I once saw a blind kid on TV that had developed a way of clicking with his mouth that he could use it to navigate sidewalks. This was pretty cool and it made me pay attention to my own sense of hearing and wondering what it must be like to use that kind of ability. I payed close attention to situations that it might be possible to hear the place of walls etc. Doing this for sometime it changed my relationship to my hearing.

I became aware when a sound is louder because additional bounces of wave energy hit my ear rather than having only the direct line-of-sight propagation. I picked up the threshold where I hear the primary sound and it's echo as simultanous sound or as two separate sounds. After paying attention to things that I theorethically knew why they would happen I could tap into kinds of "feels" in the sound. My mind somehow clicked and connected geometric forms to the echo timing profile. In understand only discrete sounds conciously but the prolonged direction-changing continous echo that a sloped wall makes I could sense intrisically. And I found out that for example claps are very directional and you can kind of like cast different claping to a wall like you would shine a flashlight.

All in all my sense of hearing became much more like my sense of seeing with good 3D-structure. Experiencing this new way of hearing was very interesting and cool. However once I got settled how to hear like a echolocator I had trouble conceptualising what it is like not to hear like it. My guess is that if you don't pay that much attention a lof information goes unextracted. But it was a big surprise that it wasn't "obvious" how much information a given hearing includes. I didn't gain a better ear. The amount of information I was receiving would have needed to stay same, but I guess I previously couldn't structure them properly.

And I realised that i had atleast two hearing modes even before this new "3D" mode. A mono mode where you can decipher what kind of sound it is and can recognise what causing it with only knowing that it is "nearby within hearing distance" and couldn't be able to face the sound and need to visually look for clues where the sound is coming. Then there is kinda "arrow" mode where you know to look at the direction where the sound is coming from. But it is kinda cool when in "3D" mode I can hear around the corner on what kind of space there is which I can't do in "arrow" mode.

Thinking about how sound waves work it kinda makes sense how the perception changes between "mono" and "arrow mode". If you are in a empty room and make big enough noise there is significant echo going from every direction. Without able to read the timing finestructure it feels like coming from everywhere. However if you in the same kind of room don't make quite as much noise then the component directly going towards you will dominate the echoes. There is also an explanation why the "arrow" isn't a pinpointer but a fuzzy approximation, when you try to read the texture/shape information as location information it will give a slightly contradictory result.

I am using language here where I first feel a certain way and then be puzzled on why it would feel this way and then start theorising in this way. I guess it's worth noting that having more theory won't give you insight in what your experience is. It was kinda mindopening to be able to target those feelings relatively theory-free and then the joy of finding the explanation. For example how sound propagation first felt "waterlike" and only afterwards confirming that that makes perfect sense as the waves are not equal in strength in all directions and do have dampening as they propagate.

I really couldn't confirm that I wasn't just reading too much into what I was supposedly experiencing, that I have just pretended to experience things while only actually wanting to experience them so. But then after I aquired the skill I passively would first pick up sounds and have 3D impressions of them when not actively pursuing to hear anything (and usually be frightened about it) and only then turning to look at them that this was a legit change in perception when the expectations formed by hearing would be confirmed by sight. For example I would drive by a post with a bike and suddenly be very aware of something square on my right, the wheel sounds giving enough echo basis that the post would pop-up against the background a lot more than it visually does. Or driving alleys making a sudden echo chamber on an otherwise echoless street. I also found out that glass sticks out a lot more than other materials (oh there is a large object to my right, oh it's just a window).

For me I have discovered what it is to be like an echolocator which I guess is supposed to be the main alien part in the bat metaphor. There is also a joke on how drugs make you "taste blue" but I have come to experience that and how it makes sense to "see sound". But the behavioural effects of this different kind of experiencing are not that telling or direct. I would not pass the vampire turing test because that isn't to the point, it would need to be refined to be that but it is not trivial how that would be made.

The operation that made me undergo this change seems to be paying attention. It doesn't seem to be that I learned a new fact. Althougth I clearly see that having atheory why I am feeling what I am feeling did have aguiding effect. Maybe call it a imagination aid? I would say it might be a deficiency in understanding and not knowledge that limits people not being able to experience bats. And it is possible for humans to understand what it is to be an echolocator. I would guess that if I had sufficiently clear descriptions on what kinds of "facets" my perceptions include I should be able to play it out how I would experience the situation had I that kind of a sense. So I think it might be possible to imagine seeing 4 primary colors but it takes skill in this "pay attention to your qualia structures" thingy that people are not in general very good at.

I talked with a woman who said she could visualize in four dimensions. I asked her how many corners a tesseract has. A correct answer would have given me very little information, but she didn't give me a correct answer.

Instead, she looked as though she was visualizing something, and then said she was trying to count the corners but couldn't keep track.

I asked her what a hypersphere looked like, and she said, "Like a regular sphere, but rounder".

I can't visualize in four dimensions, but it seems at least plausible that she could.

This seems like it's even harder to test than knowing what it's like to be a bat. On the other hand, your bat test is pretty hypothetical. How would you rate someone who showed bat-like brain activation when imagining being a bat?

I'm not sure what sort of knowledge we are talking about, and I suspect that Nagel's argument is based on an equivocation.

If we are talking about epistemic beliefs, expectations on observations, then we can certainly study the phenomenon of bat consciousness with a reductionist (that is, scientific) approach.

What we can't do is to simulate the mental states of a bat using our innate agent-simulation mental machinery, for the same reason a color-blind person can't simulate the mental state of a non-color-blind person perceiving red and green as different colors, or Mary can't simulate perceiving colors.
These experiences are a type of knowledge that can't be obtained from scientific research, even though they aren't (in principle) intrinsically epistemically meaningful: if you take Mary outside her back-and-white room, she will experience new mental states even though her epistemic beliefs essentially don't change.

It seems to me that this issue stems just from a limitation of our mental machinery, not an intrinsic flaw of the scientific method or a non-physical nature of consciousness.

Ok. The vampire know's what it is like to be a bat. Let them express the knowledge in terms of physical reductionism and Nagel is defeated by direct demonstration. Excuse them for not doing so, and nothing significant is changed; now the set of creatures who know what it is like to be a bat is expanded to (bats, vampires)

I don't know why we even talk about stuff like this. If you can simulate and run a bat brain, then bat conciousness is reductionist. If you can simulate and run a human brain, then human conciousness is reductionist.

To the best of our understanding, it takes a change to the laws of physics to make these things unsimulatable. Therefore, the prior for reductionism is high enough that you need actual evidence to counter it, not just some philosopher's rambling argument.

It may also help to taboo the word 'conciousness' in these kinds of discussions. I find that the word brings a lot of questionable baggage to the table in addition to failing to describe what is actually going on.