 The personal crisis experienced by Sanches and Descartes which motivated their epistemological inquiry concerning what could and could not be known with certainty, was responded to by means of self-reflection. In both cases, albeit with very different approaches, the self was the only legislator of certainty, and the only thing that could be counted upon in founding a science: in Descartes’ case, founded on unquestionable internal, logical principles (“I cannot doubt my own existence”), and in Sanches’ case, founded on the material things (rerum) the I perceives, albeit imperfectly. Significantly, what constituted the certainty of the internal states for Descartes did not constitute certainty for Sanches, and moreover such “certainty” could not serve as the foundation of a true science. In this respect, whereas the late sixteenth century Sanches was operating within a pre-Galilean Weltanschauung, Descartes undertaking studies in physics and astronomy that paralleled those of Galileo and “Galilean” science with its emphasis on mathematical demonstration. For Descartes, mathematics was the language of Nature, and even physical things or “natural phenomena” were mathematical, as he stated in the “Principles of Philosophy” (1644 [PWD: 247]). This was clearly not the case for Sanches, who in his letter to Clavius  (§2.2),  denied the universal and a priori truths of mathematics, reducing mathematics to mere empirical measurement. In short, if Descartes’ method culminated in the “indubitable” truths of mathematics placed at the service of sciences like physics, optics, and astronomy, Sanches’ sceptical method, with its acceptance of imperfect knowledge, culminated in the probable and empirical “truths” of the science of medicine. Descartes, the philosopher, mathematician, and physicist; Sanches, the philosopher and physician: the former emphasized the mind over the body (res cogitans over res extensa), and the latter the body over the mind.