To
philosopher David Hume, an "original existence" is a thing
which exists without requiring any importation into the human
psyche. He uses this phrase most famously to describe the human
passions, writing in
A Treatise of Human Nature, that,
A passion is an original existence, or, if you will, modification of existence, and contains not any representative quality, which renders it a copy of any other existence or modification.
Hume means here that the passions exist in a different way than representations of objects do. Whereas the Humean concept of object representation in the mind involves making a mental impression of an external object, Hume is asserting that passions--emotions--aren't representations of anything. They exist and are felt as they actually are.
If you think you're angry, you are. You can't be wrong about this because the passion of anger is, by its very nature, a phenomenon that exists inside of you. But you can be quite wrong about things which do need importation into the mind; you might think you're eating chicken but actually be eating frog legs. This could happen as a result of a mistaken impression you created in your psyche based on sensory data about the food in front of you.
But you can't be mistaken about the feeling of disgust you might feel upon learning of the true species of the animal you've eaten.
Now, your feeling of disgust might change with new information. Say, for example, someone reasons with you that frogs legs are perfectly safe to eat, and are indeed a "delicacy," and that you somehow find this comforting. But Hume would say that your feelings would not change as a result of reason; rather, they are changing as a result of the availability of new information. It is important to note that Hume believes that, "Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them."
So Hume does not believe that you can use reason to control your passions. But you can gather more information and allow the passions to evaluate this added information as it will. Reason is NOT an original existence, because it has component parts, including impressions imported from external objects. Passions, though, are base; passions are original existences.
Quotes above are from p. 266 of the Oxford University Press edition of David Hume's A Treatise of Human Nature, edited by David Fate Norton and Mary J. Norton.