THIS PAPER INVESTIGATES THE LAWS OF MOTION in Newton and Descartes, focusing initially on the first laws of each. Newton's first law and Descartes' first law were later conjoined in the minds of philosophic interpreters in what thereafter came to be called the law of inertia. Our analysis of this law will lead to the special significance of Newton's third law, and thus to a consideration of the philosophical implications of Newton's three laws of motion taken as a whole. This (...) paper also involves backward glances at an earlier tradition of physics: How do the laws of motion compare with certain basic elements of the philosophy of nature? Principal results and conclusions are: There is no one law of inertia; rather, Newton's first law is distinct from Descartes' in its implications for our understanding of nature. The concept of force entailed by Newton's three laws of motion is not the same as cause of motion as we ordinarily experience and understand it. Newton's three laws, unlike Descartes', do not rule out internal causes of motion in bodies, and are, therefore, compatible with the traditional definition of nature. The motor causality principle, often stated as "all that is moved is moved by another," is not as such incompatible with Newton's three laws of motion. (shrink)