Artificial intelligence has been obsessed with several questions from the start: Can we build a mind out of software? If not, why not? If so, what kind of mind are we talking about? A conscious mind? Or an unconscious intelligence that seems to think but experiences nothing and has no inner mental life? These questions are central to our view of computers and how far they can go, of computation and its ultimate meaning--and of the mind and how it works.

They are deep questions with practical implications. AI researchers have long maintained that the mind provides good guidance as we approach subtle, tricky, or deep computing problems. Software today can cope with only a smattering of the information-processing problems that our minds handle routinely--when we recognize faces or pick elements out of large groups based on visual cues, use common sense, understand the nuances of natural language, or recognize what makes a musical cadence final or a joke funny or one movie better than another. AI offers to figure out how thought works and to make that knowledge available to software designers.

It even offers to deepen our understanding of the mind itself. Questions about software and the mind are central to cognitive science and philosophy. Few problems are more far-reaching or have more implications for our fundamental view of ourselves.

The current debate centers on what I'll call a "simulated conscious mind" versus a "simulated unconscious intelligence." We hope to learn whether computers make it possible to achieve one, both, or neither.

I believe it is hugely unlikely, though not impossible, that a conscious mind will ever be built out of software. Even if it could be, the result (I will argue) would be fairly useless in itself. But an unconscious simulated intelligence certainly could be built out of software--and might be useful. Unfortunately, AI, cognitive science, and philosophy of mind are nowhere near knowing how to build one. They are missing the most important fact about thought: the "cognitive continuum" that connects the seemingly unconnected puzzle pieces of thinking (for example analytical thought, common sense, analogical thought, free association, creativity, hallucination). The cognitive continuum explains how all these reflect different values of one quantity or parameter that I will call "mental focus" or "concentration"--which changes over the course of a day and a lifetime.

Without this cognitive continuum, AI has no comprehensive view of thought: it tends to ignore some thought modes (such as free association and dreaming), is uncertain how to integrate emotion and thought, and has made strikingly little progress in understanding analogies--which seem to underlie creativity.

My case for the near-impossibility of conscious software minds resembles what others have said. But these are minority views. Most AI researchers and philosophers believe that conscious software minds are just around the corner. To use the standard term, most are "cognitivists." Only a few are "anticognitivists." I am one. In fact, I believe that the cognitivists are even wronger than their opponents usually say.

But my goal is not to suggest that AI is a failure. It has merely developed a temporary blind spot. My fellow anticognitivists have knocked down cognitivism but have done little to replace it with new ideas. They've showed us what we can't achieve (conscious software intelligence) but not how we can create something less dramatic but nonetheless highly valuable: unconscious software intelligence. Once AI has refocused its efforts on the mechanisms (or algorithms) of thought, it is bound to move forward again.

Until then, AI is lost in the woods.

What Is Consciousness?
In conscious thinking, you experience your thoughts. Often they are accompanied by emotions or by imagined or remembered images or other sensations. A machine with a conscious (simulated) mind can feel wonderful on the first fine day of spring and grow depressed as winter sets in. A machine that is capable only of unconscious intelligence "reads" its thoughts as if they were on cue cards. One card might say, "There's a beautiful rose in front of you; it smells sweet." If someone then asks this machine, "Seen any good roses lately?" it can answer, "Yes, there's a fine specimen right in front of me." But it has no sensation of beauty or color or fragrance. It has no experiences to back up the currency of its words. It has no inner mental life and therefore no "I," no sense of self.

But if an artificial mind can perform intellectually just like a human, does consciousness matter? Is there any practical, perceptible advantage to simulating a conscious mind?

Yes.

An unconscious entity feels nothing, by definition. Suppose we ask such an entity some questions, and its software returns correct answers.

"Ever felt friendship?" The machine says, "No."

"Love?" "No." "Hatred?" "No." "Bliss?" "No."

"Ever felt hungry or thirsty?" "Itchy, sweaty, ­tickled, excited, conscience stricken?"

"Ever mourned?" "Ever rejoiced?"

No, no, no, no.

In theory, a conscious software mind might answer "yes" to all these questions; it would be conscious in the same sense you are (although its access to experience might be very different, and strictly limited).

So what's the difference between a conscious and an unconscious software intelligence? The potential human presence that might exist in the simulated conscious mind but could never exist in the unconscious one.

You could never communicate with an unconscious intelligence as you do with a human--or trust or rely on it. You would have no grounds for treating it as a being toward which you have moral duties rather than as a tool to be used as you like.

