 Many commentators have raised questions about Descartes’ deduction of the sine law (see, e.g., Schuster 2013: 178–184). For example, what physical meaning do the parallel and perpendicular component determinations (lines AH and AC) have? Is it really the case that the speed of the ball is reduced only at the surface of impact, and not after (see Schuster 2013: 180–181)? In water, it would seem that the speed of the ball is reduced as it penetrates further into the medium. These and other questions cannot be examined in detail here. From a methodological point of view, Descartes insists that the law of refraction can be deduced from the comparisons and suppositions he employs in Optics II (see letter to Mersenne, 27 May 1638, AT 2: 142–143, CSM 1: 103), and as we have seen, in both Rule 8 and Discourse IV he claims that he can demonstrate these suppositions from the principles of physics.