 Foremost among the achievements of Descartes’ physics are the three laws of nature (which, essentially, are laws of bodily motion). Newton’s own laws of motion would be modeled on this Cartesian breakthrough, as is readily apparent in Descartes’ first two laws of nature: the first states “that each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move” (Pr II 37), while the second holds that “all movement is, of itself, along straight lines” (Pr II 39; these two would later be incorporated into Newton’s first law of motion). By declaring that motion and rest are primitive states of material bodies without need of further explanation, and that bodies only change their state when acted upon by an external cause, it is not an exaggeration to claim that Descartes helped to lay the foundation for the modern theory of dynamics (which studies the motion of bodies under the action of forces). For the Aristotelian-influenced Scholastics who had endeavored to ascertain the causal principles responsible for the “violent” motions of terrestrial bodies (as opposed to their “natural” motions to specific regions of the plenum), the explanation for these forced, unnatural motions seemed to lie in some type of internal bodily property, or external agent, that was temporarily possessed by, or applied to, a body—an explanation that accounts for the fact that the body’s motion both originates and concludes in a state of rest (since, while on the earth’s surface, the terrestrial element has no natural motions). According to the medieval “impetus” theory, for example, these violent motions occur when a quality is directly transferred to a body from a moving or constrained source, say, from a stretched bow to the waiting arrow. This property causes the observed bodily motion until such time that it is completely exhausted, thus bringing about a cessation of the violent movement (and the arrow’s fall back to earth). Implicit in the Scholastic view is the basic belief that a terrestrial body continuously resists change from a state of rest while situated upon the earth, since the depletion of the impetus property eventually effects a corresponding return of the body’s original motionless, earthbound condition. Descartes, on the other hand, interpreted the phenomena of motion in an entirely new light, for he accepts the existence of inertial motion (uniform or non-accelerating motion) as a natural bodily state alongside, and on equal footing with, the notion of bodily rest. He argues, “because experience seems to have proved it to us on many occasions, we are still inclined to believe that all movements cease by virtue of their own nature, or that bodies have a tendency towards rest. Yet this is assuredly in complete contradiction to the laws of nature; for rest is the opposite of movement, and nothing moves by virtue of its own nature towards its opposite or own destruction” (Pr II 37). While one can find several natural philosophers whose earlier or contemporary work strongly foreshadowed Descartes’ achievement in the first and second laws—namely, Galileo and Isaac Beeckman (see Arthur 2007)—the precise formulation put forward in the Principles of Philosophy is quite unique (especially as regards the second law, since both Galileo and Beeckman appear to sanction a form of circular inertial motion, which possibly betrays the influence of the Scholastic’s circular motion of the celestial element). A fascinating blending of Scholasticism and the new physics is also evident in the above quotation, since Descartes invokes the logic of contrary properties in his statement that “nothing moves by virtue of its own nature towards its opposite or own destruction”. That is, rest and motion are opposite or contrary states, and since opposite states cannot (via the Scholastic principle) transform into one another, it follows that a body at rest will remains at rest and a body in motion will remains in motion. Consequently, Descartes has employed a Scholastic/Medieval argument to ground what is possibly the most important concept in the formation of modern physics, namely inertia. Yet, it is important to note that Descartes’ first and second laws do not correspond to the modern concept of inertia, since he incorrectly regards (uniform, non-accelerating) motion and rest as different bodily states, whereas modern theory dictates that they are the same state.