Explaining consciousness -- the how and why of human feeling -- is such a hard problem that it may never yield to cognitive science
by Stevan Harnad
A Universe of Consciousness
by Gerald M. Edelman
and Giulio Tononi
Basic Books, 2000
The Mysterious Flame
by Colin McGinn
Basic Books, 1999
The Cultural Origins of Human
by Michael Tomasello
Harvard University Press, 1999
The Mind Doesn't Work That
by Jerry Fodor
MIT Press, 2000
What is the hard problem, then? It's as old as the human mind, it's probably lurking behind our ideas about religion and the immateriality and immortality of the soul, and it has been pondered since the advent of philosophy, where it is usually called the mind-body problem. Unfortunately, the word "mind" is ambiguous here, and "body" a misnomer.
The problem arises when we try to relate one sort of "thing" (mental things) with another sort of thing (physical things). We know that physical things are not just "bodies": they are matter and energy, the stuff that physicists (and chemists and biologists and engineers) study and explain to us with their familiar functional, cause-and-effect explanations (e.g., how momentum is transferred from one billiard ball to another, how chemical reactions proceed, how the liver functions). And we know exactly what mental "things" are, too. They are what is going on in our heads when we are awake: thoughts, experiences, feelings.
The problem is: How do we put those two kinds of things together? Are they both the same kind of thing? Are thoughts, experiences and feelings somehow just matter and energy? If so, How? (I pause to let the reader test whether, mirabile dictu, he can provide a satisfactory answer to this hard question where everyone else so far has failed. . . .)
Explaining How and Why We Are Not Zombies
If the mental and the physical are not the same kind of thing, what is the relation between them? We know they are exactly correlated, but correlation is not explanation. How does the mental fit into the physical world causally? Is it an extra "force," like gravitation? Those who resort to invoking the paranormal in the face of the hard problem reply "yes," and boldly proclaim the "telekinetic" power of the mind (rather like Uri Geller's spoon bending, except that it's mind over matter even when you bend the spoon with your fingers -- if you are really moving your fingers because you feel like it).
The trouble with this easy solution to the hard problem is that it has some uneasy consequences: It is at odds with the conservation of mass and energy, causal laws of physics that have an awful lot of evidence supporting them, all over the universe. To regard the mental as a telekinetic force, we have to be ready to believe that some rather remarkable things are going on on our small planet, that things move because they are willed to move, not just because of the usual transfer of energy. And what is the source of this telekinetic force? That's anyone's guess, but it can't be just our brains, because our brains, like our hearts and our livers, are just that ordinary stuff, matter and energy, described by their structure and function. (The telekineticists reason that if our brains were the cause of all our motions after all -- in other words, if telekinesis really didn't happen -- then it could never be true that we move because we feel like it; it would just feellike that was how and why we were moving.)
I will not pursue the telekinetic option any further (it is often called dualism), because, in exchange for "solving" the hard problem, it seems to raise even harder problems, pitting itself against all the rest of science. Suffice it to say that none of the authors of the books under review would endorse telekinetic dualism. They are all committed to explanations that stay within the natural bounds of matter and energy, structure and function -- bounds set by current theory and evidence in physics, biology and engineering. Yet let us admit that telekinesis certainly feels like the right explanation for our minds, and what they do, and how. It's just that it's an explanation that unfortunately does not fit with the scientific explanation of everything else -- and hence would itself stand in need of a good deal more hard scientific explanation in its own right.
