 Descartes’ correspondence with Princess Elisabeth led directly to the composition of his final book, The Passions of the Soul, a large part of which was written during the winter of 1645–6. In a prefatory letter to the book, Descartes maintains that in it he has set out to explain the passions “only as a natural philosopher [physicien], and not as a rhetorician or even as a moral philosopher” (AT XI 326/CSM I 327). On the face of it, this seems to be contradicted by the contents of the book, since much of it is devoted to understanding the passions from an ethical point of view—that is, understanding how they can be accommodated to the goal of happiness. The significance of Descartes’ remark lies in the particular account he gives of the passions. In general, they are defined as “those perceptions, sensations or emotions of the soul which we refer particularly to it, and which are caused, maintained and strengthened by some movement of the [animal] spirits” (art. 27; CSM I 338–9). Descartes’ central thesis is that the passions originate in bodily changes, which are communicated by the animal spirits to the pineal gland, and thereby give rise to affective states in the soul—affections which are referred to the soul itself and not to the body. Because the passions originate in the body, a large part of the book is devoted to differentiating the passions and accounting for their effects in physiological terms; hence Descartes’ claim to explain them as a natural philosopher. (For extended treatments of Descartes’ theory of the passions, see Brown 2006; James 1997; Kambouchner 1995a, b.)