But would a simulated human presence have practical value? Try asking lonely people--and all the young, old, sick, hurt, and unhappy people who get far less attention than they need. A made-to-order human presence, even though artificial, might be a godsend.

AI (I believe) won't ever produce one. But it can still lead the way to great advances in computing. An unconscious intelligence might be powerful. Alan ­Turing, the great English mathematician who founded AI, seemed to believe (sometimes) that consciousness was not central to thought, simulated or otherwise.

He discussed consciousness in the celebrated 1950 paper in which he proposed what is now called the "Turing test." The test is meant to determine whether a computer is "intelligent," or "can think"--terms Turing used interchangeably. If a human "interrogator" types questions, on any topic whatever, that are sent to a computer in a back room, and the computer sends back answers that are indistinguishable from a human being's, then we have achieved AI, and our computer is "intelligent": it "can think."

Does artificial intelligence require (or imply the existence of) artificial consciousness? Turing was cagey on these questions. But he did write,

I do not wish to give the impression that I think there is no mystery about consciousness. There is, for instance, something of a paradox connected with any attempt to localise it. But I do not think these mysteries necessarily need to be solved before we can answer the question with which we are concerned in this paper.

That is, can we build intelligent (or thinking) computers, and how can we tell if we have succeeded? ­Turing seemed to assert that we can leave consciousness aside for the moment while we attack simulated thought.

But AI has grown more ambitious since then. Today, a substantial number of researchers believe one day we will build conscious software minds. This group includes such prominent thinkers as the inventor and computer scientist Ray Kurzweil. In the fall of 2006, Kurzweil and I argued the point at MIT, in a debate sponsored by the John Templeton Foundation. This piece builds, in part, on the case I made there.

A Digital Mind
The goal of cognitivist thinkers is to build an artificial mind out of software running on a digital computer.

Why does AI focus on digital computers exclusively, ignoring other technologies? For one reason, because computers seemed from the first like "artificial brains," and the first AI programs of the 1950s--the "Logic Theorist," the "Geometry Theorem-Proving Machine"--seemed at their best to be thinking. Also, computers are the characteristic technology of the age. It is only natural to ask how far we can push them.

Then there's a more fundamental reason why AI cares specifically about digital computers: computation underlies today's most widely accepted view of mind. (The leading technology of the day is often pressed into service as a source of ideas.)

The ideas of the philosopher Jerry Fodor make him neither strictly cognitivist nor anticognitivist. In The Mind Doesn't Work That Way (2000), he discusses what he calls the "New Synthesis"--a broadly accepted view of the mind that places AI and cognitivism against a biological and Darwinian backdrop. "The key idea of New Synthesis psychology," writes Fodor, "is that cognitive processes are computational. ... A computation, according to this understanding, is a formal operation on syntactically structured representations." That is, thought processes depend on the form, not the meaning, of the items they work on.

In other words, the mind is like a factory machine in a 1940s cartoon, which might grab a metal plate and drill two holes in it, flip it over and drill three more, flip it sideways and glue on a label, spin it around five times, and shoot it onto a stack. The machine doesn't "know" what it's doing. Neither does the mind.

Likewise computers. A computer can add numbers but has no idea what "add" means, what a "number" is, or what "arithmetic" is for. Its actions are based on shapes, not meanings. According to the New Synthesis, writes Fodor, "the mind is a computer."

But if so, then a computer can be a mind, can be a conscious mind--if we supply the right software. Here's where the trouble starts. Consciousness is necessarily subjective: you alone are aware of the sights, sounds, feels, smells, and tastes that flash past "inside your head." This subjectivity of mind has an important consequence: there is no objective way to tell whether some entity is conscious. We can only guess, not test.

Granted, we know our fellow humans are conscious; but how? Not by testing them! You know the person next to you is conscious because he is human. You're human, and you're conscious--which moreover seems fundamental to your humanness. Since your neighbor is also human, he must be conscious too.

So how will we know whether a computer running fancy AI software is conscious? Only by trying to imagine what it's like to be that computer; we must try to see inside its head.

Which is clearly impossible. For one thing, it doesn't have a head. But a thought experiment may give us a useful way to address the problem. The "Chinese Room" argument, proposed in 1980 by John Searle, a philosophy professor at the University of California, Berkeley, is intended to show that no computer running software could possibly manifest understanding or be conscious. It has been controversial since it first appeared. I believe that Searle's argument is absolutely right--though more elaborate and oblique than necessary.

