In the “Appendix” to the Treatise, Hume claims that he has discovered a “very considerable” mistake in his earlier discussion of the self. Hume's expression of the problem is notoriously opaque, leading to a vast scholarly debate as to exactly what problem he identified in his earlier account of the self. I propose a new solution to this interpretive puzzle. I argue that a tension generated by Hume's conceptual skepticism about real “principles of union” and his account of fictions of (...) the imagination is the defect identified in the “Appendix.”. (shrink)