Only three of the books under review try to take on the hard problem directly. The neurologist Antonio Damasio of the University of Iowa College of Medicine in Iowa City comes at it via brain anatomy and physiology. The neuroscientists Gerald Edelman of the Neurosciences Institute in San Diego, California, and Giulio Tononi of the University of Pisa in Italy use computational modeling of brain function. The philosopher Colin McGinn of Rutgers University in New Brunswick, New Jersey, does not actually try to solve the hard problem, but he offers an excuse: he argues that, although the problem does have a solution, the human brain is incapable of finding it (and would not understand it if it did). The other two books do not venture to take on the hard problem at all. The philosopher Jerry Fodor, a colleague of McGinn's at Rutgers, thinks it would be futile but (unlike McGinn) does not say why (he spends his time instead trying to show why we may not even be able to solve some of the easy problems!). And the evolutionary biologist Michael Tomasello of the Max Planck Institute for Evolutionary Biology in Leipzig, Germany, does not even mention consciousness.
There are basically two ways to beg the question raised by the hard problem. One way is to change the subject, swap an easy problem for the hard one (but keep calling it the hard one anyway), and then solve that easy problem instead. The second way is simply to provide an easy solution, but interpret it as if it had solved the hard problem. Damasio does the first and Edelman and Tononi do the second.
At the outset, it looks as if Damasio will not beg the question -- as if he is not merely going to explain intelligence or language or brain function or behavior. All of those could be explained in principle even if there were no hard problem at all: If we had the very same intellectual and linguistic capacities we have, but we were not conscious (no mental states, no thoughts, experiences, feelings), there would still be the "easy" problem of explaining those capacities in terms of brain function. But that would just amount to ordinary ("easy") science. Let us call that kind of explanation a structural-functional explanation (or simply a functional explanation, for short). Functional explanations are perfectly compatible with the matter-energy explanations given by physics, biology and engineering.
What makes the hard problem hard is that giving functional explanations for bodily capacities is not all there is to it: We are not just zombies with certain intellectual and linguistic skills. We are conscious; that is, we do have mental states: thoughts, experiences, feelings. Let's call what it is that makes mental states mental "feelings," for short. If we were nonfeeling zombies, there would be no hard problem. What makes the hard problem hard is precisely the mysterious difficulty of explaining feelings functionally. So the mind-body problem is actually the "feeling-function" problem.
Why is it so difficult (if not impossible) to explain feelings in terms of function? Because a functional explanation is always a cause-effect explanation, showing how and why something works the way it does. A functional explanation is fine for ordinary, nonfeeling matter-energy. But every time we try to explain a feeling functionally, it turns out that the function alone can do the cause-effect job just fine (thank you very much!), and the feeling just falls by the wayside, unexplained.
For example, a functional explanation of pain might go something like this: Pain is a signal that tissue has been injured. It is useful to an organism's survival and reproduction for the organism to minimize tissue injury, to learn to avoid what has caused injury in the past, to avoid contact between an injured body part and other objects while the part is still damaged, and so forth. The sensorimotor and neural machinery for accomplishing all this, including the computational mechanism that would do the learning, the remembering, the selective attending and so forth, could all be described, tested, confirmed and fully understood. The only part that would remain unexplained is why pain feels like something: the functional explanation accounts for the functional facts, but the feeling is left out. And so it goes: every time you try to give a functional explanation of feeling, the feeling itself turns out to be functionally superfluous (unless you happen to be a telekinetic dualist!).
In short, we know that we are not feelingless zombies. The hard problem is explaining how and why we are not. And because how's and why's are purely functional matters, that would seem to leave only two possibilities: (1) that feelings are not functional but merely "decorative," piggybacking (for some inexplicable, because nonfunctional, reason) on certain functions ("epiphenomenalism"); or (2) that feelings are telekinetic (dualism). The hard problem is finding an explanation for feelings that is neither (1) nor (2). My own view is that this is simply impossible. How do our authors fare with this?
Motions, Emotions and Unfelt Feelings
Damasio's title, The Feeling of What Happens, makes it sound as if he will be confronting the hard problem of feeling head-on. And his book does provide a great deal of new, insightful and illuminating data and theory about the brain areas and activities correlated with feeling, particularly the feeling of the "self," and about the remarkable ways in which those areas and activities can diminish or break down in sleep, coma, vegetative states, akinetic (motionless) mutism and epileptic automatism. We are all zombies when we are in deep, dreamless sleep; are we zombies in any of the more active states, too? These questions and answers are fascinating, but they do not include the hard one.
