his assurance that he has two hands, if he is trustworthy. And if he says he knows it, that can only signify to me that he has been able to make sure, and hence that his arms are e.g. not still concealed by coverings and bandages, etc.etc. My believing the trustworthy man stems from my admitting that it is possible for him to make sure. But someone who says that perhaps there are no physical objects makes no such admission.
The idealist's question would be something like: "What right have I not to doubt the existence of my hands?" (And to that the answer can't be: I know that they exist.) But someone who asks such a question is overlooking the fact that a doubt about existence only works in a language-game. Hence, that we should first have to ask: what would such a doubt be like?, and don't understand this straight off.
One may be wrong even about "there being a hand here". Only in particular circumstances is it impossible. - "Even in a calculation one can be wrong - only in certain circumstances one can't."
But can it be seen from a rule what circumstances logically exclude a mistake in the employment of rules of calculation?
What use is a rule to us here? Mightn't we (in turn) go wrong in applying it?
If, however, one wanted to give something like a rule here, then it would contain the expression "in normal circumstances". And we recognize normal circumstances but cannot precisely describe them. At most, we can describe a range of abnormal ones.
What is 'learning a rule'? - This.
What is 'making a mistake in applying it'? - This. And what is pointed to here is something indeterminate.
 Practice in the use of the rule also shows what is a mistake in its employment.
When someone has made sure of something, he says: "Yes, the calculation is right", but he did not infer that from his condition of certainty. One does not infer how things are from one's own certainty.
Certainty is as it were a tone of voice in which one declares how things are, but one does not infer from the tone of voice that one is justified.
The propositions which one comes back to again and again as if bewitched - these I should like to expunge from philosophical language.
It's not a matter of Moore's knowing that there's a hand there, but rather