Quine propagated forward an approach to epistemology wherein there was no need of any sort of justification "beyond observation and the hypothetico-deductive method" (Quine 1981).
Quine is going after a foundationalist view of knowledge acquisition then, wherein one can attain knowledge only in a context of possessing some prior, more certain knowledge.
But while any anti-foundationalist might agree with him here, they might not agree with Quine that there is no certain knowledge period.
That is to say, there seems to be a gap between admitting that all knowledge is based on experience, or "observation and the hypothetico-deductive method", and admitting that all knowledge is uncertain.
How does Quine effectively cross that gap?