Descartes’ conception of the scope of philosophy is decidedly more far-reaching than what is today considered to constitute the proper subject matter of the discipline. Many areas of study that we now classify as distinct from philosophy were thought by Descartes and his contemporaries to fall under the broad heading of natural philosophy. Indeed, a quick look at the subjects covered by Descartes in his Principles of Philosophy (1644) reveals the wide range of phenomena that he believed to be the proper purview of the philosopher. While part I discusses the sorts of matters contemporary philosophers associate with Descartes and see as distinctly philosophical, part II would today be classified as a work in physics. Part III deals with questions concerning the moon, sun, planets, comets, and other topics that today we see as part of astronomy; and the final division, part IV, takes up all sorts of matters, none of which would be thought of as a matter for philosophical investigation today. In it, Descartes treats such diverse topics as the nature of air, the tides, the interior of the earth, qualities of various salts, and why earthquakes occur, as well as answers to several dozen questions about fire and even more about magnets (see magnetism).

In the preface of the 1647 French edition of the Principles, Descartes famously compares philosophy to a tree in order to show the internal divisions of, and the relations between, the subdisciplines of the field. In the metaphor, the “roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principled ones, namely medicine, mechanics, and morals” (AT IXB 14, CSM I 186). There is, thus, a dependence relation among the subdisciples: just as the trunk depends upon the roots for support, for example, physics relies on the principles of metaphysics. But the tree metaphor also suggests that there is an order in which philosophy ought to proceed – the roots must come before the trunk and the branches. This point comes out later in the preface, where we are given an explicit statement of Descartes’ understanding of philosophy.