Personally, I resonate with Chalmers' notion that it is logically possible to conceive of beings who are identical to us down to the most minute subatomic particle, and behave exactly like us in even the most subtle fashion.
The only difference betweeen zombie and human would be this: humans experience what it is like to be what we are; zombies don't.
Admittedly this is a strange notion to wrap our minds around. For each of us, our entire life has been lived as a conscious being with a phemomenal, as well as psychological, awareness (which for Chalmers is virtually the same thing as consciousness).
"Phenomenal" is subjective.
Our awareness/consciousness is accessible only to ourselves. No one else knows what it like to be me, just as I have no knowledge of what it is like to be someone else. For example, even though I've been married to my wife for over twenty years, and understand her very well, I don't really know what it is like to be her.
"Psychological" is objective.
This is the realm of goings-on within the brain that can be studied scientifically. Perceptions, sensations, thoughts, emotions -- all this stuff can be tracked, measured, analyzed, and such through modern imaging technology, sophisticated modeling techniques, etc.
Zombies have all that psychological stuff, but not phenomenal awareness. (Chalmers recognizes that probably no real zombie ever will exist, though science fiction stories are getting closer to reality all the time; I'm thinking The Terminator here, since cyborgs are a bit like philosophical zombies.)
They're difficult to visualize. Chalmers says:
I confess that the logical possibility of zombies seems equally obvious to me. A zombie is just something physically identical to me, but which has no conscious experience -- all is dark inside.
While this is probably empirically impossible, it certainly seems that a coherent situation is described: I can discern no contradiction in the description. In some ways an assertion of this logical possibility comes down to a brute intuition, but no more so than with the [mile-high] unicycle.
Almost everybody, it seems to me, is capable of conceiving of this possibility... At the global level, we can consider the possibility of a zombie world: a world physically identical to ours, but in which there are no conscious experiences at all. In such a world, everybody is a zombie.
Thankfully, our world is different.
As Descartes realized, the one thing we can be certain of is that we're conscious. We can be mistaken about everything else -- reality, including us, could be a super-sophisticated simulation generated by some bored galactic teenager -- but not that we're aware of something or other happening here.
Like Bob Dylan said, we just don't know what it is. Not really. Not for sure.
In some ways, and I hate to disrespect Star Trek but this is how I see it, consciousness is more of a final frontier than space exploration. In large part that's because it is both what we're most intimately acquainted with, and also what is most mysterious to us.
As noted before, there is no way to tell whether some person, animal, or plant is conscious/aware. Chalmers believes, as I do, that dogs, cats, mice, monkeys and other mammals have that what it is like to be them experience of consciousness.
Again, we just don't know what it is.
This supports the logicalness of Chalmers' zombie thought experiment. If I can't tell for sure whether my dog or wife are conscious, how can I be 100% confident that human-appearing zombies with no conscious experience aren't standing in the line with me at Starbucks?
Now, I realize that some readers of this post probably are thinking, "Who cares about all this philosophizing?" (especially those who stopped reading before they got to this paragraph).
Well, I care.
I find nothing more mysteriously marvelous than the fact that I'm aware of the world. As I've said before, I really dislike the prospect of my non-existence. But in my possibly saner moments I'm filled with gratitude (to no one in particular) that I'm able to experience life from the moment of my birth some sixty-two years ago to however long I have left before I die.
We can quibble with the fine points of David Chalmers' philosophy of consciousness. What is most important, though, is that he directs our attention to an astounding mystery which we usually take for granted: our capacity to pay attention, a.k.a. "awareness."
Which brings me to conscious thermostats, a fascinating subject that I'll have to treat more briefly than zombies, given the lateness of my blogging evening and the length of this post.
Chalmers takes seriously the possibility that the thermostat which just made our heat pump go on is conscious (in a limited, but real, sense). And not only thermostats, possibly almost everything.
To make the view seem less crazy, we can think about what might happen to experience as we move down the scale of complexity. We start with the familiar cases of humans, in which very complex information-processing gives rise to our familiar complex experiences.
...Moving down the scale through lizards and fish to slugs, similar considerations apply. There does not seem to be much reason to suppose that phenomenology should wink out while a reasonably complex perceptual psychology persists.
...As we move along the scale from fish and slugs through simple neural networks all the way to thermostats, where should consciousness wink out? ... Before phenomenology winks out altogether, we presumably will get to some sort of maximally simple phenomenology.
It seems to me that the most natural place for this to occur is in a system with a corresponding simple "perceptual psychology," such as a thermostat. The thermostat seems to realize the sort of information processing in a fish or slug stripped down to its simplest form, so perhaps it might also have the corresponding sort of phenomenology in its most stripped-down form.
As is evident from these quotations, Chalmers believes that information may be intimately related to consciousness. This leaves open the possibility of some form of "panpsychism," the view that everything has a mind. Chalmers doesn't embrace panpsychism, but he says that we need to take the possibility seriously.
If there is experience associated with thermostats, there is probably experience everywhere: wherever there is a causal interaction, there is information, and wherever there is information, there is experience. One can find information states in a rock -- when it expands and contracts, for example -- or even in the different states of an electron.
...The view that there is experience wherever there is causal intreraction is counterintuitive. But is a view that can grow surprisingly satisfying with reflection, making consciousness better integrated into the natural order. If this view is correct, consciousness does not come in sudden jagged spikes, with isolated complex systems arbitrarily producing rich conscious experiences.
Rather, it is a more uniform property of the universe, with very simple systems having very simple phenomenology, and complex systems having complex phenomenology. This makes consciousness less "special" in some ways, and so more reasonable.
It also leaves open the possibility, no matter how remote, that when I die, my consciousness isn't absolutely or completely extinguished. Perhaps some aspect of human awareness partakes in a cosmic, universal, all-pervasive consciousness.
I'm not speaking of anything other-worldly, just an expanded view of the laws of nature. As I related in my previous post about Chalmers' book, "The Conscious Mind," he sees consciousness as entirely natural (as contrasted with supernatural or mystical), yet essentially nonmaterial.
As weird as this may sound, consider...
Where do the laws of nature reside?
How does matter/energy know how to behave in accord with those laws?
What does "material" mean at the farthest reaches of quantum phenomena and the earliest instant of the big bang?
If zombies physically indistinguishable from us are logically possible, what makes us conscious, experiencing beings different from them?
Currently there aren't answers to these questions. Maybe there never will be. All we can do is keep exploring the mysteries of consciousness, which are as close to us as whatever you and I are aware of at this very moment.