 Nevertheless, even if Descartes’ mechanistic natural philosophy shunned the metaphysics of substantial forms, his underlying methodology or approach to science remained very close to the Scholastic tradition. By the time of the composition of the Principles, Descartes had formulated a method that, like the Scholastics, strived to explain natural phenomena based on the allegedly simple and irrefutable “facts” and/or observations, drawn from rational reflection on concepts or from everyday experience, about the most fundamental aspects of reality. These supposedly basic facts thereby provide the requisite metaphysical foundation for his physical hypotheses: in other words, one proceeds from our “clear and distinct” knowledge of general metaphysical items, such as the nature of material substance and its modes, to derive particular conclusions on specific types of physical processes, for instance the laws of nature. This method of conducting science is quite contrary to the modern approach, needless to say, since modern scientists do not first engage in a metaphysical search for first principles on which to base their work. Yet, this is exactly the criticism that Descartes leveled at Galileo’s physics (in a letter to Mersenne from 1638): “without having considered the first causes of nature, [Galileo] has merely looked for the explanations of a few particular effects, and he has thereby built without foundations” (AT II 380; see, also, the Preface to the French translation of the Principles, AT IXB 5–11). The structure of the Principles, Descartes’ most comprehensive scientific work, reflects these priorities: Part I recapitulates the arguments (well-known from the Meditations) for the existence of God, mental substance, and other metaphysical topics; whereas the remaining Parts proceed to explain the nature of material substance, physics, cosmology, geology, and other branches of science, supposedly based on these fundamental metaphysical truths. This preoccupation with metaphysical foundations, and the causal explanations of natural phenomena derived from them, might also account for the absence in the Principles of Descartes’ more mathematical work in physics, such as his discovery of the law of light refraction. As he argued in the Rules for the Direction of the Mind (1628), pure mathematicians are only concerned with finding ratios and proportions, whereas natural philosophers are intent on understanding nature (AT X 393–395). The development of modern physics, which is inextricably intertwined with modern mathematics, thus stands in sharp contrast to the latent Scholasticism evident in Descartes’ metaphysical approach to physics.