  The aspect of Newton's method that did produce controversy at the time was his insisting that he had established conclusions about the physical species and physical proportions of celestial forces while holding questions about their physical causes in abeyance. This was the core of the complaint by Cartesians that the Principia was a work of mathematics, not physics. For Newton's two most important critics, however, Huygens and Leibniz, the objection was not to holding the question of physical causes open, but to accepting certain conclusions that in their mind ruled out the very possibility of a proper answer to the question of physical cause. The defect in Newton's method lay in its not imposing the constraint on theory that all action be through contact, and not at a distance. The violation of this constraint lay behind Huygens's remarking,