“We cannot begin with complete doubt. We must begin with all the prejudices which we actually have when we enter upon the study of philosophy. These prejudices are not to be dispelled by a maxim, for they are things which it does not occur to us can be questioned. Hence this initial skepticism will be a mere self-deception, and not real doubt… Let us not pretend to doubt in philosophy what we do not doubt in our hearts.” – C. S. Peirce, “Some Consequences of Four Incapacities Claimed For Man”
This applies at least as much to ethics as it does epistemology.
Can we really doubt the immorality of the worst atrocities, even if we are unable to explain or account for morality?
This is a real living question, a doubt in my heart about the dubitability of morality.