 The suppositions Descartes refers to here are introduced in the course of experiment; they describe the shapes, sizes, and motions of the bodies that cause the effects observed in an experiment. Suppositions are needed because these particles are beyond the reach of observation. Some scholars have argued that in Discourse VI Descartes introduces a method distinct from the method developed in the Rules and even Discourse II. For these scholars, the method in the Rules is a priori and proceeds from causes to effects, while the method in Discourse VI is a posteriori and proceeds from effects to causes (see Clarke 1982). The latter method, they claim, is the so-called “hypothetico-deductive method” (see Larmore 1980: 6–22 and Clarke 1982: 10). Others have argued that this interpretation of both the Rules and Discourse VI suffers from a number of problems. First, experiment is in no way excluded from the method developed in the Rules. On the contrary, in both the Rules and the Essays, experiment neither interrupts nor replaces deduction; experiment structures deduction because it helps one reduce problems to their simplest component parts (see Garber 2001: 85–110). A hint of this can already be seen in the anaclastic example (see  Section 3):  the third problem in the reduction (“How is refraction caused by light passing from one medium to another?”) can only be discovered by observing that light behaves differently in a variety of transparent media. Experiment structures of the deduction. This is also the case in Descartes’ deduction of the cause of the rainbow (see  Section 7  below and Garber 2001: 91–104). Second, in Discourse VI, Descartes explicitly asserts that the suppositions introduced in the Essays can be deduced from first principles or “primary truths”, and there is no room for such demonstrations in the hypothetico-deductive method, in which hypotheses are confirmed by experience alone.