Gary Gutting has recently outlined two thought experiments contemporary philosophers like to consider in thinking about consciousness. The objectives of these experiments is to suggest that consciousness remains as mysterious and elusive as ever, that it cannot be explained as a physical process, however odd and complex the process. Of course, we have not yet done so as far as I know, but that's not the point. The point is that we cannot in principle do so.
Gutting himself is non-commital: "I myself have come to no firm conclusions about the questions raised by these thought experiments, and I would be very interested in the ideas and arguments of Stone readers." I'm willing to commit: I think these two thought experiments are incoherent.
... consider a zombie. Not the brain-eating undead of movies, but a philosophical zombie, defined as physically identical to you or me but utterly lacking in internal subjective experience. Imagine, for example, that in some alternative universe you have a twin, not just genetically identical but identical in every physical detail—made of all the same sorts of elementary particles arranged in exactly the same way. Isn’t it logically possible that this twin has no experiences?
As far as I'm concerned, the case is closed before we get to that last sentence. If I have a twin that in some alternative universe that's identical to me down to the last itty-bitty detail, then that twin will have conscious experiences just like I do? Why? Because, strange though it may seem, that's how the physical world is.
The philosophers, however, are not satisfied with that. They want to play around with "logical possibility." Here's the next paragraph:
It may, of course, be true that, in our world, the laws of nature require that certain objective physical structures be correlated with corresponding subjective experiences. But laws of nature are not logically necessary (if they were, we could discover them as we do laws of logic or mathematics, by pure thought, independent of empirical facts). So in an alternative universe, there could (logically) be a being physically identical to me but with no experiences: my zombie-twin.
Though I know something of logic, but nonetheless it is not clear to me just what is going on here. What I suspect is that this paragraph is trading on the fact that, however much some of us believe that consciousness is a physical phenomenon, we do not in fact have any well accepted accounts of just how consciousness works, in detail. Such accounts that we do have tend to be abstract and complex, making them hard to grasp. It is easy for us to think of the physical structure of this twin as being remote from, only loosely associated with, consciousness. And so we fall into the trap and grant that it is not logically connected with that physical structure.
But is that a reasonable way to speak of logic? The argument also seems to allow that those identical particles identically arranged in my identical twin do not, however, follow the same laws, whatever they may be, and so do not have the same kinds of interactions with one another. Whatever they're doing, it isn't the same as what me and my particles are doing. That, it seems to me, is more or less why my twin doesn't have conscious experience.
That all seems rather doubtful. As far as I'm concerned, asserting that the particles are the same but their interactions are not doesn't make sense. It's not logical. If the interactions are not the same, then neither are the particles. My twin may appear to be just like me, but on a deep level he is not.
Postulating alternative universes is a rather open-ended business. Asserting that my twin is "identical in every physical detail—made of all the same sorts of elementary particles arranged in exactly the same way," that has the superficial appearance of being precise. But it isn't. It's just handwaving.
That leaves us with a third and final paragraph, a short one:
But if a zombie-twin is logically possible, it follows that my experiences involve something beyond my physical makeup. For my zombie-twin shares my entire physical makeup, but does not share my experiences. This, however, means that physical science cannot express all the facts about my experiences.
As far as I'm concerned we've already dispatched the first two sentences. I just want to pick a nit with the third. Who ever said that physical science could express ALL the facts about anyone's experiences? It's not even obvious to me that physical science can express ALL the facts about anything. But that's an assertion about our ability to make assertions about the world, not about the world itself. The world is simply abundant beyond our capacity to account for it.
Roses Are Red
With this is mind, let's now look at the first thought experiment.
... consider Mary, a leading neuroscientist who specializes in color perception. Mary lives at a time in the future when the neuroscience of color is essentially complete, and so she knows all the physical facts about colors and their perception. Mary, however, has been totally color-blind from birth. (Here I deviate from the story’s standard form, in which—for obscure reasons—she’s been living in an entirely black-and-white environment.)Fortunately, due to research Mary herself has done, there is an operation that gives her normal vision. When the bandages are removed, Mary looks around the room and sees a bouquet of red roses sent by her husband. At that moment, Mary for the first time experiences the color red and now knows what red looks like. Her experience, it seems clear, has taught her a fact about color that she did not know before. But before this she knew all the physical facts about color. Therefore, there is a fact about color that is not physical. Physical science cannot express all the facts about color.
Facts are one thing. They are assertions about the world. But the world is not itself constituted entirely of assertions, though assertions do of course exist in the world. The world contains all sorts of things, quarks and quasars, seas and bacteria, drill presses and record players, mountains and bird calls and so forth. We can assert facts about any of those things and many others, whether of particular individuals or of classes and groups of individuals. But the assertions are not identical to the things and situations to which they refer.
Among the many things in the world, we find experiences. Experiences are odd sorts of things, tricky to talk about. We know that. But an experience of redness is no more identical to the various facts about redness than a log is identical to various facts about it. That's obvious in the case of a log, whose physical nature is obvious and palpable. Experience is not so palpable and yet we do know that manipulation of something that is obviously physical, the nervous system, affects experience.
Those red flowers that Mary sees upon removal of the bandage are no more identical with all the many facts she knows about color science than her hospital bed is identical to her knowledge about the materials in it, their arrangement, and so forth. Mary's (newfound) experience of red is not a fact about red or about those flowers. It's an experience, one she hadn't had before.
As far as I'm concerned, making Mary an expert in a neuroscience of color that we don't yet have, that's just a distraction. The story would be the same if Mary were a color-blind tennis pro. She gets the operation and now can see red. That's an experience, not a fact about her or about red objects.
If I had more time...
I don't think either of these thought experiments tell us anything useful about consciousness. What I've done is suggest that each gains its plausibility through a trick, though I don't believe the tricks to be deliberate attempts at deception. The philosophers who created these stories where the first ones to be tricked.
What's important is to understand how and why they tricked themselves. In one case the trick involves a poorly conceived acount of physical identity. In the other it involves a confusion between the world and explicit assertions about the world.
Why are we vulnerable to these tricks?