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Cartesian epistemology comprises three main divisions: (1) an a priori theory, discussed in Chs. 1–3, (2) a psychological theory of error explanations in judgment induced by features of our sense experience discussed in Chs. 4, 5 and 7, and (3) a theory of natural reasons (natural knowledge), discussed here. The theory of natural reasons, based on Descartes's notion of natural inclinations (natural propensities), is expressed here in terms of a series of warrant principles of which there are two main kinds: those that warrant action (reasons of goodness) and those that warrant claims for what is true (reasons of truth). This chapter traces Descartes's epistemically ambivalent attitude to cognitive dispositions from the early treatment in the Rules for the Direction of the Mind through to the final treatment in The Passions of the Soul. The chapter focuses special attention on the appearance of one kind of natural reason, reasons of truth, in the proof of the external world in Meditation VI and the striking absence of same in the proof of the external world in the Principles of Philosophy II. The chapter also considers whether natural reasons apply to particular aspects of corporeal things, concluding with a discussion of the Cartesian Circle.

It seems undeniable that we have both sense experience of primary qualities and sense experience of secondary qualities, and yet, based on a text in the Sixth Replies among others, many commentators have thought that Descartes denied precisely this for primary qualities. One of the main burdens of this chapter is to show that Descartes does have an account of the sense experience of primary qualities and that it is to be found in Descartes's account of the faculty of imagination. The chapter argues that a version of the proof of the external world depends on a special class of ideas of this faculty. There are two sides to Descartes's account: (1) the philosophical theory of ideas, including the taxonomy of perceptual responses developed in the Sixth Replies of the Meditations, and (2) the empirical theory in the Optics and the Treatise on Man, and they are both treated in depth here. Finally, the chapter argues that Descartes has an empirical and philosophical account of the phenomenological fusion of primary and secondary qualities, the former accomplished by means of images in the corporeal imagination and the mechanism of referred sensations in The Passions of the Soul.

How can a Cartesian idea represent ordinary physical objects? One possibility is that Descartes holds a theory of natural signs according to which ideas, including sensations (ideas of secondary qualities), represent states of the external world that are correlated with them. I deny that Descartes has a theory of natural signs in this sense, arguing, instead, that our perception of ordinary physical objects is achieved not through ideas, properly speaking, but through a special act of the mind (discussed in The Passions of the Soul I as ”referred sensations”) which projects (”refers”) its sensations onto objects in the external world, objects truly represented by ideas of primary qualities alone. This ”projection” comprises referral judgments that account for the characteristic phenomenology of experience of the physical world but in a way that misleads us (provides ”material for error”) to think that our ideas bear a resemblance to the underlying metaphysical reality of the physical world. Other issues discussed include Descartes's taxonomy of ideas (classification).