(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!
If I ask you to describe a chair, ultimately you'll describe it in terms of things I can perceive. "A chair is something made for sitting. Sitting is this thing I'm doing" and I can watch you sitting, therefore getting an idea of what sitting is. I can't watch your consciousness.
But what is watching someone sitting, and what is "getting an idea of what sitting is"? Those aren't things which are easy to watch.
And if you say you can notice yourself watching someone sitting and notice yourself getting an idea of what sitting is, then you can notice yourself being conscious. So there shouldn't be any difficulty figuring out whether you are conscious. The difficulty (if there is one) would be figuring out whether someone else is conscious. And it is equally difficult to know whether someone else has an idea of what sitting is.