But Bach's greatest achievement from these years was a composition not of music but of prose. The Essay on the True Art of Playing Keyboard Instruments remains the most famous treatise of its kind. The book combines technical advice about ornamentation, improvisation and the importance of correct fingering ("more is lost by incorrect fingering than can be compensated for by all the art and good taste in the world"). The now-standard practice of using thumbs in keyboard playing can be attributed to it. But the text also prescribes a philosophy of performance which for the first time placed the expression of emotion on a par with technical competence. "Since a musician cannot move others unless he himself is moved," the essay argues, "he must of necessity feel all of the affects that he hopes to arouse in his listeners." Both Haydn and Beethoven swore by it; its use remained widespread long into the 19th century.
Bach also produced an important account of performance practice in the second half of the eighteenth century, translated into English as Essay on the True Art of Playing Keyboard Instruments.
essays on the True Art of Playing Keyboard instruments
Essay on the True Art of Playing Keyboard Instruments, translated & Edited by William J Mitchell. WW Norton US, published 1949, this one printed 80s or 90s. Hardback, wrapper, 449pp, loads of musical illustrations.