 Lisa Downing (2011: 115–17) argues that the first part of Principles of Philosophy (hereafter Principles), articles 66–71 provides basis for an a priori argument which Descartes could have had in mind but never stated. The articles explain what we clearly and distinctly perceive about color, what we might mistakenly judge about it, and what we can judge about its relation to bodies without error. According to Principles 1.71, sensations of heat, color, etc. “do not represent anything located outside our thought” (CSM 1.219; AT 8.35) Based on this, Downing (2011: 118–28) constructs a valid argument to show ideas of SQs as we sense them cannot exit in a substance with the principle attribute extension. But she argues, it requires the principle attribute of a body to be absolute geometrical extension and modes to be its limits. She finds this too restrictive to be tenable because it excludes modes such as force and inertia and it is a model of the attribute-mode relation that cannot be applied to substances with the principal attribute thought.