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The point here is that in this case, and in all ordinary
cases of incredulity, the grounds for the doubt can, in principle, be
removed. As Wittgenstein would say, doubt occurs within the context of
things undoubted. If something is doubted, something else must be held
fast because doubt presupposes that there are means of removing the
doubt.^[2]
We doubt that the bird is a robin because,
at least in part, we think we know how robins typically fly and what
their typical coloration is. That is, we think our general picture of
the world is right — or right enough — so that it does
provide us with both the grounds for doubt and the means for
potentially removing the doubt. Thus, ordinary incredulity, say about
some feature of the world, occurs against a background of sequestered
beliefs about the world. We are not doubting that we have any knowledge
of the world. Far from it, we are presupposing that we do know some
things about the world. To quote Wittgenstein, “A doubt without an end
is not even a doubt” (Wittgenstein 1969, ¶ 625).

As regards the former, skeptical views typically have an epistemological form, in that they are focused on the epistemic status of certain beliefs. For example, one common variety of skepticism concerns our beliefs about the past and argues that such beliefs lack positive epistemic status – that they are not justified, or are not rational, or cannot constitute knowledge (and perhaps even all three). Where skepticism does not have this epistemological focus, then it tends to be of an ontological form in that it is directed at beliefs about the existence of some supposedly problematic entity, such as the self or God. Here the target of the skepticism is not so much one’s putative knowledge of these entities (though it may be that as well), but rather the claim that they exist at all.

As regards the latter, one can differentiate between skeptical views that are either local or radical. Local varieties of skepticism will only concern beliefs about a certain specific subject matter, such as beliefs in abstract objects or the conclusions of inductive arguments. Since ontological varieties of skepticism tend to be concerned with the existence of particular sorts of entities, they are usually (though not always) of this local form. In contrast, radical forms of skepticism afflict most of our beliefs and thus pose, at least potentially, the most pressing philosophical challenge.