Maybe we are indeed feelingless zombies when we are in the grip of an epileptic automatism; maybe we are not. (It is hard to know for sure without being the epileptic undergoing the automatism, but even if you were, you wouldn't be able to speak at the time, and afterward you wouldn't be able to recall! So, without being telepathic, no neurologist could ever know for sure whether or not a patient was in a zombie state. That is called the "other-minds" problem, the flip side of the mind-body problem.)
Damasio's functional anatomy of feeling states certainly tells us a good deal about what their brain and behavioral correlates are: when this part of the brain is active, you feel this and you can do that; when you lose this part of the brain, you can no longer feel this or do that. Such correlates are of great interest to the clinician trying to carry out diagnosis, prognosis and treatment. They are also useful to the victims of brain injury, the families of such patients and to anyone else interested in the brain. Damasio's contributions to the brain anatomy of the so-called sense of self may even help theorists design functional models that actually have the capacities that go with having a sense of self. But those are all "easy" problems. Do Damasio's findings shed any light on the hard problem of how and why we feel at all?
Alas, they do not, and I think I can pinpoint where the question gets begged: Damasio is intent on providing a bottom-up explanation of feelings, from the most primitive feeling-state of akinetic mutism to feeling-states of the highest order, such as the ones experienced by a philosopher such as Descartes, when he is reflecting on the nature of mind. But explaining the variations along such a hierarchy is the easy part; the hard part is explaining how and why any of it is felt at all. The critical transition, in other words, is between nonfeeling and feeling, and that is the transition Damasio completely overlooks.
Instead, Damasio rests his hierarchy of feeling-states on a highly nonstandard (and, I think, in the end, incoherent) notion of emotion. On the face of it, "emotion" is just a synonym for a certain kind of feeling. (Other kinds of feelings include sensations, such as seeing something blue or hearing something loud; hybrid emotion-sensations, such as feeling pain; desire states, such as wanting something; psychomotor states, such as willing an action; and complex feeling-knowing states, such as believing, doubting or understanding.) But Damasio uses emotion in an equivocal way, in an attempt to bridge the unbridgeable gap between nonfeeling and feeling. His bottom-level emotions are either just motions --in other words, movement tendencies and their underlying brain activities, in which case they are no kind of feeling at all, and leave us as clueless as before about how to bridge the gap -- or, worse, they are "unfelt feelings," which is a contradiction in terms. Either way, it is only by invoking this blurred notion of emotion that Damasio gives the (illusory) impression of having made some sort of successful transition from the unfelt to the felt.
Descartes (whom some people wrongly blame for the idea of dualism) was the subject of an earlier book of Damasio's, titled Descartes' Error. In that book Damasio argued that Descartes had made the mistake of trying to separate what in the brain is inseparable: the psychic (mind) and the somatic (body). In the workings of the brain, Damasio pointed out, there is no such duality of function. That is correct, but let us not forget that all of the brain, both structure and function, is "somatic," and that's precisely the error Damasio makes with respect to motions and emotions. The functional part of emotion, the somatic part, is indeed, as Damasio maintains, just motion. But the felt (psychic) part is something else: something 100 percent correlated with brain structure and function, to be sure -- but, again, correlation isn't explanation. Correlations need a causal explanation, and the only candidate explanation, namely telekinetic dualism, is a nonstarter. Hard luck.
Edelman and Tononi's Hermeneutics
Do Edelman and Tononi manage to do any better than Damasio? They, too, set out promising not to beg the question the way others have done before them. They want to make sure they explain the difference between real seeing and, say, the activity of an optical transducer such as a photocell. It will not do, as they correctly point out, simply to declare that one's favorite functional mechanism "feels," any more than it will do simply to declare that an optical transducer "sees." In both cases the how and why of the feeling itself must also be explained.
