Bewitched by the Word "Know�

You ask me if our friend is in town, I say yes; you ask, "Are
you sure?,� I reply, "I know it – I just installed him in our
guest room!�—What if you're mistaken?—One can't be
making a mistake about something like that!---So you"re
claiming infallibility?—Saying "I can't be making a mistake"
was just a way of saying "I know ...�; I wasn"t alleging a
ridiculous "justification� for my claim!---But only the infallible
possession of truth really justifies a claim to knowledge.—
The "I know ...� is just an instrument with a limited,
practical purpose -- a purpose which the following expansion
of the "builder's language" helps to bring out:
The number [of building stones] is sometimes estimated,
sometimes established by counting. Then the question
arises "Do you believe there are as many stones as
that?,� and the answer "I know there are – I"ve just
counted them.� But then the "I know� could be dropped.
If, however, there are several ways of finding something
out for sure, like counting, weighing, measuring the
stack, then the statement "I know� can take the place of
mentioning how I know. [Wittgenstein 1969, sec. 564]