of colours is more complicated in a normal person than in a colour blind one, the internal relations are less simple, and this is a difference of structure.
Thus the assertions of the physiologist are ordinary statements of fact and contain everything that can be said about the qualities. If the statements cannot be made with absolute certainty but only with a lesser or greater degree of probability this is not because the qualities "cannot be observed directly but must be inferred by analogy", but it is because those statements share the fate of all empirical assertions: the observations on which they are based are never complete and always subject to error, they may be corrected by subsequent and perhaps more careful investigations of reactions of the same individuals.
These reactions reveal the structure of the sense perceptions, and everything that can possibly be said about their qualities can be said in terms of those responses. As soon as you try to say anything more, as soon as you think that there is anything more to be said, namely about the "content" of the qualities, your assertions will not become less probable or more hypothetical, but they will cease to be assertions at all, the word "quality" will simply have become meaningless, you will not be making an intelligible use of it. The reason for this lies in the fact that no series of words will actually form a proposition, will have actual meaning, unless we can indicate a way of testing its truth, at least in principle. This will be explained later (section 14); at present we confine ourselves to saying that statements about the Sameness or Diversity of Qualities must by no means be interpreted as dealing with Content. Like all other propositions, they express the facts they communicate by showing forth their structures; Content is not touched upon in any way.
This is not because content were too difficult to get at, or because the right method of investigating it had not yet been found, but simply because there is no sense in asking any questions about it. There is no proposition about content, there cannot be any. In other words: it would be best not to use the word 'content' at all, there is no need for it, and my only excuse for using the word (even in the title of these lectures) is that this forbidden road seemed to me to be the easiest way of taking the reader to a point which will allow him to get a first view of the land before him. He will now be able to turn back and find the right road which will actually take him
