  Viewed retrospectively, no work was more seminal in the development of modern physics and astronomy than Newton's Principia. Its conclusion that the force retaining the planets in their orbits is one in kind with terrestrial gravity ended forever the view dating back at least to Aristotle that the celestial realm calls for one science and the sublunar realm, another. Just as the Preface to its first edition had proposed, the ultimate success of Newton's theory of gravity made the identification of the fundamental forces of nature and their characterization in laws the primary pursuit of physics. The success of the theory led as well to a new conception of exact science under which every systematic discrepancy between observation and theory, no matter how small, is taken as telling us something important about the world. And, once it became clear that the theory of gravity provided a far more effective means than observation for precisely characterizing complex orbital motions — just as Newton had proposed in the Principia in the case of the orbit of the Moon — physical theory gained primacy over observation for purposes of answering specific questions about the world.