alt Hacker News> You presumably know consciousness exists at least in the case of you.
Just because I "know" something, that doesn't make it true. Or to use the "experience therefore consciousness" version:
Experiencing something doesn't imply that either the subject or object are objectively real. Humans regularly experience unreality (books, games, movies, hallucinations), and the assumption that you exist because you feel that you do is no more valid than the assumption that Mickey Mouse is real because you feel that you have seen him.
To restate more directly, there can be no "objective" reality without others to validate it. Otherwise its only "subjective" reality.
It's been a long time, but I think Hegel really misses the mark. (You mean Hegel rather than Nagel, right?) I think he argued against doubting your own perception, but anyone who has ever taken LSD, or met a mentally ill person, can tell you that there are miles between perception and reality. The way we percieve the self is unlikely to be any more reliable than the way we percieve Mickey Mouse.
Again, I didn't say anything about "objectively real" anything.
Your knowledge of consciousness existing within you is inherently subjective. And yet it's the truest thing any one of us can possibly know (per your point, a simple molecule ingested by mouth can distort nearly everything except the fact of consciousness). No matter how distorted the external/internal data that populates consciousness, the backdrop of consciousness is certainly there.
And no, I mean Nagel, not Hegel.