But then Edelman and Tononi go ahead and beg the hard question anyway. They describe some very interesting functional networks -- "distributed, re-entrant" ones -- which, they hypothesize, have some powerful functional capacities (some already demonstrated experimentally, many of them not yet). They also describe how such networks are brain-like in many ways. This is all important and exciting, but it is still all just functional. The nagging question remains: How and why do the feelings come in (other than as the usual mysterious, unexplicated correlations)? Without an answer to that question, Edelman and Tononi's discussion is just an exercise in hermeneutics. The functional mechanism that correlates with feeling is interpreted as actually being the feeling, and hence as functionally explaining the feeling, whereas in reality it merely explains the functions that are mysteriously correlated with the feeling, nothing more.
Edelman and Tononi's network model is largely a mechanism for learning categories. A mechanism with all the functional capacities the authors attribute to their model will be an important contribution to cognitive science if it can indeed be shown to have all those capacities. But that is not what Edelman and Tononi are out to show here. In A Universe of Consciousness they simply try to persuade the reader that the functions of their network are somehow an explanation of feeling. The locus of Edelman and Tononi's question begging can, like that of Damasio's, now be pinpointed: An essential function of their model is discrimination, and their treatment of discrimination is equivocal in exactly the same way that Damasio's treatment of motions and emotions is.
To discriminate is to be able to tell things apart. Psychophysicists speak about the JND, or "just noticeable difference" -- the smallest sensory difference that people can feel. Feel? But of course psychophysics, being an ordinary functional science like all the others, really only deals with the smallest sensory difference people can detect and respond to. That could just as well apply to an optical transducer. The fact that it also happens to feel-like something when one detects those differences is another matter, and Edelman and Tononi's model comes no closer to explaining the how and why of that than an optical transducer does.
McGinn: We Don't Have the Brains
Colin McGinn suggests that the reason our species must resort to question begging in the face of the hard problem is that we just don't have the brains to solve it. Now let us immediately concede that he could be right about that -- but by the same token, the creationists could be right, too. There may be mysteries beyond the grasp of our intellects.
But why should the feeling-function problem be one of them? To make McGinn's suggestion into anything more than an arbitrary conjecture, we would have to answer a how-and-why question every bit as hard as the hard problem itself, namely, how and why is the brain unable to solve the hard problem? To this reader, McGinn's answers read like Just So stories, leaving us no less mystified than we were before being informed that our mystification was innate.
The brain does somehow cause feelings; no nondualist doubts that. The hard part is explaining how and why. McGinn is declaring, positively (but nondemonstratively), that there is an answer, but it just happens to be one that we are not equipped to grasp. By way of evidence, he gives examples of other kinds of things that our brains are not equipped to grasp. We cannot know, for instance, what it feels like to be a bat (with its extra sonar sense), any more than someone born blind can know what it feels like to see. But that's cheating! It amounts to saying that a certain feeling is simply missing from the human repertoire, and that certain feeling is what it feels like to know the solution to the feeling-function problem!
At the very least, to give his speculation some substance, McGinn would have to give some hint of what the solution to the hard problem might look like, and how and why it could be the solution, even though it did not feel like the solution. For, on the face of it, all we are asking for is a functional how-and-why explanation of something. Such explanations tend to be objective ones, which do not depend on how they "feel" to you, any more than the truth of a mathematical proof depends on whether or not it feels true to you (as Descartes also noted, famously). If there is indeed a functional explanation of feeling, it ought to be possible to at least state it (and test it, functionally), even if, because of our brain limitations, such a statement would not be sufficient to dispel the attendant mystery about the hard problem from our minds.
