we should not understand him if he were to say "Of course I may be wrong about this." We should ask "What is it like to make such a mistake as that?" - e.g. what's it like to discover that it was a mistake?
 Thus we expunge the sentences that don't get us any further.
 If someone is taught to calculate, is he also taught that he can rely on a calculation of his teacher's? But these explanations must after all sometime come to an end. Will he also be taught that he can trust his senses - since he is indeed told in many cases that in such and such a special case you cannot trust them? -
Rule and exception.
 But can't it be imagined that there should be no physical objects? I don't know. And yet "There are physical objects" is nonsense. Is it supposed to be an empirical proposition? - 
And is this an empirical proposition: "There seem to be physical objects"?
 "A is a physical object" is a piece of instruction which we give only to someone who doesn't yet understand either what "A" means, or what "physical object" means. Thus it is instruction about the use of words, and "physical object" is a logical concept. (Like colour, quantity,...) And that is why no such proposition as: "There are physical objects" can be formulated.
Yet we encounter such unsuccessful shots at every turn.
 But is it adequate to answer to the scepticism of the idealist, or the assurances of the realist, to say that "There are physical objects" is nonsense? For them after all it is not nonsense. It would, however, be an answer to say: this assertion, or its opposite is a misfiring attempt to express what can't be expressed like that. And that it does misfire can be shown; but that isn't the end of the matter. We need to realize that what presents itself to us as the first expression of a difficulty, or of its solution, may as yet not be correctly expressed at all. Just as one who has a just censure of a picture to make will often at first offer the censure where it does not belong, and an investigation is needed in order to find the right point of attack for the critic. 
 Knowledge in mathematics: Here one has to keep on reminding oneself of the unimportance of the 'inner process' or 'state' and ask "Why should