Introduction: The mind as computer



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Introduction: The mind as computer
How is it possible to have a science of the mind? To have a science of the mind, you have to be able to observe the mind, or to infer its workings from something that is observable. Let's take a look at these options in turn.
Observing the mind.
If you are a dualist, believing the mind to be immaterial, observation of the mind is problematic. Observation seems to require some sort of physical contact with the thing observed. It has to reflect light, distort a magnetic field, and collide with something. If the mental is immaterial, it seems it must be undetectable. Of course, most dualists hold that there is mind-body interaction: that pains cause screams, that intentions to move one's arm cause one's arm to move, that bodily damage causes pain, and that light causes visual experiences. This leads to the possibility of inferring the mental from its environmental or bodily causes, and from its bodily effects. But there is a principled difficulty with this idea. We may, indeed, infer a mental effect from a bodily cause, or a mental cause from a bodily effect, but we will have no idea what these mental causes and effects are like. We will, in short, have no idea what sort of mechanisms the mind harbors, and hence no idea how it works to produce the effects it does, or to be affected as it is.

To get a feel for this, consider the following analogy. How might an early chemist have explained chemical bonds? Two ideas dating from ancient times are (1) that atoms are like burrs, and stick together, or (2) that they are like pieces of a three dimensional jigsaw puzzle, or, more simply, fitted with hooks and eyes. The law of constant proportions—the fact that elements regularly bond together in fixed ratios by weight--favors (2) over one. You get H2O because each hydrogen atom has one free hook, and each oxygen atom has two free eyes. Burrs just clump willy-nilly. However, while (2) allows for H2, it ought, contrary to fact, be less stable than H20, since it ought to be easier for two hooks to come apart than for a hook to come out of an eye. And there shouldn't be any O2 at all.

You can do all this reasoning about (1) and (2) because you already understand, or can readily investigate, the properties of burrs and of hooks and eyes. (1) and (2) simply project those properties into the realm of the imperceptibly small. But suppose a defender of (2) were to respond to the problem about O2 by suggesting that very, very small eyes, unlike the medium sized ones with which we are all familiar, can hook together. Although intended to save the theory from an objection, this reply would rather undermine it altogether, for it would leave us clueless about what to expect of micro hooks and eyes, thereby destroying the theory's explanatory and predictive power.

The moral of this story is not far to seek. Inferred mechanisms, if they are to have any explanatory or predictive value, must be, to some extent anyway, understood independently of the effects they are rung in to explain. And this is where the dualist is in trouble, because mental mechanisms cannot, for the dualist, just be ordinary mechanisms that happen to be hidden away in the mind. They only occur in the mind. They occur nowhere else, and they do not operate on physical principles. So, although dualists believe in mind-body interaction, the idea that the mental can be inferred from its bodily causes and effects founders on the lack of any mental mechanisms to mediate bodily causes and effects.

The answer to this difficulty seems obvious: introspection. The mind, as we all know, can observe itself.

So the mind, according to the dualist, can be observed after all, and directly observed at that. It is just the minds of others that cannot be directly observed. We will return shortly to this idea. But first, let's see how the would-be psychologist fares if she is not a dualist, but a materialist instead.

The materialist, surprisingly, has the same problem as the dualist because of something we call Leibniz' Gap. Here is Leibniz' formulation of the Gap.

“ It must be confessed, moreover, that perception, and that which depends on it, are inexplicable by mechanical causes, that is, by figures and motions, And, supposing that there were a mechanism so constructed as to think, feel and have perception, we might enter it as into a mill. And this granted, we should only find on visiting it, pieces which push one against another, but never anything by which to explain a perception. This must be sought, therefore, in the simple substance, and not in the composite or in the machine. (Leibniz, Monodology, sec.17)

There is, as Liebniz points out in this famous passage, a gap between the concepts we use to describe the mind, and those we use to describe the brain. So even if we are convinced that the mind is the brain, or a process going on in the brain, physical observation of the brain seems to give us data in the wrong vocabulary: synapses rather than thoughts. When we look at a brain, even a living brain, we don't see thoughts. Or, not to beg the question, we don't see anything we readily recognize as thoughts. If you could put on Newton glasses and look at a billiard game in progress, you would see vectors with centers of gravity at their tails. If you could put on Psychology glasses and look at a living brain, you would, according to the materialist, see thoughts, and probably a good deal more. But to build Psychology glasses, you would need to somehow bridge Liebniz' Gap by correlating observed brain properties, events and processes with psychological properties, events and processes. It seems that the only way you could do this would be to rely on introspection to identify the psychological correlates of what you could observe in the brain.

But this puts the materialist in the same boat as the dualist: relying on introspection to generate an observational base in order to get a scientific psychology off the ground. Liebniz' Gap may only be a conceptual gap, but it seems it is no easier to see across it than it is to see across the metaphysical gap that separates the mind and body for the dualist. And so it seemed to dualist and materialist alike that psychology must be founded on introspection.


