Sidereus Nuncius was a seminal text of the Scientific Revolution. It reported celestial observations whose implications upended the geocentric cosmology few had ever doubted. But Galileo’s treatise also combined topics and methodologies that traditionally had been assigned separately to mixed mathematics and natural philosophy. Whereas the bounds between these disciplines had been weakened by earlier controversies, particularly about the regressus method and about the certainty of mathematics (i.e., the Quaestio de Certitudine), Sidereus Nuncius broke them down altogether, to the delight and dismay of readers. Galileo’s application of mathematical methods—empiricism and quantification—to natural philosophy framed the ensuing discussions, such that even those who disagreed with his conclusions responded on those grounds. Thus, the book was pivotal in the process of disciplinary admixture that reorganized the study of nature into modern science.