Wittgenstein writes in his Philosophical Investigations in paragraph 270:
Let us imagine a use for the entry of the sign "S" in my diary. I discover that whenever I have a particular sensation a manometer shows that my blood-pressure rises. So I shall be able to say that my blood-pressure rises without using any apparatus. This is a useful result.
This seems okay to me. Surely we can establish a correlation (if it exists, and otherwise prove that there is no correlation) between sensation "S" and a rise in blood-pressure. Moreover, it is very well imaginable that if I sense "S" I can prevent better some of the nasty and potentially dangerous side effects of high blood-pressure by taking some medication more timely. In this manner the sensation is useful, beyond the mere information of having high blood-pressure, something you could not care less for if without consequence. But now Wittgenstein continues...
And now it seems quite indifferent whether I have recognized the sensation right or not. Let us suppose I regularly identify it wrong, it does not matter in the least. And that alone shows that the hypothesis that I make a mistake is mere show.
I do not understand how this continuation follows from his preceding remarks. It is rather counterintuitive and calls for an explanation I think. Naturally, if I am not able to distinguish between a multitude of sensations, how can I then possibly observe a correlation (except in the case that all those sensations indicate high blood-pressure, but then it is still useful, since obviously we do not always have high blood pressure)?