Philosophers say it as a philosophical opinion or
conviction that there are sense data. But to say
that I
believe that there are sense data comes to saying
that I
believe that an object may appear to be before our
eyes even when it isn't. Now when one uses
the word “sense datum”, one should be clear about
the peculiarity of its grammar. For the idea in
introducing this expression was to model expressions referring
to “appearance” after expressions referring to
“reality”. It was said,
e.g., that if two things
seem to be
equal, there
must be two somethings which
are
equal. Which of course
118.
means nothing else but that we have decided to use such an
expression as “the appearances of these two things are
equal” synonymous with “these two things seem to
be equal”. Queerly enough, the introduction of
this new phraseology has deluded people into thinking that they had
discovered new entities, new elements of the structure of the
world, as though to say “I believe that there are
sense data”
were similar to saying
“I believe that matter consists of
electrons”. When we talk of the equality of
appearances or sense data, we introduce a new usage of the word
“equal”. It is possible that the
lengths A and B should appear to us to be equal, that
B and C should appear to be equal, but that A and
C do not appear to be equal. And in the new
notation we shall have to say that though the appearance (sense
datum) of A is equal to that of B and the appearance
of B equal to that of C, the appearance of A is
not equal to the appearance of C; which is all right if you
don't mind using “equal”
intransitively.