Searle asks us to imagine a program that can pass a Chinese Turing test--and is accordingly fluent in Chinese. Now, someone who knows English but no Chinese, such as Searle himself, is shut up in a room. He takes the Chinese-understanding software with him; he can execute it by hand, if he likes.

Imagine "conversing" with this room by sliding questions under the door; the room returns written answers. It seems equally fluent in English and Chinese. But actually, there is no understanding of Chinese inside the room. Searle handles English questions by relying on his knowledge of English, but to deal with Chinese, he executes an elaborate set of simple instructions mechanically. We conclude that to behave as if you understand Chinese doesn't mean you do.

But we don't need complex thought experiments to conclude that a conscious computer is ridiculously unlikely. We just need to tackle this question: What is it like to be a computer running a complex AI program?

Well, what does a computer do? It executes "machine instructions"--low-level operations like arithmetic (add two numbers), comparisons (which number is larger?), "branches" (if an addition yields zero, continue at instruction 200), data movement (transfer a number from one place to another in memory), and so on. Everything computers accomplish is built out of these primitive instructions.

So what is it like to be a computer running a complex AI program? Exactly like being a computer running any other kind of program.

Computers don't know or care what instructions they are executing. They deal with outward forms, not meanings. Switching applications changes the output, but those changes have meaning only to humans. Consciousness, however, doesn't depend on how anyone else interprets your actions; it depends on what you yourself are aware of. And the computer is merely a machine doing what it's supposed to do--like a clock ticking, an electric motor spinning, an oven baking. The oven doesn't care what it's baking, or the computer what it's computing.

The computer's routine never varies: grab an instruction from memory and execute it; repeat until something makes you stop.

Of course, we can't know literally what it's like to be a computer executing a long sequence of instructions. But we know what it's like to be a human doing the same. Imagine holding a deck of cards. You sort the deck; then you shuffle it and sort it again. Repeat the procedure, ad infinitum. You are doing comparisons (which card comes first?), data movement (slip one card in front of another), and so on. To know what it's like to be a computer running a sophisticated AI application, sit down and sort cards all afternoon. That's what it's like.

If you sort cards long enough and fast enough, will a brand-new conscious mind (somehow) be created? This is, in effect, what cognitivists believe. They say that when a computer executes the right combination of primitive instructions in the right way, a new conscious mind will emerge. So when a person executes the right combination of primitive instructions in the right way, a new conscious mind should (also) emerge; there's no operation a computer can do that a person can't.

Of course, humans are radically slower than computers. Cognitivists argue that sure, you know what executing low-level instructions slowly is like; but only when you do them very fast is it possible to create a new conscious mind. Sometimes, a radical change in execution speed does change the qualitative outcome. (When you look at a movie frame by frame, no illusion of motion results. View the frames in rapid succession, and the outcome is different.) Yet it seems arbitrary to the point of absurdity to insist that doing many primitive operations very fast could produce consciousness. Why should it? Why would it? How could it? What makes such a prediction even remotely plausible?

But even if researchers could make a conscious mind out of software, it wouldn't do them much good.

Suppose you could build a conscious software mind. Some cognitivists believe that such a mind, all by itself, is AI's goal. Indeed, this is the message of the Turing test. A computer can pass Turing's test without ever mingling with human beings.

But such a mind could communicate with human beings only in a drastically superficial way.

It would be capable of feeling emotion in principle. But we feel emotions with our whole bodies, not just our minds; and it has no body. (Of course, we could say, then build it a humanlike body! But that is a large assignment and poses bioengineering problems far beyond and outside AI. Or we could build our new mind a body unlike a human one. But in that case we couldn't expect its emotions to be like ours, or to establish a common ground for communication.)

Consider the low-energy listlessness that accompanies melancholy, the overflowing jump-for-joy sensation that goes with elation, the pounding heart associated with anxiety or fear, the relaxed calm when we are happy, the obvious physical manifestations of excitement--and other examples, from rage to panic to pity to hunger, thirst, tiredness, and other conditions that are equally emotions and bodily states. In all these cases, your mind and body form an integrated whole. No mind that lacked a body like yours could experience these emotions the way you do.

No such mind could even grasp the word "itch."

In fact, even if we achieved the bioengineering marvel of a synthetic human body, our problems wouldn't be over. Unless this body experienced infancy, childhood, and adolescence, as humans do--unless it could grow up, as a member of human society--how could it understand what it means to "feel like a kid in a candy shop" or to "wish I were 16 again"? How could it grasp the human condition in its most basic sense?

