Privacy of sense data.
I see dark red where he sees light red.
     We interpose an object between the physical object and the seeing subject.
     As we only know about his seeing this sense data by what he shows, we are inclined to say that we don't see his sense datum. Or that we don't know whether we see it.
     Why shouldn't he always see green when I see red? It would seem that we must admit this possibility. Compare: “Of course we never know whether new circumstances wouldn't show that after all he saw what we see.”
     “Can't I imagine all blind people to see.” Can't I imagine Mr W. now seeing & now blind but always behaving as he does? For this seems to be the source for saying that it must make sense. What do we imagine & how does the image give sense to the sentence.
     “Surely what I have he can have”. “Remember that we admit that the other can have pain without expressing it.” So if this is conceivable why not that everybody has constant pain?
     It seems that we must be able to imagine this & that therefore the proposition makes sense independent of any expression of pain.
     “As I myself can see so I can imagine him to do what I do. I can imagine him to play the same role in the act of seeing which I play. How are we to make the substitution.
     “Can't he all along mean something different by ‘green’ than I do? Surely I can imagine that! Surely I know what it is like to have the impression green!” I have at least a private explanation of the word, though this may not be transmittable to anyone else! But is this private explanation an explanation at all? Is it a justification?
     “The sense datum is private” is a rule forbidding e.g. “They had the same sense datum || different sense data”. It may or may not allow “he guessed that the other had the sense datum so & so.” It may only allow “The other looked at the chair had a sense datum & said …”. Use?
     “Surely I distinguish between having pain & showing it & showing it without ….
     This can't be merely a matter of using different expressions!?”
     Do we always distinguish between ‘mere behaviour’ & ‘real experience’?
     You could of course always express yourself in the form of the supposition that the other is no automaton.
     “Saying that I lie is justified by a particular experience.”
     Is toothache a behaviour? Is moaning the same as to say “I moan”.
     Our point is only that we can't explain a word by a private experience which can't be shown.
     Moaning isn't a description. Do I talk about L.W. if I say that I have toothache?

     “If a body constantly changed its weight, it would make no sense || nonsense to talk about ‘its weight’”, “If the chair behaved in this manner, we wouldn't talk about ‘a chair’”.
     “If the natural colour only lasted for half an hour, it would lose its point to talk about the ‘natural colour’ of an object.”
     Certain expressions have their sense in a certain behaviour being the rule & not the exception.
     The importance of these remarks: that it often seems as though grammar said something about objects, whereas it treats of samples. But isn't it in the nature of the world that there is such an object as blue? Isn't it in the nature … that there is a foot, a unit, etc.?
Objectification

     Grammar & the nature of objects.
     What do we call the nature of an object?
     “What is the nature of this→?”
     Determining objects by pointing in various ways.
     Having in common.
     The object which I mean.
“What is the nature of space?” of what? “What is the nature of colour?”
“What I mean must have this grammar.” What || The name of the object which I am talking about must have …”.
Pointing to colour, to the physical body.
Sense data, origin of the idea. Philosophy since Descartes. Concentrating one's attention. Looking & seeing.
“Only sense data are real.”
Is a cube symmetrical or not?
“I can't know what he feels.” “Only he can know what he feels.”
“Only you can know what you feel!”
     Privacy of sense data
Private language ¤