Qualities, Properties, and Laws in Newton's Induction

Newton's argument for universal gravitation in the Principia eventually rested on the third Rule of Philosophizing, which warrants the generalization of qualities of bodies. An analysis of the rule and the history of its development indicate that the term 'quality' should be taken to include both inherent properties of bodies and relations among systems of bodies, generalized into 'laws'. By incorporating law-induction into the rule, Newton could legitimately rebuff objections to his theory by claiming that universal gravitation was justified by his method even if he could not specify the cause of gravity.