"If a blind man were to ask me "Have you got two hands?"
I should not make sure by looking. If I were to have
any doubt of it, then I don't know why I would trust my
eyes. For why shouldn't I trust my eyes by looking to
find out whether I see my two hands? What is to be tested
by what? (Who decides what stands fast?)
"And what does it mean to say that such and such stands
fast?" ( Wittgenstein:On Certainty)