 Locke may have regarded Newton as a fellow enthusiast for the experimental philosophy, but there are reasons to think that his embrace of the mechanical philosophy presented him with a difficulty in interpreting the consequences of Newton’s theory of universal gravity in the Principia. For Newton’s theory seemed to be in tension with a mechanist constraint on views of causation, at least from Locke’s own point of view. In the first edition of the Essay, Locke articulated a central tenet of mechanist thinking about causation when he wrote: “bodies operate by impulse and nothing else” (Essay, II.viii.11). Impulse refers here to contact action. In correspondence with Locke that would prove to be influential, Bishop Edward Stillingfleet questioned this view from the Essay, contending that Locke must jettison the idea of human liberty if he insists that bodies can operate solely by impulse, presumably on the grounds that the human will cannot be understood to cause bodily action in that manner. In a famous exchange, Locke responded in part by reformulating his commitment to the mechanist view that all causation involving material bodies must be by contact (impulse) alone: