  Because Newton's theory of comet trajectories depended only on that part of the theory of gravity that was least controversial — inverse-square centripetal accelerations everywhere around the Sun — it did not provoke much philosophical resistance. The success of the method provided evidence that these centripetal forces act equally on comets, contrary to Hooke's proposal in his Cometa of 1678 that comets must consist of a fundamentally different kind of material from the planets insofar as they do not respond to the forces directed toward the Sun in the same way. The success of the method also provided strong evidence that inverse-square forces toward the Sun hold throughout the space surrounding it, for not only do comets traverse the spaces between the planet orbits, but also their trajectories, in contrast to those of the then known planets, are often highly inclined with respect to the plane of the ecliptic. Most of all, however, the success of the method provided the most compelling evidence against not only Cartesian vortices, but all theories claiming that the planets are carried around the Sun by fluid vortices. Corollary 3 to Proposition 39 in all three editions summarizes the argument: