Introspection is "looking within". Why should science not be interested in that? It is an aspect of reality. It is not more or less real than galaxies or atoms. I know that it is a very perplexing one when one holds a physicalist metaphysical commitment, which is easy to confuse with some notion of "no-nonsense modern scientific standard", and so there is a temptation to pretend the undeniable is not there, or that it is "ill defined" in some way.

Think about what things "cannot be doubted", with all the brain-in-a-vat types of caveats. It's not trying to be a scientific definition. It operates earlier on the epistemological ladder than science can be meaningfully applied, and that might well be the only reasonable place to define consciousness. (I still can't call it a great definition, even if it did perfectly correspond with the concept. Too indirect.)

There are lots of statements we can form that "make sense" on a linguistic level. It's easy to convince yourself of something when the only standard is "linguistic plausibility." Consciousness is presumably a physical process. When you say "It operates earlier on the epistemological ladder than science can be meaningfully applied", I just don't know what that means. You're going to have to give me examples of what other beliefs we hold that occupy that space. Justified belief about reality has to be based on measurement (science).

If consciousness isn't a physical process, then you've lost me again. People have discussed these things for hundreds of years.

> You're going to have to give me examples of what other beliefs we hold that occupy that space.

Yeah, there's not a lot down there, mostly your assumptions about your sense inputs corresponding to some kind of causally consistent external reality. It's the same region as the lead up to what you seem to take as an axiom, "Justified belief about reality has to be based on measurement".

I think I just experienced how much self-deception there is about the world. So it's not really an axiom. There's no shortage of metaphysical ideas from the past, from well-intentioned people who thought they could intuit the world, that we have had to throw out.

> I think I just experienced how much self-deception there is about the world.

This is not actually a proof. It is, however, exactly the kind of soft reasoning that motivates reasonable axioms. I'm not saying it's a bad axiom, I'm saying you should know what you're doing. That way when you run into a domain where it doesn't apply very well, you know where and how to back up and restart.