(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!
Indeed.
"Subjective perception," is opposite, in the relevant way, to "objective description."
Suppose there were two kinds of things, physical and non-physical. This would not help in any way to explain consciousness, as long as you were describing the physical and non-physical things in an objective way. So you are quite right that subjective is not the opposite of physical; physicality is utterly irrelevant to it.
The point is that the word consciousness refers to subjective perception, not to any objective description, whether physical or otherwise.
No, physical things have objective descriptions.
Can you find another subjective concept that does not have an objective description? I'm predicting that we disagree about what "objective description" means.