(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!
Can you figure out whether there are chairs in your house? How? Suppose you say that there are. How do you know they are chairs and not something else? If you answer those questions, we can continue in the same way and ask how you know those answers are right and what they mean. You will never be able to explain any concept without using other concepts, and we can always say, "but what are those things?"
I would say there is no difference; consciousness is no harder to recognize than chairs (and in fact a bit easier.) If you think there is a difference, what is it?
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.