Duties concern things that are voluntary. I do think that if you have a moral duty to bring me back the book you borrowed, that implies, roughly, that your doing so depends on your wanting to do so: if you want to bring me the book, you will. This is not the case if you are stuck at some airport due to a snowstorm, far away from me. This, however, is not the same as "ought" implying a metaphysical "can".

Moral virtues and intellectual virtues are very different from each other, and moral virtue has to do with motivation, not cognition. Moral virtue requires a human level of intelligence, but it doesn't require that one be an intelligent human.

When you find yourself reluctant to sit on a chair because it had unexpectedly collapsed in the past you might shake your head and think "there, I'm so irrational!". But your reluctance to sit on a probably rickety chair is not irrational - you think it's irrational because you have a false view of what irrationality is.

I don't think any of my desires or beliefs or other mental states are external to me. Many people will occasionally feel alienated from the motives for an action - "whatever possessed me to do that?". Note, however, that some people feel alienated from the white hairs that recently appeared on their heads - "who put them there?", they might ask the mirror - but the white hairs are still theirs. Similarly, I might feel alienated from an action or a mental state because it does not fit with my visceral self - image.

If I am told my loving you had to happen because of some Freudian stuff about my childhood, that might be degrading or deflationary, but if I am told it had to happen because you are such a wonderful person that I couldn't possibly resist your charms, or because we are so incredibly compatible, then necessity seems very romantic.

If I make a stupid decision but don't execute it because I'm, say, lazy, then I'm lucky, not rational. However, at other times a person acts for good reasons just as she does what she thinks she shouldn't do, not knowing that they are good reasons. Just like sometimes we are a lot less rational than we think we are, it is also true that sometimes we are a lot more rational than we think we are.

Duties concern things that are voluntary. I do think that if you have a moral duty to bring me back the book you borrowed, that implies, roughly, that your doing so depends on your wanting to do so: if you want to bring me the book, you will. This is not the case if you are stuck at some airport due to a snowstorm, far away from me. This, however, is not the same as "ought" implying a metaphysical "can".