A new book, Seeing Red: A Study in Consciousness, by Nicholas Humphrey, posits that consciousness is the metaphysical result of that classic evolutionary tale of stimulus and response.
First, imagine the evolutionary history of mankind, our Promethean rise from single cell to Cro-Magnon man to Paris Hilton. In the beginning, our microscopic fathers were subjected to a number of stimuli. Let's focus on Humphrey's arbitrary example of "a toxic substance." Naturally, the response to this for any creature should be "move away from the toxic substance." So some stimulus (S) enters the environment of the being (B), which triggers a response (R).
Suffice it to say that this response is at first random - or, at the very least, ignorant of consequence (so much for "naturally" after all!) Much like the child who touches the hot stove, we must learn our responses categorically, and in sequence. This is the role of evolution in the process - creating a learned history of general stimuli and response (pain and pleasure being the two major building blocks.)
Humphrey's theory is that consciousness evolved out of this learned history by taking a shortcut: rather than respond directly to the stimulus, our ancestor's primitive brain began responding to the message within it which conveyed the stimulus. It was at this crucial moment in our adaptive history that the brain became completely internalized and separate from all of our sensory organs.
This form of detached representation and isomorphism is common in humans today. It's how we can look at the letter C and know it is pronounced "SEA"; know it's a programming language; know it's a musical note; a letter grade in school; the roman numeral 100; etc. Indeed, this last abstraction is important: C standing in for 100 is a form of shorthand. That is what consciousness is, Humphrey states: shorthand for stimulus and response.
This becomes even more useful when you consider both the complexity of the message being received, the redundancy of information in the message, and the ability of the brain to process virtually all information provided to it. Let's imagine, for argument's sake, that instead of the word "RED", we had used the word "ANTIDISESTABLISHMENTARIANISM" to describe the color we refer to as "red." Now when our eye senses the color "ANTIDISESTABLISHMENTARIANISM" it begins communicating a signal of stimuli to our brain, like typing on a telegraph:
At the first arrival of that A in the signal stream, our brain does not respond to the stimulus, but rather heads to its trusty "Color Spectrum Index", contained within itself. It pulls out Volume "A", by which time the N has arrived down the pipe. It turns to the section marked "AN", then "ANT", then "ANTI", and so on, and by the end of the message, it has the correct perception which responds to the red sensation our eyes are receiving, and it places this in the mind's eye at the correct level. In fact, this disjunction between perception and sensation can be heightened by the medium of the message itself. What if all messages relating to the color blue were transmitted in Greek, while messages relating to green were transmitted in Chinese? You could look at the first character of the message and know what color you were dealing with, without resorting to any sort of lookup. This would require extra translators, but if the messages were otherwise similar (having a structure indicating brightness, saturation, hue, opacity, etc.), the lookup table for the result would be optimized at a level greater than that of the actual message itself.
Of course, all of this takes place virtually instantaneously, over millions of colors in the spectrum, without any delay or even perception of delay. But within this system we can imagine all kinds of ways to improve it, and our brain has, according to Humphrey, met the challenge. It stores away the most common stimuli in an even shorter shorthand, and reads this "Popular Favorites Index" alongside the more comprehensive "All Possible Stimuli Index." It can identify new stimuli by comparing it to existing stimuli-response messages and picking out the most similar ones. And perhaps best of all the brain needs no actual stimulus to generate responses, suggesting that consciousness is the ultimate self-contained evolutionary goal.
In a recent New York Times Book Review, John Searle tut-tutted Humphrey for deigning to separate sensation and perception. And whether or not our brain entirely dismisses stimuli, or merely takes educated guesses until the stimuli becomes perfectly clear is up for doubt even within Humphrey's own work. But the idea that consciousness is self-contained, and is simply a metaphysical construct of stimulus and response is a powerful one, and I think one that Searle, with his years of AI research, could possibly learn from and try to implement in the thinking machines of tomorrow.