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
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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.