But perhaps McGinn means something even stronger: not just that we lack the sense to see that something is a solution to the hard problem even when it is staring us in the face, but that we even lack the means to state that solution. But that would be very odd, because it would be a limit not just on the nature of our brains, but on the expressive power of language and mathematics (both of which, though rooted in our brains, have universal, brain-independent powers, too). I may not be able to feel what it is like to be a bat, but surely I should be able to state all the functional facts about it (in fact, that's exactly how we understand the bat's sonar sense, and there is absolutely no mystery there, just a feeling that we know perfectly well that we humans happen to lack).
No, I don't think McGinn's conjecture helps us with the hard question at all. If the question is How and why do we feel, then his reply that we are not equipped to know simply raises another question, just as hard: How and why not?
Before leaving the hard problem and moving on to the two books that address easier problems, I will venture an answer: It is not because we have the wrong brains. It's because of the nature of functional explanation, the nature of feeling, and possibly also the nature of causality. The only alternative to telekinesis (which gives feelings an independent causal power of their own) is that feelings do not have an independent causal power of their own. They just are. (We know they exist; that's not in dispute.) Moreover, they pose no problem to the rest of science if they are merely epiphenomena, that is, side effects of matter and energy, structure and function, not causes in their own right.
Make no mistake: We are no less mystified by my own conclusion that the "function" of feelings is merely decorative, but at least epiphenomenalism renders moot any further how-and-why questions. And it implies that the reason the hard problem is insoluble is that (1) telekinesis itself is false and (2) feeling is immune to nontelekinetic, functional explanation (hence it is inexplicable).
Tomasello: Pantomime vs. Propositions
The question Tomasello is trying to answer is unapologetically one of the "easy" ones: How and why does our species, and no other, master language? In the past, other theorists have begged the question of consciousness by suggesting that having consciousness and having language are somehow one and the same thing, but Tomasello will have no part of that view. He recognizes that animals not only have feelings but that they are also very smart. So in many ways the question about language is: How and why do we differ from other animals in that respect? What is the functional specialization that makes us capable of language, and them incapable?
To answer that question Tomasello studies the behavioral, social, conceptual and communicative capacities of apes and of children both before and after the age at which children acquire language. His comparative studies implicate a few critical capacities: the capacity to imitate others; the capacity to "mind-read" (to sense what others are seeing, wanting or thinking); and the capacity to monitor and coordinate joint attention with others (to sense that you and another being are looking at or thinking about the same thing, and to sense that the other being senses that too). (Damasio's mechanisms for the sense of self would come in handy here.) No nonhuman species has that set of capacities in full, and even human children do not have it until the age when language usually begins. So Tomasello concludes that those are the capacities that make up the functional basis of language.
These findings are very important, and, as Tomasello shows, the capacities he has isolated form a basis for human culture. But do they explain language? Has Tomasello really pinpointed the functional basis for language (apart from grammar), here? I would like to suggest that he has not. For human language is, among other things, the capacity to express any proposition with a string of symbols -- "The cat is on the mat," "Feeling cannot be explained functionally," "2 + 2 = 4" -- plus the capacity to understand symbol strings as expressing propositions.
But if you look closely at the capacities Tomasello has singled out (and even if you design a functional model that implements those capacities), you will find that you have a mechanism that is capable of producing and sharing social pantomime. Such a mechanism could act out present and future scenes, draw people's attention to this or that, share all the kinds of data that can be shared through this kind of joint activity -- but this does not provide a clue about how to get from pantomime to propositions. Even acting out the cat's being on the mat is simply that: a pantomime of the cat being on the mat, in much the way that the cat's actually being on the mat is a pantomime of itself.
In short, entities with Tomasello's functional capacities remain in the analogue world of events, copies of events, and reenactments. Such capacities may well be necessary preconditions for language. But there is no language proper until we make the transition from this analogue world of social imitation to the arbitrary, symbolic world of propositions. Perhaps Tomasello's functional resources need to be augmented with Edelman and Tononi's category-learning network. If that network has the power they say it does, it should be able to learn to detect and identify cats and mats and "on-ness." So far that would just name them. But if it can also string those names into propositions that describe events and can be construed as either true or false, then we may indeed be closer to the functional substrate for language capacity.
