attentively into his face. - So I don't know, then, that there is a sick man lying here? Neither the question nor the assertion makes sense. Any more than the assertion "I am here", which I might yet use at any moment, if suitable occasion presented itself. - Then is "2x2=4" nonsense in the same way, and not a proposition of arithmetic, apart from particular occasions? "2x2=4" is a true proposition of arithmetic - not "on particular occasions" nor "always" - but the spoken or written sentence "2x2=4" in Chinese might have a different meaning or be out and out nonsense, and from this is seen that it is only in use that the proposition has its sense. And "I know that there's a sick man lying here", used in an unsuitable situation, seems not to be nonsense but rather seems matter-of-course, only because one can fairly easily imagine a situation to fit it, and one thinks that the words "I know that..." are always in place where there is no doubt, and hence even where the expression of doubt would unintelligible.
We just do not see how very specialized the use of "I know" is.
- For "I know" seems to describe a state of affairs which guarantees what is known, guarantees it as a fact. One always forgets the expression "I thought I knew".
For it is not as though the proposition "It is so" could be inferred from someone else's utterance: "I know it is so". Nor from the utterance together with its not being a lie. - But can't I infer "It is so" from my own utterance "I know etc."? Yes; and also "There is a hand there" follows from the proposition "He knows that there's a hand there". But from his utterance "I know..." it does not follow that he does know it.
That he does know remains to be shown.
It needs to be shown that no mistake was possible. Giving the assurance "I know" doesn't suffice. For it is after all only an assurance that I can't be making a mistake, and it needs to be objectively established that I am not making a mistake about that.
"If I know something, then I also know that I know it, etc." amounts to: "I know that" means "I am incapable of being wrong about that.