The difficulty which we express by
saying “We can't know what he sees when he
(truthfully) says that he sees a blue patch” arises
from the idea that “knowing what he sees”
means:
102.
“seeing that
which he also sees”; not however in the sense in which we
do so when we both have the same object before our eyes: but
in the sense in which the object seen would be an object, say, in
his head, or in
him. The idea is that the same
object may be before his eyes and mine, but that I can't
stick my head into his (or my mind into his, which comes to the
same) so that the
real and
immediate object of
his vision becomes the real and immediate object of my vision,
too. By “I don't know what he
sees” we really mean “I don't know
what he looks at”, where “what he looks
at” is hidden and he can't show it to me; it is
before his mind's eye. Therefore, in
order to get rid of this puzzle, examine the grammatical difference
between the statements “I don't know what he
sees” and “I don't know what he
looks at”, as they are actually used in our
language.