Obviously, if you say you are absolutely certain that everything we think is either false or unknown, including your own certainty of this, no one will ever be able to "prove" anything to you, since you just said you would not admit any premise that might be used in such a proof.

But in the first place, such a certainty is not useful for living, and you do not use it, but rather assume that many things are true, and in the second place, this is not really relevant to Less Wrong, since someone with this certainty already supposes that he knows that he can never be less wrong, and therefore will not try.