Fodor's Skepticism about Explaining the Mind
Grounding language in category learning, however, is an enterprise about which our last author, Jerry Fodor, is somewhat skeptical. To understand what Fodor is driving at, you have to know where he is coming from. Although Fodor and McGinn are both philosophers, Fodor's work is in part inspired by the monumental work on grammar by the linguist Noam Chomsky of the Massachusetts Institute of Technology. Chomsky showed that much of the human capacity for grammar, rather than being learned, arises from a complex inborn structure in the brain. Furthermore, that inborn "universal grammar" probably did not evolve in the usual way, the way that fins or wings did; instead, universal grammar has somehow been an intrinsic part of the structure of matter ever since the big bang, or possibly even a necessary part of the eternal Platonic world of logic and mathematics, constraining matter whenever it is configured into a mechanism capable of language.
Now, this view of Chomsky's is highly controversial, but it has a great deal of evidence supporting it. It does look as if universal grammar is not and cannot be learned by the language-learning child (as Chomsky has long been pointing out, the trial-and-error possibilities are far too large, and the child's actual learning time and experience far too small); for similar reasons, it is hard to imagine how universal grammar could have evolved in the usual way (but that conclusion is perhaps not as firmly based on evidence as the fact that universal grammar cannot be learned in childhood).
Fodor, impressed by the innateness of one function of the mind, universal grammar, generalizes to other functions that go far beyond the evidence for universal grammar: Fodor thinks that most categories ("cat," "mat," "object," "number" and so forth) are innate and unlearned, just as universal grammar is. All we learn is what names to call them; their meanings are already innately in our heads, like placeholders merely waiting for labels. If this is true, it is bad news for Edelman and Tononi's category-learning networks, because it leaves them precious little to do: most of the category structure of the world would somehow have to be built into them in advance.
But is there any reason to believe that Fodor's assertion is true? Is there any evidence that there are not examples enough, and time, for children (and adults) to learn all the kinds of things there are in the concrete and abstract world by trial and error, guided by feedback that indicates when they get it right and wrong? I think there is no such evidence. But then why does Fodor believe that what is true of grammar might be true of meaning, too?
I think the answer is related to yet another "easy" problem, the symbol-grounding problem: Symbols alone do not mean anything. Ignorant of Chinese, you would look in vain for the meaning of any Chinese word in a Chinese-Chinese dictionary: It's all in there, and yet it isn't! You look up a definition, and it's just more meaningless symbols, even though, for a Chinese speaker who does not know the meaning of that particular word but does know the meaning of the words used to define it, the definition is enough to convey the new meaning.
This example illustrates both the power and the limitation of language. In principle, you can find out anything and everything from strings of words expressing propositions, but you can't start from scratch: some of the words have to be "grounded" in something other than just more meaningless words. How are those basic words to be grounded? Edelman and Tononi's networks, linked to the world through sensors and effectors, sound like a good start, although one would be well-advised to build in the functions Damasio describes for internal sensorimotor maps and the self, as well as the functions Tomasello describes for social communication.
Such a system would then ground some of its symbols directly, in the capacity to detect, discriminate, categorize and manipulate the things they stand for in the outside world. Other symbols could then be grounded indirectly, through propositions that define them in terms of already grounded symbols.
Fodor thinks that kind of mechanism is a nonstarter, for pretty much the same reason that the "associationism" of seventeenth-century philosophers was a nonstarter: Thought and meaning arise not merely through the association of "ideas." Thought has structure over and above mere association in time and space. Symbols and computation can perhaps capture some of the structure of thought, but Fodor, although he is a functionalist and a computationalist (computation is his "language of thought"), doubts that they can do the whole job. His doubts are based in part on worries about "holism" (the view that symbols are local things, but meanings are not) and in part on "abduction" (how can a symbol system find the best theory to explain any set of data unless the answers are all already built into it in advance?). So Fodor would not believe in the computational component of such a hybrid symbol-grounding system, either.
