 The awakening of Descartes’ interest in physics is often dated to late 1618, when Descartes first met Isaac Beeckman, an amateur scientist and mathematician who championed the new “mechanical” philosophy. The mechanical philosophy’s explanation of natural phenomena, which Descartes quickly adopted, rejected the use of Scholastic substantial forms (see  Section 2).  Rather, the mechanical approach favored a contact or impact model of the interaction of small, unobservable “corpuscles” of matter (which possess only a limited number of mainly geometric properties, such as size, motion, shape, etc.). Over the course of the next decade, Descartes worked on a large number of problems in both science and mathematics, with particular emphasis on the theory of light, mechanics (including hydrostatics), and the free-fall of terrestrial bodies. Much of Descartes’ output at this time is both highly mathematical and concerns only specific physical problems, not unlike the work of his contemporary Galileo. One of the accomplishments of these years includes his discovery of the law of refraction, often called Snell’s law: when light passes from one medium into another, the sine of the angle of incidence maintains a constant ratio to the sine of the angle of refraction. By the beginning of the 1630s, however, Descartes embarked on a more ambitious plan to construct a systematic theory of knowledge, including physics. The result was The World (1633), an important text in that it essentially contains the blueprints of the mechanical/geometric physics, as well as the vortex theory of planetary motion, that Descartes would continue to refine and develop over the course of his scientific career. Before publishing the treatise, however, he learned of the Church’s (1633) condemnation of Galileo for promoting Copernicanism, which prompted Descartes to withdraw his work from publication (since Descartes also advanced Copernicanism in The World). In the 1630s, the publication of the Geometry, the Optics, and the Meteorology, along with a philosophical introduction, Discourse on the Method (1637) further presented Cartesian hypotheses on such topics as the law of refraction, vision, and the rainbow. Yet, besides a brief sketch of his metaphysics and physics in the Discourse (Parts IV and V), a comprehensive treatment of his physics had to await the 1644 publication of the Principles of Philosophy. This work not only represents Descartes’ most fully developed and exhaustive investigation of physics, it also provides the metaphysical underpinnings of his physical system (in Part I). As an embodiment of his mature views, the Principles will thus form the basis of our examination of Cartesian physics.