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".

Sometimes I suspect that there are two prototypes of philosophers who write about humans - I call them "celestials" and "terrestrials", without implying that celestials have their heads in the clouds or that terrestrials have theirs buried in the ground. The difference between these two types is not so much in their theories but in whether or not they would find it a very sad thing if it turned out that the only way a human is superior to a wolf is this: the human brain is significantly more capacious and complex.

Suppose whether or not someone tells me a lie depends only on whether he wants to, but he is morally indifferent, he doesn't care much about the truth or about me, and his self interest, which he worships, tells him to lie, and so it comes about that given his psychology, it is a forgone conclusion that he will lie to me. I think in this case he is still blameworthy, and that implies, among other things, that he did something he ought not do.

Sometimes we deliberate - for example when we plan a long trip or - if we are not math wizards - when we solve long division problems. However, if we deliberated every time we acted we would never get through the day. Most of the time, we act for reasons without deliberation. I am not just talking about cases of simple, habitual action, like brushing your teeth, but also about more sophisticated action.

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".

The fact that only humans above a certain age can be morally virtuous, rather than babies or cats, means that that being moral requires some cognitive ability. If virtue is about desires, it is worth remembering that you can't desire some things without being able to conceive of them. Suppose a virtuous person will desire to make people happy and desire to tell the truth. You can't desire to make people happy without having the concept "happy" and you can't desire to be truthful if you don't have have the concept "lie", so a cat or a baby cannot desire these things.

When I see two owls and then two more owls and conclude that I see four owls, I am responding to reasons, and it better not be my choice to believe that two plus two always equals four. If I am a rational person, I will have that belief by necessity, whether I wanted to or not. So it's not that strange to discuss responding to reasons out of necessity.