A mind-in-a-box, with no body of any sort, could triumphantly pass the Turing test--which is one index of the test's superficiality. Communication with such a contrivance would be more like a parody of conversation than the real thing. (Even in random Internet chatter, all parties know what it's like to itch, and scratch, and eat, and be a child.) Imagine talking to someone who happens to be as articulate as an adult but has less experience than a six-week-old infant. Such a "conscious mind" has no advantage, in itself, over a mere unconscious intelligence.

But there's a solution to these problems. Suppose we set aside the gigantic chore of building a synthetic human body and make do with a mind-in-a-box or a mind-in-an-anthropoid-robot, equipped with video cameras and other sensors--a rough approximation of a human body. Now we choose some person (say, Joe, age 35) and simply copy all his memories and transfer them into our software mind. Problem solved. (Of course, we don't know how to do this; not only do we need a complete transcription of Joe's memories, we need to translate them from the neural form they take in Joe's brain to the software form that our software mind understands. These are hard, unsolved problems. But no doubt we will solve them someday.)

Nonetheless: understand the enormous ethical burden we have now assumed. Our software mind is conscious (by assumption) just as a human being is; it can feel pleasure and pain, happiness and sadness, ecstasy and misery. Once we've transferred Joe's memories into this artificial yet conscious being, it can remember what it was like to have a human body--to feel spring rain, stroke someone's face, drink when it was thirsty, rest when its muscles were tired, and so forth. (Bodies are good for many purposes.) But our software mind has lost its body--or had it replaced by an elaborate prosthesis. What experience could be more shattering? What loss could be harder to bear? (Some losses, granted, but not many.) What gives us the right to inflict such cruel mental pain on a conscious being?

In fact, what gives us the right to create such a being and treat it like a tool to begin with? Wherever you stand on the religious or ethical spectrum, you had better be prepared to tread carefully once you have created consciousness in the laboratory.

The Cognitivists' Best Argument
But not so fast! say the cognitivists. Perhaps it seems arbitrary and absurd to assert that a conscious mind can be created if certain simple instructions are executed very fast; yet doesn't it also seem arbitrary and absurd to claim that you can produce a conscious mind by gathering together lots of neurons?

The cognitivist response to my simple thought experiment ("Imagine you're a computer") might run like this, to judge from a recent book by a leading cognitivist philosopher, Daniel C. Dennett. Your mind is conscious; yet it's built out of huge numbers of tiny unconscious elements. There are no raw materials for creating consciousness except unconscious ones.

Now, compare a neuron and a yeast cell. "A hundred kilos of yeast does not wonder about Braque," writes Dennett, "... but you do, and you are made of parts that are fundamentally the same sort of thing as those yeast cells, only with different tasks to perform." Many neurons add up to a brain, but many yeast cells don't, because neurons and yeast cells have different tasks to perform. They are programmed differently.

In short: if we gather huge numbers of unconscious elements together in the right way and give them the right tasks to perform, then at some point, something happens, and consciousness emerges. That's how your brain works. Note that neurons work as the raw material, but yeast cells don't, because neurons have the right tasks to perform. So why can't we do the same thing using software elements as raw materials--so long as we give them the right tasks to perform? Why shouldn't something happen, and yield a conscious mind built out of software?

Here is the problem. Neurons and yeast cells don't merely have "different tasks to perform." They perform differently because they are chemically different.

One water molecule isn't wet; two aren't; three aren't; 100 aren't; but at some point we cross a threshold, something happens, and the result is a drop of water. But this trick only works because of the chemistry and physics of water molecules! It won't work with just any kind of molecule. Nor can you take just any kind of molecule, give it the right "tasks to perform," and make it a fit raw material for producing water.

The fact is that the conscious mind emerges when we've collected many neurons together, not many doughnuts or low-level computer instructions. Why should the trick work when I substitute simple computer instructions for neurons? Of course, it might work. But there isn't any reason to believe it would.

My fellow anticognitivist John Searle made essentially this argument in a paper that referred to the "causal properties" of the brain. His opponents mocked it as reactionary stuff. They asserted that since Searle is unable to say just how these "causal properties" work, his argument is null and void. Which is nonsense again. I don't need to know anything at all about water molecules to realize that large groups of them yield water, whereas large groups of krypton atoms don't.

