I argue that Hume's characterization of the passions as "original existences" is shaped by his preoccupation with Stoicism, and is not (as most commentators suppose) a ridiculous or trifling remark. My argument has three parts. First, I show that Hume's description of the passions as "original existences" is properly understood as part of his argument against the possibility of passions caused by reason alone (rational passions). Second, I establish that Hume was responding to the Stoics, who claimed that a rational passion is caused by a special type of impression that accurately represents its cause—the divine reason administrating the cosmos—in virtue of resembling it. Third, I argue that Hume rejects the Stoics' claim by appeal to the following feature of his general account of impressions and ideas: for Hume, ideas are exact "copies" or representations of the impressions that caused them, but simple impressions are "originals" that do not represent in virtue of resembling their external or physiological causes. On my reading of Hume, the 'original existence' passage therefore signals his rebuke of a key claim in the Stoics' theory of rational passions.