This point is very hard to explain so I’ll probably refine this post over time.
Shared experiences are not what they seem. When you think you are experiencing the same thing as someone else – e.g. staring at a sunset, this is not true. Even superficially speaking, you’re looking at it from a different angle, you may rub your eye and the colours may change, you may be wearing sunglasses. When the other person closes their eyes, you still see the scene.
But it’s more fundamental than that. Even if you were both at the same point in space or could somehow hook your optic nerves up to the same pair of eyes, the subjective experience would still be an exclusive one-man show.
How would you ever be able to talk about a comparison between subjective experiences? I can compare things within my experience. E.g. I can check for myself if the light from my computer screen is brighter than light from yours.
I can also check with someone else that agreed linguistic relationships are the same. E.g. I can ask some else if what he calls “sunlight” is what he calls “brighter yellow” than what he calls “urine”, but I still can’t compare experiences themselves. Why? If you think it’s possible, tell me an experiment which would be able to tell me if your experience of green is the same as my experience of green. Such an experiment would be able for example to tell me if your green was my red and vice versa. This is obviously not possible by asking questions, any kind of questions, because every time you say green, I would think of my green, and you would think of your green (red).
Compare it to a signalling system. Say Alice is communicating through an interpreter Ingrid to Bob and Carol. Bob can’t hear what Ingrid says to Bob and Carol, but he can see what they do. How could Alice test whether Ingrid is talking the same language to Bob and Carol? Not possible. Same with subjective experience. We can only communicate about it to each other through a shared language which has only a conventional relationship to the experience itself.
My feeling is when examining experience closely however, that my experience is unique to my consciousness. I can’t explain this but I can refute any possible proofs that experience is shared. No wonder we often feel so inexplicably lonely…