How would she know with certainty whether the thoughts (chemical secretions?) in her head correspond to external realities?
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).