Why the Cognitive Spectrum Is More Exciting than Consciousness
To say that building a useful conscious mind is highly unlikely is not to say that AI has nothing worth doing. Consciousness has been a "mystery" (as Turing called it) for thousands of years, but the mind holds other mysteries, too. Creativity is one of the most important; it's a brick wall that psychology and philosophy have been banging their heads against for a long time. Why should two people who seem roughly equal in competence and intelligence differ dramatically in creativity? It's widely agreed that discovering new analogies is the root (or one root) of creativity. But how are new analogies discovered? We don't know. In his 1983 classic The Modularity of Mind, Jerry Fodor wrote, "It is striking that, while everybody thinks analogical reasoning is an important ingredient in all sorts of cognitive achievements that we prize, nobody knows anything about how it works."

Furthermore, to speak of the mystery of consciousness makes consciousness sound like an all-or-nothing proposition. But how do we explain the different kinds of consciousness we experience? "Ordinary" consciousness is different from your "drifting" state when you are about to fall asleep and you register external events only vaguely. Both are different from hallucination as induced by drugs, mental illness--or life. We hallucinate every day, when we fall asleep and dream.

And how do we explain the difference between a child's consciousness and an adult's? Or the differences between child-style and adult-style thinking? Dream thought is different from drifting or free-­associating pre-sleep thought, which is different from "ordinary" thought. We know that children tend to think more concretely than adults. Studies have also suggested that children are better at inventing metaphors. And the keenest of all observers of human thought, the English Romantic poets, suggest that dreaming and waking consciousness are less sharply distinguished for children than for adults. Of his childhood, Wordsworth writes (in one of the most famous short poems in English), "There was a time when meadow, grove, and stream, / The earth, and every common sight, / To me did seem / Apparelled in celestial light, / The glory and the freshness of a dream."

Today's cognitive science and philosophy can't explain any of these mysteries.

The philosophy and science of mind has other striking blind spots, too. AI researchers have been working for years on common sense. Nonetheless, as Fodor writes in The Mind Doesn't Work That Way, "the failure of artificial intelligence to produce successful simulations of routine commonsense cognitive competences is notorious, not to say scandalous." But the scandal is wider than Fodor reports. AI has been working in recent years on emotion, too, but has yet to understand its integral role in thought.

In short, there are many mysteries to explain--and many "cognitive competences" to understand. AI--and software in general--can profit from progress on these problems even if it can't build a conscious computer.

These observations lead me to believe that the "cognitive continuum" (or, equally, the consciousness continuum) is the most important and exciting research topic in cognitive science and philosophy today.

What is the "cognitive continuum"? And why care about it? Before I address these questions, let me note that the cognitive continuum is not even a scientific theory. It is a "prescientific theory"--like "the earth is round."

Anyone might have surmised that the earth is round, on the basis of everyday observations--especially the way distant ships sink gradually below (or rise above) the horizon. No special tools or training were required. That the earth is round leaves many basic phenomena unexplained: the tides, the seasons, climate, and so on. But unless we know that the earth is round, it's hard to progress on any of these problems.

The cognitive continuum is the same kind of theory. I don't claim that it's a millionth as important as the earth's being round. But for me as a student of human thought, it's at least as exciting.

What is this "continuum"? It's a spectrum (the "cognitive spectrum") with infinitely many intermediate points between two endpoints.

When you think, the mind assembles thought trains--sequences of distinct thoughts or memories. (Sometimes one blends into the next, and sometimes our minds go blank. But usually we can describe the train that has just passed.) Sometimes our thought trains are assembled--so it seems--under our conscious, deliberate control. Other times our thoughts wander, and the trains seem to assemble themselves. If we start with these observations and add a few simple facts about "cognitive behavior," a comprehensive picture of thought emerges almost by itself.

Obviously, you must be alert to think analytically. To solve a set of mathematical equations or follow a proof, you need to focus your attention. Your concentration declines as you grow tired over the day.

And your mind is in a strange state just before you fall asleep: a free-associative state in which, rather than following from another logically, one thought "suggests" the next. In this state, you cannot focus: if you decide to think about one thing, you soon find yourself thinking about something else (which was "suggested" by thing one), and then something else, and so on. In fact, cognitive psychologists have discovered that we start to dream before we fall asleep. So the mental state right before sleep is the state of dreaming.

Since we start the day in one state (focused) and finish in another (free-associating, unfocused), the two must be connected. Over the day, focus declines--perhaps steadily, perhaps in a series of oscillations.

Which suggests that there is a continuum of mental states between highest focus and lowest. Your "focus level" is a large factor in determining your mode of thought (or of consciousness) at any moment. This spectrum must stretch from highest-focus thought (best for reasoning or analysis) downward into modes based more on experience or common sense than on abstract reasoning; down further to the relaxed, drifting thought that might accompany gazing out a window; down further to the uncontrolled free association that leads to dreaming and sleep--where the spectrum bottoms out.

