alt Hacker News> I'm sensing redness here and now, so the sensation of redness exists very clearly tied to a particular point in spacetime. In what sense is the qualia of redness not firmly anchored in spacetime? Of course, you could talk about the concept redness, like the concept Pi, but even then, these concepts exist in the mind of a human thinking about them, still tied to a particular location in spacetime.
But qualia are inherently subjective. You can correlate brain activity (which exists at a position in spacetime) to subjective experience, but that experience is not related to spacetime.
Said otherwise: imagine you are in the Matrix at a coffee shop and sense redness, but your brain is actually in a vat somewhere being fed fake sensory input. "Where" is the redness? You would clearly say that it arises in your brain in the coffee shop. Imagine then the vat is moved (so its position in spacetime changes), your brain is rolled back to its previous state, and then fed the same sensory input again. Where is the redness now?
You can't differentiate the two sensations of redness based on the actual position of the brain in spacetime. For all intents and purposes, they are the same. Qualia only depend on the internal brain state at a point in time and on the sensory input. Spacetime is nowhere to be found in that equation.
> The two brains don't receive the same sensory inputs
But let's say they do. Identical brains, identical inputs = identical qualia. What differentiates both consciousnesses?
> But let's say they do. Identical brains, identical inputs = identical qualia. What differentiates both consciousnesses?
I'll start with this, because it should help with the other item. We know there are two identical consciousnesses exactly because they are separate in spacetime. That is, while I can send the same input to both and get the same mind, that's not the interesting thing. The interesting thing is that I also can send different inputs, and then I'll get different minds. If it really were a single consciousness, that would be impossible. For example, you can't feed me both pure redness and pure greenness at the same time, so I am a single consciousness.
Here is where we get back to the first item: if we accepted that qualia are not localized in spacetime, we'd have to accept that there is no difference between me experiencing redness and you experiencing redness. Even if you consider that your qualia are separate from my own because of our different contexts, that still doesn't fully help: perhaps two different beings on two different planets happen to lead identical lives up to some point when a meteorite hits one of the planets and gravely injures one of their bodies. Would you say that there was a single consciousness that both bodies shared, but that it suddenly split once the meteorite hit?
Now, that is a valid position to take, in some sense. But then that means that consciousness is not continuous in any way, in your view. The day the meteorite hit planet A is not special in any way for planet B. So, if the single consciousness that planet A and planet B shared stopped that day, only to give rise to two different consciousnesses, that means that this same phenomenon must happen every day, and in fact at every instant of time. So, we now must accept that any feeling of time passing must be a pure illusion, since my consciousness now is a completely different consciousness than then one that experienced the previous minute. While this is a self-consistent definition, it's much more alien than the alternative - where we would accept that consciousness is tied in spacetime to its substrate.