up any spatial structures, and, therefore, also the structure of the system of colours, because it can be represented by a spatial picture like that of the double cone or a similar device.)
The description of a coloured object does not communicate content to any one, whether blind or seeing, but leaves it to him to provide it from his own stock. You will probably say that only the seeing person will really provide "colour", whereas a blind man will provide some other content, and you will assert that the latter, although he will think that he understands the description perfectly well, is in reality very far from it, because the "true" interpretation must be given in terms of "colours", and nothing else can take their place.
I answer that you are quite right if by "colour" you mean something which has to do with vision, i.e. involves the use of certain particular sense organs called "eyes". You are at liberty to say, by way of definition, that an interpretation shall be acknowledged as true only in the case of a person capable of using his eyes in a normal way. This would be perfectly legitimate If I should ask you whether or not Mr. X could properly understand a description of, say, a coloured picture, you could submit him to certain experiments (which would consist in observing his reactions to colours presented to his eyes), and the results would enable you to answer my question with either yes or no (in the latter case you would declare Mr. X to be blind or colour-blind).
Nothing can be said against this procedure, which, as we know, is actually used in certain tests, but I cannot agree with you if you think that it is based on anything more than an arbitrary, though very sensible, definition. I suspect that you are inclined to argue somewhat on the following line: "If I observe a man using his eyes in a similar way in which I use my own I am justified in believing that he experiences in his consciousness exactly the same kind of sensations as I do when the same objects are presented to my eyes, so that he will be able to fill a given structure with the same content which I have in mind when I try to communicate to him what I have seen. I must necessarily regard his interpretation as the only right one, because only he can use the right content for it".
This argument speaks of visual sensations not only as of something which has certain relations to sense organs (or, which would amount to the
