 Hume’s ninth rule also has Newtonian debts. Since the time of Aristotle, many philosophers have asserted proportionality between cause and effect in one fashion or another. Hume’s ninth rule echoes, for instance, a principle Leibniz uses quite frequently: the principle of the equality of cause and effect. It is the basis of arguments Leibniz gives for his conservation principles (e.g., Specimen Dynamicum). But Leibniz’s formulation is in terms of equality and not proportionality. Unlike Leibniz and other Cartesians, Newton preferred to reason in terms of proportionality. Although Hume’s ninth rule is not derived from any of Newton’s official Rules of Reasoning, Newton does implicitly use a rule like Hume’s ninth throughout the Principia. For example, the importance of a proportion between cause and effect is emphasized throughout the treatment of the behavior of bodies in resisting fluids (e.g., the Scholium to Proposition 40, Book 2, Section 7, especially Experiment 14), which, given the importance, for Hume, of treating money as a fluid (“Of the Balance of Trade”, EMPL 312–315; see Schabas 2001) may have caught Hume’s attention. Hume also treats resistance in his footnote to the EHU, 7.1.15. Moreover, there is a prominent place in the Principia (the Scholium following Proposition 69, Book I, Section 11), where Newton implicitly transforms a version of Hume’s seventh rule into Hume’s ninth rule in which proportionality is emphasized. In this Scholium, Newton starts employing the language and emphasizing the importance of proportionality. This is not ruled out by Newton’s own “rules”, but it was not emphasized there. This passage could have attracted Hume’s attention for two reasons: it is one of the few explicit methodological passages in the Principia, and it offers an account of what Newton means by “attraction”. Hume was almost certainly aware of this passage because the proper meaning of “attraction” is discussed, with explicit appeal to Newton’s intentions, in Hume’s footnote to EHU 7.1.25. Hume makes the ninth rule explicit and uses it as a constraint with which competing theories (in political economy, the argument from design, etc) can be ruled out. Hume’s use of it in his attack on the design argument is an instance where Hume attacks Newton with Newtonian tools.