We can understand epistemic naturalism as the view that there are cases in which we are justified in holding a belief and cases in which we are not so justified, and that we can distinguish cases of one sort from cases of the other with reference to non-normative facts about the mechanisms that produce them. By my lights, Hume is an epistemic naturalist of this sort, and I propose in this paper a novel and detailed account of his epistemic naturalism. On my account, which I call the determinacy account, Hume characterizes epistemic justification in terms of the mind's feeling determined by the relation of cause and effect to move from one impression (or idea) to an(other) idea. I find a statement of this account, which Hume applies initially to what he calls the second system of realities, in Treatise 1.3.9. After rejecting other accounts of Hume's epistemic naturalism, I show how the determinacy account handles the cases Hume considers later in Treatise 1.3.