Johann Sebastian Bach was one of the most widely sought music teachers in Germany during his lifetime. This study examines how young musicians in J. S. Bach’s circle learned the art of composition, complementing previous scholarship emphasizing Bach’s importance as a keyboard pedagogue. Copying and studying music in manuscript was the main way to learn to compose in the eighteenth century, both in a guild apprenticeship and in a Latin school education. While a few sources with clear significance for compositional study under Bach have received scholarly attention, the importance of apprentices’ scribal copies has been overlooked. Yet, the preparation of performance parts, transcriptions and arrangements, the emulation of models, the correction and revision of scores—these kinds of copying activities gave apprentices access to written music and the opportunity for study. Writing music from the mind without the aid of instruments was increasingly valued by composers in the early-eighteenth century, and was emphasized by late-eighteenth century commentators’ portrayal of Bach as an original genius. The centrality of writing emerges from the humanistic education that musicians received within the cantorate system. Just as humanist authors practiced rhetorical techniques of imitation, expansion, adaptation, and elaboration, so composers learned to use the conventions of musical writing to expedite compositional invention and development. Copyists, likewise, used their conceptual grasp of compositional structure to expedite the copying of an exemplar. The early-eighteenth-century concept of genius, as acumen in the study of models and fecundity in their imitation, is the basis for the late-eighteenth-century concept of the original genius, despite the later emphasis on individuality and novelty. Musicological discussions of recent decades have shed light on the importance of improvisation, breaking down the distinction between performer and composer. The importance of writing need not minimize the importance of embodied musical skill: writing is a kind of embodied knowledge. Considering music notation and the act of writing as forms of compositional thought, not merely as the means of inscription, is analogous to Derrida’s deconstruction of the speech-writing binary for language.