Let's take mathematics as a first case in epistimology.  If you were a 
defender of mathematics and were defending the position that a=a, you would
(as a matter of proving it) first have to make the position understandable, 
and then show that it is true.  On the first front, you would have to ditch
mathematics altogether and end up saying something like "It means that a
thing is always the same as itself" (which is why mathematics relys on the
form of existence and nature for it's truth). But then, having said this,
how do you prove it without begging the question?  Any assertion you might
make in it's defense would rely on the assertions meaning being the same as
itself.  In mathematics, it's an axiom-- a starting place, immune from
questioning or need of proof.  It's better elucidated version (a thing is
the same as itself), having the same meaning, can also be called an axiom,
or, an item of faith-- a starting place.

   Now, how would you think the mathematicians decided on this particular
axiom?  What makes the Rationalists (and we Realists) take on it's meaningful 
version?  Ought we doubt it?  Is nothing absurd?

   Another such axiom, an axiom of science, as a matter of fact, is the
position that the things we perceive exist as we perceive them.  That is,
there is no such thing as an 'empirical fact which supports a theory' if
there is also no such thing as an empirical fact.  Likewise, how do you
form a theory about the origin of the universe if you don't know whether
or not there IS a universe?  Lastly, we talk about the scientific method
and about people who believe in it-- well, what makes you think there ARE
'people' or methods they believe in?  Ought we doubt perception?  Does
nothing exist?

    Putting it together now, how do we answer nihilism on the one hand, and
solipsism on the other?  Descartes gave up questioning his reason (he refused
to face nihilism) and used 'cognito ergo sum' to arrive at the same place the
solipsists did.  He then used the ontological argument in a certainly creative
way, to try and rise above solipsism.  The ontological argument, however, and
his subsequent reasoning though failed, and so, in the end, on both fronts,
Descartes failed in this mission.

   So, where are we now?  Certainly the solipsists and the nihilists are foolish
and quite wrong, but how do we come to know this?  By what method do we come to
know that a thing is the same as itself, and that our perceptions are of real
things?  Do we go the way of the skeptics, using reason and perception to defend
their skepticism of each (certainly a nihilistic position if ever there was 
one)? And suppose there is such a method.. how do we avoid begging the question
by assuming that the method is the same as itself, and that there exists things
to use the method on?  This is a rhetorical question, obviously-- there cannot
exist a method which isn't itself (we would not even not even have a subject
for this predicate) without reason and perception (Realism in other words) as
a starting point.

   However, we are still left answering how we know that the solipsists and
the nihilists are wrong.  Without a method to rely on, we must use the only
non-method at our a-prior disposal-- faith.  Now, it should be noted that 
I am not defending positivism here.  The positivists have a cavalier attitude
about what they have faith in; almost anything will do-- this is not what I
suggest.  What I _do_ suggest is that we have faith in that which is evidently
true, that which we can not even bring ourselves to question.

    Does this include many items of common sense?  No.  You'll find that people
have little problem doubting much of what seems to be common sensical to them,
once they are asked about it.  Even then, you'll find that the things they 
really _do_ believe, those few items that really _are_ faith, can be ill placed
at times, but then, were faith an infallible method, it would require the same
defense the scientific method does.  As it is, we only need note that faith is
required for reason and perception (for the starting points of truth and
epistimology) just as you need axioms for there to be truth in mathematics.
Everything else, including common sense as previously noted, ought be 
questioned. 

    Faith is required for truth, but not truth for faith (unfortunately).


   Q: What about the scientific method?  It's self correcting after all!

   If the method is used to defend itself, then it begs the question
obviously.  If some other method is used to back up the scientific 
method, then it too needs defense.


   Q: What about the silly things people have believed in the past?  That the
earth is flat, for instance, or that the sun is moving when it rises.

   None of those are evident.  I've never seen the whole earth, so any such
assumption would be falsely assumed.  As to the other, even the ancients riding
their horses knew that it was them and not the landscape that moved, and so
any such assumption regarding the meaning of the perception of the sun rising
would also be fallaciously drawn.  In other words, I need only answer that
these are not self evident (remember the distinction I drew above with
positivism).