If a skeptic refuses to accept that any statement beats its negation, she will not be completely justified in believing anything. Even if she is incorrect in denying that anyone has knowledge, she is correct in denying that she does. But if she is correct in what she accepts, we would not be completely justified in accepting what we do, and the skeptic would be victorious. So victory depends on whether what we accept is correct, especially what we accept about our trustworthiness. We cannot prove our case, but only argue from within our acceptance system, which is the very thing under attack by the skeptic.
Skepticism is avoided through a theory of justification without a guarantee of truth.
This is all we should really expect. Its competitors would be beaten or neutralized on the basis of the verific system and ultrasystem. Lacking satisfaction in proving the skeptic wrong, we may know that the skeptical hypotheses are false. Some externalists have advanced a similar, though distinct, argument. They reject a closure principle under the known conditional.