I've actually been wondering this ever since I read Descartes (I adore how pretentious that sounds). So for the whole intellectualism/skepticism/agnosticism whatnot, one must eliminate all a priori assumptions, no? But how does one get by with eliminating the assumption that the human intellect is in any way useful at discovering the truth? I mean, why should we possibly assume that that which seems logical to us has any correlation with reality? And if the answer is that it has seemed in the past to do so, then we just get circular because 1)why should we assume that 'past performance gaurantees future results' and 2) how do we know that the reality that was previously conceived actually meant anything?
Meaning that: how is it possible to eliminate one enormous a priori assumption when making any attempt to understand anything?