Why 'we don't know the sun will rise' is a different kind of ignorance

Philosophers use "skepticism" in a narrower sense than the rest of us. The interesting versions, according to the Stanford Encyclopedia of Philosophy, claim we do not know propositions we ordinarily take ourselves to know.

That has to be separated from ordinary skepticism — the claim that we don't know things we'd happily admit we don't know. Not knowing whether the Milky Way contains an even number of stars is the ordinary kind. Not knowing that the sun will come out tomorrow is the philosophical kind, because the whole point is that it targets something you thought was settled.

The encyclopedia's example is worth sitting with. Most of us don't even believe the star-count claim, and knowledge is widely taken to require belief, so there's nothing to unsettle. Even someone who happened to believe it correctly wouldn't know it, because they aren't justified in believing it — though they aren't justified in disbelieving it either.

That last bit is the useful machinery: belief and disbelief are both doxastic attitudes, and suspending judgment, or never considering the proposition at all, is another option entirely. Philosophical skepticism is interesting precisely because it goes after a proposition you can't comfortably file under "never thought about it."