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