Even a thinker who finds some truths beyond doubt can ask a narrower question: whether our perceptual beliefs reach a world outside the mind at all. The skeptic argues that no experience distinguishes a veridical perception from a matching dream or deception, so the existence of mind-independent things is never secured by experience.
This is a tighter question than whether knowledge is possible at all: it grants that some things may be known while denying that the external world is among them. The broader dispute over certainty and suspension is treated under The Possibility of Knowledge.