  The added evidence supplied by the theory of comets highlights a sometimes overlooked aspect of Book 3. The development of evidence for the theory of gravity in it does not end with the “deduction” of the law of universal gravity at the beginning, but continues all the way through the Book. During the eighteenth century attention focused overwhelmingly on the evidence supplied by Newton's theory of the figure of the Earth and the variation of surface gravity, the theory of the tides, the quantitative derivations of select lunar inequalities, the derivation of the precession of the equinoxes, and the theory of comets. This suggests that, both then and now, the “deduction” of universal gravity should not be read in isolation from the rest of Book 3, but instead the entire Book should be seen as offering a sustained evidential argument for the theory. Read this way in the context of the rest of the Book, the “deduction” is most appropriately viewed as intended to establish universal gravity, but only provisionally, as a theory on which further research is to be predicated, research that will continue to bring evidence to bear on the theory.