Thanks to the panelists for their many fascinating contributions.
I'd like to address the issue of the brain's complexity. I was one of the first who argued that we need to look at the quantum substrate of the brain in order to get a handle on how consciousness hooks up to the gray matter.
My efforts were guided by the fact that, throughout science, we often find simple mechanisms underlying the diversity of natural phenomena. For example, and broadly speaking, all of life is encoded by way of four amino acids.
Where is the simplicity underlying the storied complexity of the brain?
I believe the answer lies in quantum field theory (QFT). That might sound like trading one complexity for another, but bear with me for a moment.
In a classic article in Scientific American, Freeman Dyson wrote: "There is nothing else except these fields: the whole of the material universe is built of them."
This would seem to clarify matters in a wondrous manner. On a mind/brain identity theory, such as we find in Chalmers, e.g., it would seem to follow that our sensory fields just are quantum fields and, more particularly, photon fields.
Indeed, if we accept the premise given by Dyson, we would seem to have rather few choices in the matter. So, what is a field, exactly?
Nobelist Gerard 't Hooft supplies an answer: "A field is simply a quantity defined at every point throughout some region of space and time."
Curiously, we often speak of our visual fields. Is this a coincidence? Or might there be more to it? Wittgenstein makes an interesting point:
"A speck in the visual field, though it need not be red must have some color; it is, so to speak, surrounded by color-space. Notes must have some pitch, objects of the sense of touch some degree of hardness, and so on."
Now, many otherwise reliable textbooks will tell you that color, as "everyone knows," is just the frequency of light. Or maybe its wavelength -- or a trick performed by the brain. It's a little unclear just what everyone knows.
Well, if the authors of those texts had done their homework a little better, they might have discovered what the real brains had to say on the subject. In his day, Schrödinger was the foremost authority on what is called color science. This is what he wrote:
"If you ask a physicist what is his idea of yellow light, he will tell you that it is transversal electromagnetic waves of wavelength in the neighborhood of 590 millimicrons. If you ask him: But where does yellow come in? he will say: In my picture not at all, but these kinds of vibrations, when they hit the retina of a healthy eye, give the person whose eye it is the sensation of yellow."
"In my picture not at all..." We would seem to have come to a dead end, but not to worry. Maxwell, Schrödinger, Weyl and Feynman all tell us that color behaves like a vector -- and that's interesting, because photons also behave like (state) vectors.
Where does all this leave us? There is much more that must be said on the subject, but the picture which emerges is one where neurons embody matrices in the form of operator fields which act on input vectors to yield output vectors.
Or, as I like to say, neural form follows quantum function. Simple!