Are there grounds for all this skepticism on Fodor's part about the only explanatory resources that cognitive science has at its disposal? It is certainly true that cognitive science has not even come close to solving any of its "easy" problems, such as explaining the functional basis of language or meaning or any other life-size piece of human intellectual capacity or brain function. But it's also hard to know how fast cognitive science ought to be explaining the mind, based on scientific track records elsewhere. Edelman and Tononi have to be given the time to demonstrate whether or not their networks can do what associationism could not. Their networks are, after all, operating on sensorimotor and symbolic inputs, not "ideas" (whatever those are).
And if symbols have their limitations, they also have their powers. No one can say in advance what hybrid systems can or cannot accomplish if their symbols are grounded in the sensorimotor world via category-learning networks. Changing the definition of just one word in a dictionary propagates "holistically" to every other definition in the dictionary in which that word figures. Change the sensorimotor grounding and the holistic effects could be even more dramatic.
So there's no a priori reason to doubt that the "easy" problems can be solved using cognitive science's current functional tools. If what you want to know, however, is how and why it feels like something to be a system that has and exercises all those remarkable functional capacities, I am afraid you will be disappointed. That is one unsolved mystery we may all just have to learn to live with.
Stevan Harnad is a professor of cognitive science at the University of Southampton in England. His papers on categorization, communication and cognition are archived at http://www.cogsci.soton.ac.uk/~harnad.
Cangelosi, A. & Harnad, S. (2000) The Adaptive Advantage of Symbolic
Theft Over Sensorimotor Toil: Grounding Language in
Perceptual Categories. Evolution of Communication (Special Issue on Grounding)
Chalmers, D. (1995) Facing Up to the Problem of Consciousness. Journal of Consciousness Studies http://www.u.arizona.edu/~chalmers/papers/facing.html
Chomsky, N. (1959) A Review of B.F. Skinner's Verbal Behavior. Language
Damasio, A.R. (1994) Descartes' Error: Emotion, Reason, and the Human Brain. Avon Books.
Fodor, J. A. (1975) The language of thought. New York: Thomas Y. Crowell.
Fodor, J. A. (1985) Precis of The Modularity of Mind. Behavioral & Brain Sciences 8:1-42.
Harnad, S. (1987) The induction and representation of categories. In:
Harnad, S. (ed.) (1987) Categorical Perception: The Groundwork of Cognition.
New York: Cambridge University Press.
Harnad, S. (1990) The Symbol Grounding Problem. Physica D 42: 335-346.
[Reprinted in Hungarian Translation as "A
Szimbolum-Lehorgonyzas Problemaja." Magyar Pszichologiai Szemle XLVIII-XLIX (32-33) 5-6: 365-383.]
Harnad, S. (1996) The Origin of Words: A Psychophysical Hypothesis In
Velichkovsky B & Rumbaugh, D. (Eds.)
Communicating Meaning: Evolution and Development of Language. NJ: Erlbaum: pp 27-44.
Harnad, S. (2000) Correlation Versus Causality: How/Why the Mind/Body Problem Is Hard. Journal of Consciousness Studies 7(4): 54-61. http://www.cogsci.soton.ac.uk/~harnad/Papers/Harnad/harnad00.mind.humphrey.html
Harnad, S. (2000) Minds, Machines, and Turing: The Indistinguishability of Indistinguishables. Journal of Logic, Language, and Information 9(4): 425-445. (special issue on "Alan Turing and Artificial Intelligence") http://www.cogsci.soton.ac.uk/~harnad/Papers/Harnad/harnad00.turing.html
Tomasello, M., Kruger, A. C. & Ratner, H. H. (1993) Cultural learning.
Behavioral & Brain Sciences 16:495-552.