Newton's defence, in the Principia, of the existence of a universal force of gravity immediately gave rise to a wave of philosophical doubts and objections. His own remarks on the nature of gravitation are not easily amenable of a consistent, uniform interpretation. This paper begins by reviewing briefly these remarks. Its primary objective is, however, to examine how this important scientific issue contributed to demarcate two main epistemological positions on the status of scientific hypotheses transcending immediate experience. In Newton's time, two exponents of these positions were, respectively, Locke and Berkeley. Intriguingly, Newton fuelled both the Berkeleyan, instrumentalist interpretation, and the Lockean, realist interpretation. On the one hand, immediately after offering the definitions of "quantities of centripetal forces" (Definitions 6-8), he warned that he was treating these forces "mathematically", without pronouncing on its physical status. This remark lends support to Berkeley's anti-realist interpretation of forces, as Berkeley himself was keen to point out. But in the General Scholium, at the end of the book, Newton declared that he could "explain" certain important phenomena of motion by the force of gravity, although he had not yet been able to explain the cause of this force, adding, famously, that he would "feign no hypotheses" about this issue. A natural, realist interpretation of this statement is that Newton believed that he could infer, from the phenomena, the existence of gravity, as a real, causal physical agent, but that he had not yet succeeded in discovering the cause of this cause. In discussing the shortcomings and advantages of these opposing views, we indicate that although the realist interpretation appears to do more justice to the actual development of physics after the Principia, Berkeley's interpretation has the philosophical merit of representing a firmer adherence to empiricism, a position valued, in one way or another, by all parties involved in the dispute on the nature of gravitation.
Force of gravity; Newton; Locke; Berkeley; Scientific realism; Instrumentalism