"Fare well!" "A whole world of pain is contained in these words." How can it be contained in them? — It is bound up in them. The words are like an ac… - Ludwig Wittgenstein

"303. "I can only believe that someone else is in pain, but I know it if I am." — Yes: one can make the decision to say "I believe he is in pain" instead of "He is in pain". But that is all. — — What looks like an explanation here, or like a statement about a mental process, is in truth an exchange of one expression for another which, while we are doing philosophy, seems the more appropriate one.
Just try — in a real case — to doubt someone else's fear or pain."

Let us not forget this: when 'I raise my arm', my arm goes up. And the problem arises: what is left over if I subtract the fact that my arm goes up from the fact that I raise my arm?

If you tried to doubt everything you would not get as far as doubting anything. The game of doubting itself presupposes certainty.