The aim of this article is to analyse Descartes’ inference from knowing to being (a nosse ad esse valet consequential. While recalling the major interpretations of this principle, the article concentrates on the polemical framework of which it is part, namely the discussion with the Jesuit Bourdin, the author of the Seventh Objections. By means of an analysis of the concept of res cogitans understood as an affirmation of the immateriality of the soul, the target of Bourdin’ s criticism is doubt, which is accused of unduly authorising the passage from a mental negation of material beings to their negation in being. Against this Descartes develops the outline of an epistemological option that is idealist, but not reductionist, an option from ...