(This post grew out of an old conversation with Wei Dai.)
Imagine a person sitting in a room, communicating with the outside world through a terminal. Further imagine that the person knows some secret fact (e.g. that the Moon landings were a hoax), but is absolutely committed to never revealing their knowledge of it in any way.
Can you, by observing the input-output behavior of the system, distinguish it from a person who doesn't know the secret, or knows some other secret instead?
Clearly the only reasonable answer is "no, not in general".
Now imagine a person in the same situation, claiming to possess some mental skill that's hard for you to verify (e.g. visualizing four-dimensional objects in their mind's eye). Can you, by observing the input-output behavior, distinguish it from someone who is lying about having the skill, but has a good grasp of four-dimensional math otherwise?
Again, clearly, the only reasonable answer is "not in general".
Now imagine a sealed box that behaves exactly like a human, dutifully saying things like "I'm conscious", "I experience red" and so on. Moreover, you know from trustworthy sources that the box was built by scanning a human brain, and then optimizing the resulting program to use less CPU and memory (preserving the same input-output behavior). Would you be willing to trust that the box is in fact conscious, and has the same internal experiences as the human brain it was created from?
A philosopher believing in computationalism would emphatically say yes. But considering the examples above, I would say I'm not sure! Not at all!
No, meaning the material, physical world. I'm glad you agree it's there. Frankly, I have not a slightest clue what "exterior" means. Did you draw an arbitrary wall around your brain, and decided that everything that happens on one side is interior, and everything that happens on another is exterior? I'm sure you didn't. But I'd rather not answer your other points, when I have no clue about what it is that we disagree about.
No, you can tell if their brain is active. It's fine to define "consciousness" = "human brain activity", but that doesn't generalize well.
It's where you are willing to look, as opposed to where you are not. You keep insisting that cosnciousness can only be found in the behaviour of someone else: your opponents keep pointing out that you have the option of accessing your own.
We don't do that. We use a medical definition. "Consciousness" has a number of uses in science.