Privacy of sensedata. I must bore you by a repetition of what I said last time. We said that peo one reason for introducing the idea of the sense datum was that people, as we say, sometimes see different things, colours e.g. when the looking at the same object. Cases in which we say “he sees dark red objects whereas I see light red”. This We then are inclined to talk about an object other than the physical object which the person sees who is said to see the phys. obj.. It is further clear that we only gather from the other persons behaviour (e.g. what he tells us) what that obj looks like & so it lies near to say that he has this object before his mind's eye & that we don't see it. Though we can also say that we might have it before our mind eye as well without however knowing that it is he has it before his minds eye. The ‘sense datum’ ˇhere is the way the physical object appears to him. In other cases no phys object enters.
  Now I must draw your attention to one particular difficulty about the use of the ‘sense datum’. We said that there were cases in which we should say that the person sees green what I see red. Now the question suggests itself: if this can be so at all, why shouldnt
it be al[l|w]ays the case? It seems, if once we have admitted that it can happen under peculiar circumstances that it may allways happen. But then it is clear that the very idea of seeing red loses its use if we can never know if the other doesnt see something utterly different. So what are we to do: Are we to say that this can only happen in a limited number of cases? This is a very serious situation. – We introduced the expression that A sees something else than B & we mustn't forget that this had only sense use under the circumstances under which we introduced it. Concider the prop: “Of course we never know whether new circs wouldn't show that after all he saw what we see” Remember that this hole notion need not have been introduced. “But can't I imagine all blind men to see as well as I do & only behaving differently; & on the other hand imagine them really blind? For if I can imagine these possibilities then the question even if never answerable makes sense.” Imagine a man say W. now blind, now seeing & observe what you do? How do these images give sense to the question? They don't & you see that the expression stands & falls with its usefulness
“But I know The idea that the other person sees something else than I is only introduced because of to account for certain expressions; whereas it seems that this idea can exist without any reference to expressions. “Surely what I have he too can have.”
  “And remember that we admit that the other may have pain without showing it! So wh if this is conceivable, why not that he never shows that he has pain; & why not that everybody has pain constantly without showing it; or that even things have pain?!” What strikes us is that there seem to be a few usefull applications of the idea of the other persons having pain without showing it & a vast number of useless applications, applications which look as though they were no applications at all. And these latter applications seem to have their justification in this that we can imagine the other person to have what we have & in this way the prop that he has toothache seems to make sense appart from any expression at all. “Surely”, we say, “[We| I] can imagine him to have pain or to see, etc..” Or “As I can see myself so I can imagine him to do the same”. In other words I
can imagine him to play the same role in the act of seeing which I play. But does saying this determine what I mean by “he sees”?
  We arrive at the conclusion that imagining him to have pain (etc.) does not fix thes sense of the sentence “he has pain”.
  “He may all along mean something different by ‘green’ than I mean.” Evidence (Verification). But there is this consideration: “Surely I mean something particular, a particular impression & therefore he may have an other impression; surely I know what that would be like!” “Surely I know what it is like to have the impression I call ‘green’!” But what is it like? You are inclined to look at a green object & to say “it's like this!”. And these words though they don't explain anything to anybody else seem to be at any rate an explanation you give yourself. But are they?! Will this explanation justify your future use of the word ‘green’? In fact seeing green doesnt allow you to make the substitutions of someone else for you and of red for green.
  “The sensedatum is private” is a rule
of grammar, it forbids to use such expressions as “they saw the same sensedatum” it may (or may not) allow such sentences as “he guessed that the other had a sensedatum of this … kind”. It may only allow expressions of the form: “The other looked round, had a sensedatum and said …”. You see that the use of this word in such a case has no use at all. But if you like to use it, do! –
  “But surely I distinguish between having toothache & expressing it, & merely expressing it; & I distinguish between these two in myself.” “Surely this is not merely a matter of using different expressions, but there are two distinct experiences!” “You talk as though the case of having pain & that of not having pain were only distinguished by the way in which I expressed myself!”
  But do we always distinguish between ‘mere behaviour’ & ‘experience & behaviour’? If we see someone falling into flames & crying out do we say to ourselves: “there are of course two cases …”? Or if I see you here before me do I distinguish?? Do you? You
can't! That we do in certain cases doesn't show that we do in all cases. This to some of you must sound silly & superficial; but it isn't. When you see me do you see one thing & conjecture another? (Don't talk of conjecturing subconsciously!) But supposing you expressed yourself in the form of such a supposition wouldn't this come to adopting a ‘façon de parler’?
  Can we say that ‘saying that I lie is justified by a particular experience of lying’. Shall we say ‘… by a particular priv. experience’? or ‘… by a part. priv. exp. of lying’? or ‘by a part. priv. exp. characterized in such & such ways’?
  “But what, in your opinion is the difference between the mere expression & the expression & the experience?”