  Descartes famously writes in the preface to the Principles that “the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other special sciences, which may be reduced to three principal ones, namely medicine, mechanics, and morals” (AT VIIIa, 14; CSM 1, 186). Metaphysics is his starting point, but what Descartes means by metaphysics, and what falls under the rubric of metaphysics for him, is crucial to understanding his project, and for understanding Du Châtelet's conceptual relation to him. One of the central innovations Descartes makes in the field of metaphysics is to include metaphysical claims about the mind of the knowing subject (Hatfield 1990, 11–17). This element of metaphysics leads Descartes very quickly into a discussion of first principles of knowledge, such as his belief that the knowing subject is capable of rationally intuiting further metaphysical principles because God has implanted innate ideas about them within human souls.