Ludwig Wittgenstein, answered from the texts and cited to the page.
The diary case is where it begins. Suppose I want to keep a record of a certain sensation — I associate it with the sign "S" and write this sign in a calendar for every day on which I have the sensation.1 Now: what is the criterion for whether I have written "S" correctly on a given occasion? What distinguishes a correct application from an incorrect one?
Nothing. Whatever is going to seem correct to me is correct — and that only means that here we cannot talk about "correct" at all.2 The inward ceremony of concentrating attention on the sensation and, as it were, pointing to it inwardly, looks like a definition but is not one. A definition serves to lay down the meaning of a sign; but this process can only "commit to memory" the connection between sign and sensation if there is already a criterion for remembering it correctly.3
There is none. The balance on which impressions are weighed is not the impression of a balance.4 Consider what is needed even before naming can begin. When we say that someone has "named his pain," we forget that much must already be prepared in the language for mere naming to make sense.5 The grammar of the word "pain" — its role in expressing, ascribing, treating, sympathizing — is what has been prepared; it indicates the post where a new word is stationed.
Strip that preparation away and you do not have a more primitive kind of naming; you have nothing that functions as naming at all. The blood-pressure case makes this vivid from another direction. Suppose I learn to report that my blood pressure is rising by noticing sensation S, confirmed by a manometer. Now it seems quite indifferent whether I have recognized the sensation correctly or not — if I regularly make a mistake in identifying it, this makes no difference at all.6
And this alone shows that the supposition of a mistake was merely sham. We turned a knob that looked as if it could adjust something in the mechanism; it was a mere ornament, not connected with the mechanism at all. The word "sensation" itself belongs to our common language, not to a language intelligible only to me.7 So its use stands in need of a justification everyone understands.
And it would not help to retreat and say: he has something, and that is all that can be said — because "has" and "something" also belong to our common language.8 One reaches the point where one would like to emit an inarticulate sound. But such a sound is an expression only in a particular language-game, which now has to be described. None of this denies that I have my own pains and you do not feel them.
The essential thing about private experience is not that each person possesses their own specimen, but that nobody knows whether other people have the same thing or something else.9 That asymmetry is real. What the argument denies is that inner naming — associating a sign with a sensation in the absence of any criterion of correctness — could constitute a language.
Understanding is not a private inner episode of grasping; it is something exhibited in practice, tied to criteria in behaviour.10 Sounds which no one else understands but which I appear to understand might be called a "private language" — but appearing to understand is not understanding.11
I want to keep a diary about the recurrence of a certain sensation. To this end I associate it with the sign 'S' and write this sign in a calendar for every day on which I have the sensation.Philosophical Investigations, pp. 123–124
One would like to say: whatever is going to seem correct to me is correct. And that only means that here we can't talk about 'correct'.Philosophical Investigations, pp. 123–124
But 'I commit it to memory' can only mean: this process brings it about that I remember the connection correctly in the future. But in the present case, I have no criterion of correctness.Philosophical Investigations, pp. 123–124
Are the rules of the private language impressions of rules? The balance on which impressions are weighed is not the impression of a balance.Philosophical Investigations, pp. 124–125
when one is doing philosophy, one gets to the point where one would like just to emit an inarticulate sound... one forgets that much must be prepared in the language for mere naming to make sense. And if we speak of someone's giving a name to a pain, the grammar of the word 'pain' is what has been prepared here; it indicates the post where the new word is stationed.Philosophical Investigations, pp. 123–124
Suppose that I regularly make a mistake in identifying it, this does not make any difference at all. And this alone shows that the supposition of this mistake was merely sham. (We, as it were, turned a knob which looked as if it could be used to adjust something in the machine; but it was a mere ornament not connected with the mechanism at all.)Philosophical Investigations, pp. 126–127
What reason have we for calling 'S' the sign for a sensation? For 'sensation' is a word of our common language, which is not a language intelligible only to me. So the use of this word stands in need of a justification which everybody understands.Philosophical Investigations, pp. 124–125
it would not help either to say that it need not be a sensation; that when he writes 'S' he has Something and that is all that can be said. But 'has' and 'something' also belong to our common language.Philosophical Investigations, pp. 124–125
The essential thing about private experience is really not that each person possesses his own specimen, but that nobody knows whether other people also have this or something else.Philosophical Investigations, pp. 126–127
What we call 'understanding' is not the behaviour — whatever it may be — that shows us the understanding, but a state of which this behaviour is a sign.Philosophical Grammar, pp. 84–85
And lastly, criteria for his understanding the word correctly. In the second case, one might speak of a subjective understanding. And sounds which no one else understands but which I 'appear to understand' might be called a 'private language'.Philosophical Investigations, pp. 126–127