In the first 100 years of the reception of Newton’s 1687 Principia (e.g. Huygens, Cotes, Leibniz, Euler, and Kant), as in the last 60 years of its reception (e.g. Koyré, Stein, Densmore, Harper, Smith, and Biener/Smeenk), commentators have observed that Newton must presuppose that Law III applies to gravitational interactions in order to deduce the law of universal gravity.  Apart from issues of apparent circular reasoning, mechanical doctrines of explanation find this presupposition unsettling: how to conceive, and how to justify, that bodies interact immediately at a distance?  Newton’s own silence on these questions gives the impression either that he cannot address them, or that his ‘mathematical’ treatment of forces is conjectural and disconnected from empirical evidence.  Based on unpublished manuscripts from 1685 of Newton’s first derivation of universal gravity, I argue Newton had already posed these questions to himself and framed a novel way of answering them.  At the heart of this framing is a distinction between what I call ‘active and passive quantities of matter’, a modern distinction no prior commentator has had reason to believe Newton was in the position to make.  I will show that this distinction provides a new way to understand Newton’s mathematical theory of forces, its responsiveness to empirical evidence, and its role in the deduction of universal gravity. After setting out my argument, I will briefly sketch five lines of research into which my interpretation leads: (1) the influence of magnetic theory on Newton; (2) Newton’s place among paradigms of mathematical and mechanical explanation; (3) a new ‘relativistic’ interpretation of Newtonian absolute space; (4) Newton’s empiricism, rules of reasoning, and the role of analogical evidence; (5) closer scrutiny of the deductive logic of the Principia.