Hume clearly distinguishes between better and worse causal inferences, notably in his “Rules by which to judge of causes and effects” in Treatise 1.3.15. Although Hume describes these rules as “all the logic I think proper to employ” in his philosophy (Treatise 1.3.15.11), the literature has paid relatively little attention to it. This chapter will investigate these eight rules, as well as their basis in Hume’s discussion of general rules. The paper then examines two controversial causal inferences in light of these rules by which to judge of causes and effects: first, the postulation of the calm passions; second, the missing shade of blue. As Hume himself recognizes, the correct application of these rules can be an enigmatic affair. Nevertheless, I find that there is reason to think that while Hume abides by these rules in his postulation of the calm passions, the missing shade of blue constitutes a gross violation of these same rules.