To question that error is impossible, what must we do?

Question

As we have learned from Descartes (even though he himself seems sometimes to lose sight of this lesson in the later stages of the Meditations), it is very hard to find beliefs for which there is not some possible way in which the proposition in question could be false in spite of the reasons or justification for thinking that it is true. Given possibilities like the evil genius, it is doubtful whether any beliefs about the material world outside of our minds or about the past will count as knowledge, according to the strong conception. Indeed, contrary to Descartes, it can even be questioned whether beliefs about our own states of minds will constitute knowledge according to this strong standard: is it really impossible (given my evidence or basis, whatever exactly it is) that I could be mistaken about whether I am experiencing a specific shade of color or about how severe a sensation of pain is?
(Laurence BonJour: Epistemology, 2nd ed)

BonJour writes "it can even be questioned" — but what does this mean? If we want to question that we can know something with absolute infallibility, what must we do?

Just questioning something à la "entering a state of not being convinced", surely can't be enough, or can it?

• Do we actually have to present a situation where error does occur, like Descartes' evil genius?

• Do we have to refute the arguments that try to prove the infallibility of a certain belief?
  But at some point we might be confronted with a "simple intuition of the mind" as Descartes categorizes his cogito. What then? Simply claiming "I do not have this intuition!" would be extremely unsatisfactory.

• Do we just have to question the arguments for the infallibility of a certain belief? But this ends in an infinite regress, if we don't know what "questioning" means.

Answer 1

In my experience, "It can even be questioned" is a very low but very important bar in philosophy. In the most literal sense, it means someone who is asking such a question cannot be dismissed on the grounds that they must assuredly be wrong without even looking at the proof itself. Contrast this to one of the millions of 1=0 "proofs" out there. I can dismiss most if not all of them without actually finding a flaw in their logic, simply because I know there must be one there if I dig at it (typically a well hidden divide by zero).

Arguments that question whether we can know something with absolute infallibility must be accepted or dismissed by the quality of the proof itself rather than simply dismissing them on their final outcome. It is certainly worth noting that BonJour is not stating a proof that absolute infallibility cannot exist, merely that the arguments against it are strong enough that they should not be dismissed in hand. It says that such an assumption should deserve at least a well thought out lemma in your paper, rather than just a footnote stating that the proof of this assumption is left as an exercise to the reader.

Answer 2

Doubt, Arendt points out is the foundation of Modern Western Philosophy; but by this she does not mean everything is doubted, or made doubtful; it's a new beginning and a new faith, and it's this faith she says that has carried out the tremendous work in the modern era of thinking - a new critical spirit of enquiry.

Had doubt qua doubt been properly implanted in this spirit, then like Hamlet who doubted himself and everything around him, nothing substantial would have been achieved.

Descarte himself begins dubito ergo sum (I doubt therefore I am), and later merely says cogito ergo sum (I think therefore I am); and one might profitably say that the synthesis of dubito (I doubt) and cogito (I think) is I enquire (and of which I don't know the Latin - perhaps someone can fill in?)