Structuralism
Unquestionably the most significant introspectionist program in the United States was the "structuralism" of E. B. Titchener. Titchener was concerned to establish the claim that the "new psychology" imported from Germany had made psychology a rigorous empirical science. Lacking a nontrivial account of science, Titchener supported his claim by emphasizing the analogies between psychology as he saw it and an established experimental science--namely, physical chemistry. To understand Titchener's vision of psychology, therefore, we do well to examine his model.

The core of physical chemistry in Titchener's time was the periodic table. The periodic table allowed one to explain analytically an enormous number of the chemical properties of compounds. It provided a list of chemical elements--i.e., components whose further analysis is not theoretically significant for the explanation of properties in the intended domain--together with a specification of the chemically important properties of those elements. With these resources, it was possible to derive laws of composition--which compounds are possible--and laws of instantiation--which properties a compound will have given its constituents and structure--for a large number of the empirically established chemical properties of substances. Titchener's idea was to provide for psychology what the periodic table provided for chemistry, thereby making it possible to explain the properties of mental events and processes by analyzing them into psychological "elements": mental events that could not be further analyzed.

Since Titchener's elements are not things or substances but events, his program requires some account of the origin of elements. He needed a general recipe for answering this question: Why do we have just these elements present (in consciousness) at this time rather than some others? There appear to be just three possible sources of elemental mental events: either a current element is the effect of one or more previous elements, or it is the effect of extramental stimuli, or both. Titchener allowed all three possibilities, but he concentrated mainly on the second, probably because (i) extra mental events are more open to experimental manipulation, and (ii) under the influence of empiricist philosophy, Titchener believed that perception is the most significant source of events in the mind.

The object of psychological theory, then, is to explain the origin and properties of the contents of consciousness--e.g., feeling anger, the visual image of a pouncing cat, or the experience of voluntary action. Suppose the feeling of anger has properties Q and R (as revealed by introspection). To explain why anger has these properties, we are to proceed by analyzing this feeling into its elements--call them x, y, and z. Then, appealing to the properties of these elements and the laws of composition, we endeavor to show why anger must have the properties Q and R. To explain why S was angry at some particular time, we explain the occurrence of x, y, and z (tokens of the mental element-types that make up anger) as effects of previous mental events and/or current extramental stimuli. (Perhaps we shall also need to explain why conditions were propitious for the combination of x, y, and z into anger. Compare: many chemical combinations require a fair amount of heat, or a catalyst, to take place.)

The project, then, was to discover the fundamental and introspectively unanalyzable elements of consciousness and to formulate the principles of combination whereby these elements are synthesized into the complex and familiar experiences of ordinary life. Every compound mental state or process was to be explained compositionally, the characteristics of the whole derived from the characteristics of the parts and mode of combination. Not surprisingly, introspectionists spoke of mental valences, of mental equilibrium, and of mental elements neutralizing each other.

The fundamental issues of mental analysis and synthesis never made significant progress, however, and the reason is fairly clear. There were simply no technologies or experimental procedures for analysing a mental event or process: nothing like qualitative analysis in chemistry. This was a fatal defect. You get a large explanatory payoff from the strategy of explaining the observed properties of things in terms of the properties of their elemental constituents and their mode of combination only if the properties of the compound differ significantly from those of the constituents. Salt is nothing like chlorine or sodium. But this means that simply observing salt will tell you nothing about its constituents. You need to be able to analyze it--break it up into its components and isolate them for study. Lacking anything comparable to the laboratory tools of analytical chemistry, the student of introspective psychology was, in the end, simply left with passive introspective observation, and this mean that no significant analysis could be forthcoming. Everything, in effect, was an unanalyzable element.

Introspection as a method thus sorts ill with the explanatory strategy of the theory. This strategy was to explain analytically the properties of the complex contents of consciousness, and perhaps the capacities of the mind required for it to have such contents. Since introspection is, at best, a form of observation, it can hope to yield data--the properties of conscious contents--but it cannot hope to yield analyses. The elements and their properties will not be "visible" when compounded unless we assume that there is no serious composition at all. If we assume a bushel basket theory of consciousness, a step even Hume did not take, then the properties of anger, say, will simply be the union of the properties of the elements in consciousness when one is angry.

So the analysis of consciousness bogged down for lack of analytical tools. But the correlative project to explain the elements of consciousness as responses to perceptual stimulation did not, for here introspectionists had an experimental paradigm in the Weber-Fechner experiments. Here is Titchener's textbook description of a typical variation.

