@FBM,
Quote:Why is it necessary to believe in first-hand experience?
It is not necessary, but you mentioned first-hand experience as the source for your knowledge, a source supposedly better than belief. You said "it's not belief if it is based on experience". But in order to base anything on experience, you must believe that experiences can be had (ergo that there is a self, a world, and senses to link those two), and that knowledge can be derived from said experience (another fairly tightly-packed assumption, which we can unpack another day).
I am getting a bit tired of stating the obvious here, and several times, but in a nutshell: no mental life can be have without a number of mental axioms, which everybody has to go by. Even if you don't believe in them (or to say it more precisely, even if you shroud them in a postulated haze of disbelief), you do in effect "go by" them. E.g. you say "I think", and even think "I think", before apologizing for using such simplistic language but you only do so because you can't come up with an alternative... And truly you can't. You can't operate without the axioms I mentioned up-thread and not become crazy.
In a smaller nutshell: beliefs are healthy to have and unavoidable. You might as well be upfront about the stuff and values you care about, and stand up for them. That will avoid you a lot of unnecessary paradoxes and relieve some highly uncomfortable philosophical posturing.