Both Hume and Kant regard the motive of duty as a special one. Hume thinks that it has to be explained in terms of other motivations: self-interest or benevolence. Kant thinks that to explain the motive of duty in terms of any other motivation would be to distort its character and so he analyzes it in terms of the form of a law rather than other motivations.
We had a very fruitful, sophisticated discussion of these two thinkers. In a way, it was more sophisticated than I had hoped since we went past the point that I wanted to identify.
The point that I had hoped to identify is that Kant and Hume have different opinions about what the central case of moral virtue is.
For Hume, it’s benevolence: the desire to help others. The benevolent person wants to help others and feels good about doing so.
For Kant, the central cases involve overcoming one’s natural inclinations. Benevolence, like self-interest, might lead one to do one’s duty but it isn’t the same thing as the motive of duty and actions motivated by it rather than the motive of duty lack moral worth.
Kant agrees with Hume that the benevolent person has good qualities. He just denies that these qualities are relevant to the special value of moral worth.
Hume agrees with Kant that a person can be motivated to do the right thing against his natural inclinations (3.2.1,8). He also agrees that a virtuous person may not bring about any good results. He just regards these as special cases.
His description of the first case is too brief to say exactly what he had in mind with any precision. But it seems at a minimum to involve the point that it is sometimes possible to do what you are required to do, even if you lack virtuous motivations. His general disinclination towards thinking that we might be motivated simply to comply with rules gets in the way here, I think: he’s looking for a motive when, for the purposes of justice, at least, we’re usually satisfied with behavior alone. That said, the case he uses to illustrate the point, gratitude, does typically involve the appropriate motivation: if you write me a thank you note without actually feeling grateful for my gift, that’s better than nothing, but it’s not good enough.
The virtuous but ineffectual person is easier to explain. We identify character traits as virtues because they typically produce good effects. Someone might have those character traits but not bring about good effects for perfectly good reasons. Hume’s only point is that these have to be special cases because we only identify the trait as good in the first place dues to its good effects. From Hume’s perspective, Kant is right to say that the effects of one’s behavior do not always matter as far as virtue is concerned, but he draws an inappropriate conclusion from this observation, namely, that the effects never matter.
Back to my main point. If you think that Hume is right about the central cases for virtue, you may well think that Kant’s characterization of the good will and its value is mistaken. If you think that Kant is right about the central cases, you will have the opposite opinion.
I don’t know that there’s any way to prove that one side or the other is correct here. I just wanted to point out what I regard as the most basic point on which the two sides come apart.