125. The feeling of the unreality of one’s surroundings. This feeling I have had once, and many have it before the onset of mental illness. Everything seems somehow not real; but not as if one saw things unclear or blurred; everything looks quite as usual. And how do I know that another has felt what I have? Because he uses the same words as I find appropriate.

But why do I choose precisely the word “unreality” to express it? Surely not because of its sound. (A word of very like sound but different meaning would not do.) I choose it because of its meaning. But surely I did not use the word to mean: a feeling. No; but I learned to use it with a particular meaning and now I use it spontaneously like this. One might say — though it may mislead — : When I have learnt the word in its ordinary meaning, then I choose that meaning as a simile for my feeling. But of course what is in question here is not a simile, not a comparison of the feeling with something else.

[...]

133. “But depression is surely a feeling; you surely don’t want to say that you are depressed and don’t feel it? And where do you feel it?” That depends on what you call “feeling it.” If I direct my attention to my bodily feelings, I notice a very slight headache, a slight discomfort in the region of the stomach, perhaps a certain tiredness. But do I mean that, when I say I am severely depressed? — And yet I say again: “I feel a burden weighing on my soul.” “Well, I can’t express it any differently!” — But how remarkable that I say it that way and cannot express it differently!

134. My difficulty is altogether like that of a man who is inventing a new calculus (say the differential calculus) and is looking for a symbolism.

—Wittgenstein, Remarks on the Philosophy of Psychology, Vol. 1