Descartes’s Method: The Formation of the Subject of Science provides a systematic interpretation of Descartes’s method in Rules for the Direction of the Mind and related texts. The book reconstructs Descartes’s method in its entirety and concretely demonstrates both the efficacy of the method in the sciences as well as the unity of the method from Rules for the Direction of the Mind (1620s) to Principles of Philosophy (1644). The principal thesis of the book is that Descartes’s method is a problem-solving cognitive disposition (or habitus) that can be actualized in a variety of well-defined ways, depending always on the nature of problem. The book divides into five parts and eleven chapters. Parts I–II (Chapters 1–4) develop an interpretation of the historical and conceptual foundations of Descartes’s method (its operations and acquisition), while the remainder of the book (Parts III–V, which include Chapters 5–11) demonstrates the fruits of the method in solutions to problems in the sciences (above all, mathematics and optics).