Although the question of a connection between Bach’s music and the discipline of rhetoric has been raised repeatedly in the past, the proposed solutions have rarely taken into account the particular kind of rhetorical thinking prevalent in the eighteenth century. In this article, I show that a notion of rhetoric initially developed by Erasmus of Rotterdam and perpetuated in seventeenth-century writings, which focused on argumentative procedures involving variation and amplification, continued to underlie poetic and musical theory in Bach’s time. By articulating fundamental creative patterns that came to underpin a variety of disciplines, this Erasmian model can provide the starting-point for a reassessment of rhetorical techniques in Bach’s music, shifting the focus from isolated moments of affective decoration to the formal-expressive trajectories that shape the layout of whole pieces. Constituted in the interplay of compositional processes and their listening reception, these trajectories emerge as the result of the skilful arrangement of musical phrases into individual and flexible large-scale designs that often leave aside or undercut the supposed structural conventions of concerto or aria forms. The first movement of the third ‘Brandenburg Concerto’, bwv1048, serves as an example of how an awareness of these seventeenth-century rhetorical and musical legacies makes possible a thorough reconsideration of Bach’s compositional strategies.