Why did Hume think this?  Hume holds that “reason is the discovery of truth and falsehood” (p. 458).  Since beliefs aim to correctly represent the world—and the beliefs that succeed in representing the world are true, while the beliefs that fail to do so are false—they can be assessed as reasonable or unreasonable. But passions, as well as intentions and actions, are “original existences.” They don’t purport to represent or “copy” some other “existence” (p. 415). Specifically, they don’t aim to correctly represent the world, and so they can’t be true or false.  Hence, they can’t be assessed as reasonable or unreasonable. 