 On the whole, the vortex theory offered the natural philosopher a highly intuitive model of celestial phenomena that was compatible with the mechanical philosophy. The theory was regarded as superior to Newton’s theory of universal gravitation since it did not posit a mysterious, occult quality (gravity) as the cause of the planetary orbits or the free-fall of terrestrial objects. The vortex theory likewise provided a built-in explanation for the common direction of all planetary orbits. Additionally, the vortex theory allowed Descartes to endorse a form of Copernicanism (i.e., sun-centered world) without running afoul of Church censorship. Since the alleged motion of the earth was one of the Church’s principal objections to Galileo’s science, Descartes hoped to avoid this objection by placing the earth at rest within a vortex band that circled the sun, such that the earth does not undergo a change of place relative to the containing surface of the neighboring material particles in its vortex band (Pr III 24–31; and section 3). Through this ingenious maneuver, Descartes could then claim that the earth does not move—via his definition of place and motion—and yet maintain the Copernican hypothesis that the earth orbits the sun. “The Earth, properly speaking, is not moved, nor are any of the Planets; although they are carried along by the heaven” (Pr III 28). In the long run, however, Descartes’ vortex theory failed for two fundamental reasons: first, neither Descartes nor his followers ever developed a systematic mathematical treatment of the vortex theory that could match the accuracy and predictive scope of the (continuously improving) Newtonian theory; and second, many attempts by Cartesian natural philosophers to test Descartes’ various ideas on the dynamics of circularly moving particles (e.g., by using large spinning barrels filled with small particles) did not meet the predictions advanced in the Principles (see Aiton 1972).