Method-to find the numerical expression of Weber's law for noise-An ivory ball is let fall from two different heights, upon a hard-wood plate. The difference of intensity between the two sounds(i.e., the difference between the two heights of fall) must be slight. The two sounds are given in irregular order in different experiments (to avoid the influence of expectation), and the subject is required to say, in each case, whether the second is louder than the first. In 100 experiments, he will give a certain number of right answers, and a certain number of wrong.

The method assumes that if the two sounds are just noticeably different in intensity, the subject will give about 80% right and 20% wrong answers. This proportion is calculated by what mathematicians call the 'law of probability.' Now suppose that a certain difference gave 70 right and 30 wrong answers in 100 experiments. We could calculate, by aid of the integral calculus, how much larger the difference must have been to give 80 right and 20 wrong-i.e., to be just noticeable. The calculated difference (difference of height of fall) is the numerator, and the original intensity (height of fall) of the weaker sound, the denominator, of the fraction which expresses Weber's law. [Titchener, 1897,pp. 81-82]


Here is a different and more fundamental description:
Suppose that we are investigating the intensity of noise. We shall begin with a stimulus of moderate intensity: say, the noise made by the fall of an ivory ball upon a wood plate from a height of 90 cm. We will call the intensity of this sensation 1. If we gradually increase the height of fall, we shall reach a point at which the noise of the fall is just noticeably greater than the original noise. We may call the intensity of this second sensation 2. If we further increase the height of fall, we shall presently get a noise, 3, which is just noticeably louder than 2; and so on. Now what are the different heights of fall-i.e., intensities of stimulus-necessary to arouse sensations of the intensities 2, 3, 4, etc.? .. . An addition of 30 cm. suffices to raise the intensity of sensation from I to 2; but if we are affected by the stronger stimulus 120 cm., we must add more than 30 to it to change intensity 2 to intensity 3. In other words: change in the intensity of sensations does not keep even pace with change in the intensity of the stimuli which occasion them.

Experiment enables us to replace this general statement of the relation of sensation intensity to stimulus intensity by a definite scientific law. If sensations are to increase in intensity by equal amounts, their stimuli must increase by relatively equal amounts.[Titchener, 1897, pp. 79-80]


This sort of experiment, begun by Weber (1834) and refined and expanded by Fechner (1860), led to Fechner's law (mistakenly called Weber's law by Titchener and Fechner-compare Gregory, 1981, pp. 500-505). Here was a law and a procedure for testing and refining it. Does the law hold for all sensation? What are the values of the constants of proportionality for each case? Work on these matters proceeded apace.

But this apparent bright spot in the program proved to be an Achilles' heel. Although the analysis of consciousness languished, the fact was that there were no generally accepted or clearly articulated canons for the evaluation of structural-analytic explanations envisioned above for complex mental processes like anger, especially as applied to consciousness. Hence there was no way to determine whether the trouble was merely practical or deeply conceptual. On the other hand, there did exist well-articulated and generally accepted canons for the evaluation of the sort of explanations Fechner's law was used to construct, since the idea is to explain an event in consciousness--e.g., a discernible difference in loudness--as an effect of an external cause--a change in stimulus strength. Critics knew how to hunt down and articulate problems with this sort of paradigm, and they did so.

We can bring out the problem in a few lines. The canon requiring independent access to causes and effects applies to the Weber-Fechner experiments as follows. Suppose we find a subject whose responses don't fit the law? Is the subject (A) misdescribing his/her sensations, or (B)psychologically idiosyncratic? For that matter, how do we know that a subject responding normally is not in fact (C) psychologically idiosyncratic but systematically misdescribing experience, rather than (D)psychologically normal? We cannot compare a subject's descriptions with what is supposed to be described. Our only access to the sensations of the subject are (i) inference from established connections with responses, including verbal reports, and (ii) inference from established connections with stimuli. Obviously we cannot establish connections of the sort required unless we can, at least sometimes, distinguish(A) from (B) and(C) from (D). But we cannot make these distinctions unless we can establish the connections. Since the accuracy of introspective observation cannot be checked, it seems it cannot play the role required of scientific observation. With introspection ruled out of court, the only way to measure sensation intensity is to measure stimulus intensity and then calculate sensation intensity using Fechner's law, but if we do this, we are using Fechner's law to define sensation intensity, and we cannot then turn around and pretend to explain the intensity of a sensation by appeal to stimulus intensity and Fechner's law. Once introspection is disqualified, we have no access to sensation intensity other than the very law that is supposed to explain it.
Behaviorism.
Thus it was that introspectionists became vulnerable to a powerful methodological attack. The methodology of explanation by causal subsumption had been well entrenched by Bacon, Berkeley, Hume, Mach, and Mill. The empiricist doctrine was (and is) that causal laws have no explanatory power unless the causes and effects they subsume are knowable independently of each other. It is ironic that this doctrine was so fatal to introspectionism, for introspection was held to be the only avenue to noninferential knowledge by the very empiricist philosophers who developed the line of argument that killed introspectionist psychology. Locke's inverted spectrum problem returned to haunt those who attempted to pick up where Book Two of the Essay Concerning Human Understanding left off. The inverted spectrum problem turned on the conceptualist assumption that linguistic training would inevitably disguise sufficiently systematic "psychological" differences. This is just the possibility raised by (A) and (C).

