This chapter considers Descartes' systematic doctrines on the nature of the mind and its ideas. It combines Descartes' statements on sensation and perception for hints about how to apply such principles. Outside the Meditations and Principles, Descartes discusses the anatomy, physiology, and mental operation of the senses in the Dioptrics and Passions. Descartes further develops the notion of ideas as images by explaining that differences in the objective reality of ideas amount to differences in what those ideas represent. The author favors an interpretation in which, for Descartes, all sensory ideas represent by resemblance, different kinds of sensory ideas vary in cognitive value, externalization arises through spatial localization, and, with sensory ideas of color and the like, as materially false they do not intrinsically misrepresent but afford occasion for false judgments, which arise as merely apparent, and so not actually legitimate, teachings of nature.