Good, in-depth analysis.
“Are p-zombies in the least bit analogous to a million-sided polygon (i.e., a megagon) when it comes to conceivability leading to possibility?” That’s the key question. Between the two extremes, a round square and a megagon, it seems to me that p-zombies belong in the megagon extreme, as they seem conceptually coherent. It may be difficult to imagine them, but they don’t appear to defy logic.
One could start with an extremely simple example to better visualize it. Let’s say you have a device or organism that constricts with light and expands in darkness, like our pupils. That’s all it does. So the question is what mechanism it has that makes it behave so. It could consist of a basic photo sensor and a simple algorithm. That’s all that’s needed (no inner experience of any sort). But it could also be that it’s designed to feel pain when the sensor detects light, and that if it feels pain then it constricts (just a slightly less simple algorithm, really). One may wonder why on earth pain would be needed if the outcome would be the same, but that’s the whole conundrum of what exactly consciousness is for—a different subject. The point here is that we can thus conceive—with hardly any difficulty, besides—of a non-conscious mechanism that behaves exactly like a conscious one. Applying this logic to the tremendously complex and sophisticated ways in which humans behave may seem like a stretch, but it still falls within megagon territory—that is, it is conceptually coherent.
“Kripke stressed our ability to imagine a pain state without its correlated brain state”. I guess we can imagine it, but that would be in the round-square end of the conceivability spectrum. I think what we do when we imagine a p-zombie is that we conceive of a being whose outward manifestations are, in all details, indistinguishable from those of a conscious being. Such conception does not, must not, include the brain states of both entities. If their brain states were assumed to be equally indistinguishable, it would be incoherent to think that their inner experiences are different, that is, that one is conscious and the other isn’t. The p-zombie thought experiment only makes sense if limited to outward behavior.
“That said, the central position taken in this piece is that if that link between conceivability and metaphysical possibility is — at least in some cases — broken, then that will be (at least partly) due to the fact that such conceivings aren’t really genuine conceivings at all.” So what’s your opinion on p-zombies? Do you think they constitute, in your words, a genuine conceiving?