Does the fact that such knowledge-claims are fallible really mean that they are dubitable?
That is, can I really ‘doubt’ that I have a hand when I am perceiving my hand under conditions I believe to be optimal? (the reason why Descartes introduces the argument from dreaming lies in the point he is making about when he says that he would be insane,)
It is indeterminate what people can doubt (or even "doubt"). What is determinate is whether a person can rationally
doubt he has a hand. A related (although different question) is whether a person is
doubting just because he sincerely says he is doubting. For instance, when a person uses his hand to pick up an object, and asserts that he doubts that he has a hand, is he really doubting that he has a hand, or does he just think he is, but really is not?