What, if anything, changes between Hume's moral theory as presented in the Treatise of Human Nature and then in the Enquiry concerning the Principles of Morals? This question has received increased attention, especially focused on Hume's presentation of sympathy and humanity, and the connection of those principles to Hume's account of moral sentiments. While there is a strong consensus that Hume is making important stylistic changes to the presentation of his views, scholars are divided on the question of whether there (...) are also substantive changes to the views. This paper sides with those who hold that there are substantive changes, offering new evidence for that position. I argue that Hume revised his Treatise account of sympathy to better accommodate cases of affective difference and incapacity. After presenting these cases, I show that Hume's Treatise account of the sympathy mechanism and its role in the production of moral sentiments is more seriously threatened by the problem of affective difference and incapacity than the corresponding account in the second Enquiry. This gives us persuasive evidence in support of the interpretations that hold that there are substantive changes in the second Enquiry, while also helping to explain Hume's motivation for such changes. (shrink)