 The rule is that we should treat well-confirmed propositions as true (or nearly true) until there are deviations that promote new research, which, in turn, lead us to refine our original propositions or reject them for new ones (Smith 2002). But while one has a theory, one must not be distracted by possible differing explanations for the found regularities until one has empirical reason. One accepts a theory as true as a means to developing a better theory. As Newton writes in the “Preface” to the Principia, “the principles set down here will shed some light on either this mode of philosophizing or some truer one” (emphasis added). Thus, Newton accepts that physical inquiry is forward-looking and may be open-ended; not only its theories may evolve, but also its methods. Rule IV implicitly accepts that the future may bring surprises and new evidence, and, thus, anticipates Hume’s fallibilistic insights. As commentators on Hume have noted, this strand of Hume’s skepticism is shared by Newton. It is less clear if Hume learned it from Newton because there is no evidence that he was acquainted with Newton’s fourth rule of reasoning (added to the third edition of the Principia); there is no equivalent in Hume’s rules of reasoning to Newton’s fourth rule. In the section on Hume’s Rule of Reasoning  (§4.5),  the significance of the absence of an equivalent to Newton’s rule 4 in Hume’s thought is explored.