*Descartes’s Method* develops an ontological interpretation of Descartes’s method as a dynamic and, within limits, differentiable problem-solving cognitive disposition or *habitus* , which can be actualized or applied to different problems in various ways, depending on the nature of the problem. Parts I-II develop the foundations of an habitual interpretation of Descartes’s method, while Parts III-V demonstrate the fruits of such an interpretation in metaphysics, natural philosophy, and mathematics. The first book to draw on the recently discovered Cambridge manuscript of Descartes’s *Rules for the Direction of the Mind* , Descartes’s Method concretely demonstrates the efficacy of Descartes’s method in the sciences and the underlying unity of Descartes’s method from *Rules for the Direction of the Mind to Principles of Philosophy* (1644).