From Ayn Rand's essay "Philosophical Detection."
"Don't be so sure -- nobody can be certain of anything." Bertrand Russell's gibberish to the contrary notwithstanding, that pronouncement includes itself; therefore, one cannot be sure that one cannot be sure of anything. the pronouncement means that no knowledge of any kind is possible to man, i.e., that man is not conscious. Furthermore, if one tried to accept that catch phrase, one would find that its second part contradicts its first: if nobody can be certain of anything, then everybody can be certain of everything he pleases -- since it cannot be refuted, and he can claim he is not certain he is certain(which is the purpose of that notion).
She's absolutely correct about Russell. But her statement doesn't address the problematic "gap" between the "knower and thing known." I don't recall her addressing the issue in any of her writings.