Over at Ayn Rand Contra Human Nature, Daniel Barnes (who I understand is at least sympathetic to critical rationalism) provides the following quotation from Rand:
“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). (Ayn Rand, “Philosophical Detection”, Philosophy: Who Needs It? p.14)
Barnes provides a subjectivist account of the comprehensively critical rationalist’s response, but it should be objectified to explain its true strength: talking about the status of mental states tells us nothing about the epistemological status of statements. Is the statement “Nobody can be certain of anything,” when rephrased as “We cannot know of any statement as justified” justified, true, or believed?
Someone may in due time develop a powerful objection to the statement, but so far none have been raised and survived even the most superficial criticism. If some statement is found that is known to be justified, then the sentence would be false. This negates the first criteria of justification.
It’s then permissible to adopt the statement as true. Adopting a statement as true, provisionally as it may be, does not make the statement true. This means that we cannot know (in the sense of having sufficient reasons) that the statement is true. The statement is then not self-refuting, but a conjecture about a critical failure in justificationist theories of knowledge: they are dealing with a pseudo-problem, a ‘false consciousness’ about ‘knowledge.’
It is not my duty to justify conjectures, for the conjecture asserts that very impossibility. To anyone that adopts the conjecture, according to their own lights, such an activity would be a waste of time, for it is prima facie absurd. It is my duty, and the duty of others, to criticize conjectures.
I do believe the statement is true, in much the same way I believe that I exist, but my beliefs, my psychological states, are unimportant–truth is primary.