This paper explores the ontological categories in which Hume’s texts seem to justify placing his central terms of art, impression, and idea. The options of impressions/ideas as “acts” (or “states”) and as “objects” (inner mental particulars) are discussed, with reference to interpretations forwarded in the secondary literature as well as to Hume’s texts. Variants of both these options are explored and assessed, as are relations between the categoreal type for impressions and ideas and Hume’s views on the “external world.” I argue as well that there is an interesting, though elusive, alternative which most commentators neglect, but which appears in later empiricist philosophy, viz., that Hume intends impressions/ideas to be a new category of item, intermediate between act and object. I conclude that while some Humean texts suggest such a view, the likeliest interpretation is a version of the “act” or “state” construal.

In Treatise 1.1.7, Hume poses the problem of how to understand the “distinction of reason” that figures in the philosophies of the medievals, Descartes, and the Port Royalists. The problem in a nutshell is that a distinction of reason is supposed to be a distinction in thought between things that are inseparable in reality; yet according to Hume’s own principles, whatever things are distinct are separable in thought and therefore also in reality. It follows that things inseparable in reality should be neither distinguishable in thought nor distinct, period, so a distinction of reason ought on Hume’s principles to be impossible. Yet Hume goes on to try to make room for it, to the consternation of many commentators. I argue that he can indeed make room for it; the key is to recognize that ‘distinction of reason’ is an incomplete symbol.