loose sheet of paper, or even a bill of fare, to note down his thoughts. In a corner of his room lay a large pile of note-books, into which he had copied in ink his first rough pencil-sketches. Many of these sketch-books have been fortunately preserved, and they are among the most remarkable relics we have of any man of genius. They prove above all things that rapidity of work is not a test of musical inspiration, and that Carlyle was not entirely wrong when he defined genius as "an immense capacity for taking trouble." In the "Fidelio" sketch-book, for example, sixteen pages are almost entirely filled with sketches for a scene which takes up less than three pages of the vocal score. Of the aria, "O Hoffnung," there are as many as eighteen different versions, and of the final chorus, ten; and these are not exceptional cases by any means. As Thayer remarks: "To follow a recitative or aria through all its guises is an extremely fatiguing task, and the almost countless studies for a duet or terzet are enough to make one frantic." Thayer quotes Jahn's testimony that these afterthoughts are invariably superior to the first conception, and adds that "some of his first ideas for pieces which are now among the jewels of the opera are so extremely trivial and commonplace, that one would hardly dare to attribute them to Beethoven, were they not in his own handwriting." On the other hand these sketch-books bear witness to the extreme fertility of Beethoven's genius. Thayer estimates that the number of distinct ideas noted in them, which remained unused, is as large as the number which he used; and he refers to this as a commentary on the remark which Beethoven made toward the close of his life: "It seems to me as if I were only just beginning to compose." And Nottebohm, who has studied these sketch-books more thoroughly than any one else, thinks that if Beethoven had elaborated all the symphonies which he began in these books we should have at least fifty instead of nine.