Low focus means that your tendency (not necessarily your ability) to free-associate increases. A wide-awake person can free-associate if he tries; an exhausted person has to try hard not to free-associate. At the high end, you concentrate unless you try not to. At the low end, you free-associate unless you try not to.

Notice that the role of associative recollection--in which one thought or memory causes you to recall another--increases as you move down-spectrum. Reasoning works (theoretically) from first principles. But common sense depends on your recalling a familiar idea or technique, or a previous experience. When your mind drifts as you look out a window, one recollection leads to another, and to a third, and onward--but eventually you return to the task at hand. Once you reach the edge of sleep, though, free association goes unchecked. And when you dream, one character or scene transforms itself into another smoothly and illogically--just as one memory transforms itself into another in free association. Dreaming is free association "from the inside."

At the high-focus end, you assemble your thought train as if you were assembling a comic strip or a story­board. You can step back and "see" many thoughts at once. (To think analytically, you must have your premises, goal, and subgoals in mind.) At the high-focus end, you manipulate your thoughts as if they were objects; you control the train.

At the bottom, it's just the opposite. You don't control your thoughts. You say, "my mind is wandering," as if you and your mind were separate, as if your thoughts were roaming around by themselves.

If at high focus you manipulate your thoughts "from the outside," at low focus you step into each thought as if you were entering a room; you inhabit it.That's what hallucination means. The opposite of high focus, where you control your thoughts, is hallucination--where your thoughts control you. They control your perceived environment and experiences; you "inhabit" each in turn. (We sometimes speak of "surrendering" to sleep; surrendering to your thoughts is the opposite of controlling them.)

At the high-focus end, your "I" is separate from your thought train, observing it critically and controlling it. At the low end, your "I" blends into it (or climbs aboard).

The cognitive continuum is, arguably, the single most important fact about thought. If we accept its existence, we can explain and can model (say, in software) the dynamics of thought. Thought styles change throughout the day as our focus level changes. (Focus levels depend, in turn, partly on personality and intelligence: some people are capable of higher focus; some are more comfortable in higher-focus states.)

It also seems logical to surmise that cognitive maturing increases the focus level you are able to reach and sustain--and therefore increases your ability and tendency to think abstractly.

Even more important: if we accept the existence of the spectrum, an explanation and model of analogy discovery--thus, of creativity--falls into our laps.

As you move down-spectrum, where you inhabit (not observe) your thoughts, you feel them. In other words, as you move down-spectrum, emotions emerge. Dreaming, at the bottom, is emotional.

Emotions are a powerful coding or compression device. A bar code can encapsulate or encode much information. An emotion is a "mental bar code" that encapsulates a memory. But the function E(m)--the "emotion" function that takes a memory m and yields the emotion you in particular feel when you think about m--does not generate unique values. Two different-seeming memories can produce the same emotion.

How do we invent analogies? What made ­Shakespeare write, "Shall I compare thee to a summer's day?" Shakespeare's lady didn't look like a summer's day. (And what does a "summer's day" look like?)

An analogy is a two-element thought train--"a summer's day" followed by the memory of some person. Why should the mind conjure up these two elements in succession? What links them?

Answer: in some cases (perhaps in many), their "emotional bar codes" match--or were sufficiently similar that one recalled the other. The lady and the summer's day made the poet feel the same sort of way.

We experience more emotions than we can name. "Mildly happy," "happy," "ebullient," "elated"; our choice of English words is narrow. But how do you feel when you are about to open your mailbox, expecting a letter that will probably bring good news but might be crushing? When you see a rhinoceros? These emotions have no names. But each "represents" or "encodes" some collection of circumstances. Two experiences that seem to have nothing in common might awaken--in you only--the same emotion. And you might see, accordingly, an analogy that no one else ever saw.

The cognitive spectrum suggests that analogies are created by shared emotion--the linking of two thoughts with shared or similar emotional content.

To build a simulated unconscious mind, we don't need a computer with real emotions; simulated emotions will do. Achieving them will be hard. So will representing memories (with all their complex "multi-media" data).

But if we take the route Turing hinted at back in 1950, if we forget about consciousness and concentrate on the process of thought, there's every reason to believe that we can get AI back on track--and that AI can produce powerful software and show us important things about the human mind.

David Gelernter is a professor of computer science at Yale University and a national fellow of the American Enterprise Institute.