the perceptive faculties could be tested (including utterances of speech) would be exactly the same for the two individuals. Both of them would say that they were seeing with their eyes and hearing with their ears, they would call the objects and their qualities by the same names, their judgments about all similarities and differences of sounds, colours, sizes etc. would agree in every respect, in short, they would understand each other perfectly. And yet in spite of all this the content of all their experiences and thoughts would be absolutely and incomparably different (I am always using the language of the metaphysician), they would be living in two entirely different worlds of content.
Thus we see that there may be complete understanding between individuals even if there is no similarity between the contents of their minds, and we conclude that understanding and meaning are quite independent of Content and have nothing whatever to do with it.
This result remains valid (although it should be formulated in more correct language), and we see that wherever words like "colour", "sound", "feeling" etc. occur in our sentences they can never stand for Content. They have meaning only is so far as they stand for certain structures. The structures corresponding to the word "colour" occur, as we know empirically, in connection with the use of the organs called "eyes". People who do not possess these organs or lack the capacity of using them in the ordinary way are called "blind" or "colour blind" etc. And if we assert that e.g. colour blind persons are not able properly to understand a proposition about colours we assert nothing but that certain structures do not occur in their experience — a fact which shows itself in the set of their responses — , and we do not assert anything about their inability of filling structures with the "right content".
In so far as a blind man is actually capable of building up structures identical with those of the colour system, he does understand communications concerning coloured things, and in so far he is in possession of the logical form of colours. He is not able to use his knowledge in the same way as a normal person — he cannot, for instance, be a painter — but that is not on account of lack of some particular content, but because the different structures which play important parts in his life do not have the same connections and relationships among each other as exist in the life of a seeing person. We must not fall into the error to say that this is so because his optical
