Why Computers Will Never Pass the Turing Test
What's happening when we talk is that my words trigger in you memories of sensations and experiences that you have sensed and experienced. I can't tell you about a novel experience without relating it to something you yourself have already experienced.
For example, if I want to tell you what it's like to be out in a sleet storm, I can tell you that it's wet and that it's cold. You already know what both those things are, so now you have a pretty good idea what a sleet storm is like. I can also tell you that sleet stings when it hits your face, and you'll understand, because you already know what a "stinging" sensation feels like.
On the other hand, if you've been blind since birth, I cannot help you to understand what red is. I can tell you about red —it has a long wavelength, it comes in about five hundred variants — but you will have no understanding of what red is, because color is, at base, a sensation and not a physical reality. Light at 7000 angstroms is not red. It's not even "light." It's just an electromagnetic wave of a certain frequency. "Light" is what it becomes when it goes through an eye and into a brain, and "red" is how a brain happens to perceive light at 7000 angstroms. Red isn't what it is. Red is what it feels like.
Similarly, if you're deaf I can tell you about A-flat or D-sharp, but that's all you'll know: what you can say about them, without really knowing what they are. A-flat isn't a physical construct. It's a sensation provoked by a physical construct. (This is very similar to Niels Bohr's take on quantum physics: We can talk knowledgeably about electrons, how they'll behave in certain situations and how one gets from point A to point B, but as soon as we try to describe what en electron is, or how it got from A to B, we're just fooling ourselves.)
In order for me to tell you what red is or what A-sharp is, I have to first get you to conjure up a related experience or sensation and then tell you how this new one is different. You also have to have experience of the difference. If I want to describe chocolate ice cream, I can tell you to think of vanilla, which you already know, and then tell you to imagine that you've taken a spoonful and then eaten a chunk of chocolate and then let them both melt together in your mouth. That will give you a pretty good notion of chocolate ice cream. But if you've never had vanilla ice cream or have never eaten chocolate, you won't get it.
We can communicate facts anew, but we cannot communicate novel experiences unless they can be related in terms of modification to experiences already experienced.
But a computer has never had an experience or a sensation. All it "knows" is what has been described to it, with no relation to earlier experiences of its own, only to words it has previously been supplied with. The computer can say things about things, but doesn't really know the thing except in terms of what can be said about it. In order to teach the computer novel things, we have to simulate experiences and sensations, and this will always fall greatly short of the mark, because language doesn't actually describe experience and sensation; all it does is evoke experiences and sensations we've already had. Since the only thing you can program a computer to experience is words, and since words are inadequate descriptors of experience and sensation, the computer can never truly understand them.
Suppose I want to tell a computer what it's like to hold a baseball. What is a baseball? It's a round object about four inches in diameter. Even a human can only experience it's surface, so let's talk about that. It's made of leather and red thread. Thread? Yes, kind of like string. String? Sort of two dimensional, of a certain elasticity. What's that? Its length can change under tension. You said "red." What's that? It's a color. Which is? Well, there are about 16,000 of those that the human eye can see. See? So color doesn't really exist "out there?" It's an artifact of brain and eye? A sensation? Yes. So what's the sensation of color like? Well, it comes into your eye, stimulates rods in the retina, which send electrical signal to the brain, where neural this happens and neural that happens…
So we can go about two hundred layers down trying to describe what thread is, and ultimately fail because, at some point, we have to relate it to some other sensation the computer has had, and we can't. Then we pop back up and try to do the same thing for leather, for hardness, for roughness, for heft, etc., going down two hundred levels for each of those, with assorted branches, only to find during each descent that we always hit a wall and never quite get there, because the computer has never held a baseball, or held anything, or known red or rough or hard or heavy, all of which can be quantified with the exception of "feel."
Because language is ultimately about the evocation of experience and sensation, no matter how good a computer might get at faking that it has had experiences and sensations, you'll always be able to trip it up. It's like trying to get a blind person to understand color or a deaf person to understand music: You can throw words at them all day long but they'll never get it, and it won't take you long to find out.
To make it even worse, there are other things critical to language that the computer will never get, such as the mood or personality of the speaker/writer, sarcasm, irony, hyperbole, artistic license, subtle metaphor and the like. Imagine this:
You: The president would just as soon that somebody drop a nuke on New York and vaporize it.
Computer: Do you mean that?
You: Yeah, right.