  No work of science has drawn more attention from philosophers than Newton's Principia. The reasons for this, however, and consequently the focus of the attention have changed significantly from one century to the next. During the 20th Century philosophers have viewed the Principia in the context of Einstein's new theory of gravity in his theory of general relativity. The main issues have concerned the relation between Newton's and Einstein's theories of gravity and what the need to replace the former with the latter says about the nature, scope, and limits of scientific knowledge. During most of the 18th Century, by contrast, Newton's theory of gravity remained under dispute, especially because of the absence of a mechanism — in particular, a contact mechanism — producing gravitational forces. The philosophic literature correspondingly endeavored to clarify and to resolve, one way or the other, the dispute over whether the Principia should or should not be viewed as methodologically well founded. By the 1790s Newton's theory of gravity had become established among those engaged in research in orbital mechanics and physical geodesy, leading to the Principia becoming the exemplar of science at its most successful. Philosophic interest in the Principia during the 19th Century therefore came to focus on how Newton had achieved this success, in part to characterize the knowledge that had been achieved and in part to pursue comparable knowledge in other areas of research. Unfortunately, a very large fraction of the philosophic literature in all three centuries has suffered from a quite simplistic picture of the Principia itself. The main goal of this entry is to replace that simplistic picture with one that does more justice to the richness of both the content and the methodology of the Principia.