ABSTRACT
This chapter clarifies the role of the four Rules for the Study of Natural Philosophy in the argument for universal gravity as it is presented in Book 3 of the Principia. It specifically illuminates how Newton uses the Rules to extend to celestial bodies what has been empirically and experimentally confirmed of terrestrial bodies and thereby highlights Newton’s use of empirical and experimental evidence to justify claims about bodies on which no experiments can be performed. The various instances at which Rules 1, 2, and 4 are applied in the argument for universal gravity are discussed in the opening section of the chapter. In the second section, consideration is given to the single instance at which Newton uses Rule 3 to argue for universal gravity. The chapter concludes with a discussion of the advantages and disadvantages of applying the long-standard One-Set Reading of Rule 3 to key selections from the Principia.
