Example: I know there is a table in front of me (both sides agree: if I know anything, I at least know that)
Skeptic rejects that, and therefore rejects that you know anything
Many different skeptical arguments, often of the form “There exists a scenario in which you are having the same phenomenological experience yet there is no table, so how can you know whether you are in that scenario vs the genuine experience scenario?”
Examples: dreaming, evil genius, brain in vat (Putnam)