(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!
There is a big gap between "difficult" and "impossible". If a thing is "difficult to measure", then you're supposed to know in principle what sort of measurement you'd want to do, or what evidence you could in theory find, that proves or disproves it. If a thing is "impossible to measure", then the thing is likely bullshit.
What understanding exactly? Besides "I'm conscious" and "rocks aren't conscious", what is it that you understand about consciousness?
In the case of consciousness, we are talking about subjective experience. I don't think that the fact that we can't measure it makes it bullshit. For another example, you might wonder whether I have a belief as to whether P=NP, and if so, what that belief is. You can't get the answer to either of those things via measurement, but I don't think that they are bullshit questions (albeit they are not particularly useful questions).
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