This article discusses the change in the relationship between natura and ars that occurred when Cartesian language theories penetrated conceptions of rhetoric in France during the 17th century. In part one, the article considers the reception of Descartes by the French universities and collèges in order to explore what I call the epistemic transfer between Aristotelianism and the new Cartesian philosophy. The focus then shifts to Bernard Lamy’s conception of rhetoric as presented in his principal work De l’art de parler (1675). Based upon the Cartesian theory of passions, Lamy’s book redefines rhetoric in a way that has been labelled in current research as a ‘grammar of affects’. The fundamentals of the ars rhetorica are thus substantially altered, however without eliminating the power of the rhetorical tradition. The third section turns to the prize contests of the French academies in the 18th century in order to show the extent to which the transformation of rhetorical theory affected the broader practice of eloquence. Moreover, the rhetorical prize contests demonstrate an explicit reflection on the diverging modes of knowledge production that are essential, on the one hand, to the exact sciences and, on the other, to the textual tradition. Particularly striking is the critique of scientific method that was developed by the defenders of rhetoric in the concours académique.