 On Hume’s interpretation, Newton’s second rule is the only rule that is said to be derived (un-controversially) “from experience” (THN 1.3.15.6). It is a crucial rule because the fifth and sixth rules are, by Hume’s lights, extensions of the fourth (THN 1.3.15.7–8). Moreover, the conjunction of Hume’s fourth and seventh rules produces a new rule: “An effect always holds proportion with its cause” (“Of Interest”, EMPL 297). We can call this Hume’s ninth rule. If one assumes (or prescribes) that linear causal relationships are the only possible ones, this new rule allows Hume to rule out competing claims that posit the existence of causal relationships that are not “proportional”. Hume can use the rule as a constraint on theory. It plays a prominent role in Hume’s political economy when he rejects Mercantilism and, more important for our present purposes, in his criticism of the use of analogy in the Newtonian argument from Design (see EHU XI, and Dialogues Parts V–VII).