 These four rules are best understood as a highly condensed summary of the method described in the Rules (see Gilson 1987: 196–214; Beck 1952: 149; Clarke 1982: 181; Garber 2001: 39; Newman 2019: 85). Descartes’ abridgment of the method in Discourse II reflects a shift vis-à-vis the idea of a “theory” of method. As he famously put it in a letter to Mersenne, the method consists more in practice than in theory (letter to Mersenne, 27 February 1637, AT 1: 349, CSMK 3: 53), and to learn the method one should not only reflect on the rules of the method, but also see how they function in solutions to particular problems. No matter how detailed a theory of method may become, there is no way to prepare oneself for every eventuality that may arise in the course of scientific inquiry, and there is certainly no way to codify every rule necessary to the solution of any and all problems. Every problem is different. By the late 1630s, Descartes decided to reduce the number of rules and focus on the application of the method rather than on the theory of the method. In  Sections 6–9,  I follow Descartes’ advice and examine how he applies the method in solutions to particular problems in optics, meteorology, geometry, and metaphysics. First, though, the role played by experiment in Descartes’ method needs to be discussed in more detail.