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.