Whatever we may think of this critique, the mere certainty that it could be formulated was eventually enough to kill any psychology based on introspection. And just as Berkeley's critique of representational realism seemed to point inevitably to a single alternative (if we know about tables and can know only about ideas, then tables are ideas), so this critique of introspectionism seemed to point inevitably to a single alternative. Introspection isn't genuine observation, so the Weber-Fechner law cannot be about consciousness. It is obviously about something, though, for experimenters certainly observe and record something in the sort of experiment described by Titchener in the passages above. What? Since experimenters record what their subjects say, the Weber-Fechner law must correlate stimuli with "introspective behavior." This is verbal behavior, per accidens, in the usual experimental set-up, but button pressings would do just as well.

This bit of diagnostics would probably have sufficed to produce behaviorism eventually, but a number of other factors conspired to guarantee a quick takeoff. Two, I think, are especially worthy of note. First, pragmatism was in vogue in United States philosophical circles, and pragmatists emphasized the importance of understanding connections with action to the understanding of traditional philosophical problems involving mental states and processes. According to Dewey, for instance, the central mistake of empiricists and rationalists alike is the supposition that knowledge and belief can be understood independently of action, and treated as antecedent conditions to be investigated in their own right. Refuting this supposition was always a central theme in Dewey's writings. It isn't a long step from this to the doctrine that talk of mental states is just shorthand for talk of identifiable behavioral patterns. John Watson, the founder of behaviorism. was a student at Chicago at a time when the influence of Dewey's pragmatism was very strong there.

The other significant factor was Pavlov's discovery of stimulus substitution. This was an important discovery in its own right, but in the intellectual climate we have been describing, it had a special significance, for it seems to account for the sort of phenomenon generally attributed to the association of ideas, without recourse to ideas. Someone taken with the empiricist critique of introspection, and the pragmatist treatment of doxastic states, could hardly have failed to conclude that Pavlov had shown that it was stimuli and responses, not ideas, that were associated. This had to seem a major breakthrough, for association was the only principle of learning on hand.

Analysis in the Behaviorism of Watson. By itself, stimulus substitution has no chance of explaining complex behavior, or the introduction of new responses. Pavlovian conditioning can simply link new stimuli to responses already in the organism's repertoire, and experimentation quickly revealed that the stimuli and responses involved had to be rather short and simple. The new Pavlovian principle of association, though experimentally demonstrable, seemed to be no more explanatory than the old version.

Watson brightened this dim scene with a simple strategy: analyze an extended behavioral pattern into a sequence of responses to stimuli produced by execution of the previous response. Consider playing a tune from memory on the piano. Initially we have a set of connections between perceiving a written note (stimulus) and striking the appropriate key. Now striking a particular key produces a corresponding stimulus-visual and kinesthetic-that always immediately precedes striking the next key specified by the score. Thus repetitious playing from the score should produce stimulus substitution-perception of a previous response substituting for perception of the next note in the score. When substitution is complete, the score will be unnecessary.

This analysis fails for a number of rather boring reasons-e.g., it runs afoul of the fact that people can play more than one tune from memory without difficulty even though the different tunes share notes. But the strategy was promising and exciting because the problem of explaining acquisition and exercise of a complex behavioral capacity is reduced to the problem of analyzing the capacity into simple antecedently explained capacities. A compellingly general picture of psychological change emerges. An organism begins with a genetically determined endowment of S-R connections, some of which, perhaps, emerge only at certain maturation stages. This basic endowment is expanded via stimulus substitution. The resulting connections are then combined in more or less complex sequences to yield an organism with a respectably complex behavioral repertoire.

This picture is still the operative picture underlying behaviorist psychology. The shifts have been shifts in detail. First, contemporary behaviorism is far more liberal about genetic endowment than was Watson--the tabula isn't nearly so rasa. Second, and more important, classical Pavlovian conditioning is supplemented by operant conditioning. Since operant conditioning builds on emitted behavior rather than on preexisting S-R connections, this change affects the assumptions about genetic endowment. Also, since operant conditioning produces "shaping," contemporary behaviorism has a source of novel, unanalyzable, behaviors available as building blocks. But the basic Watsonian picture remains: significant psychological change is the result of composition of antecedently explained (via genetics or shaping) behaviors. Hence, psychological explanation must proceed by analyzing observed behaviors into more tractable components. Thus it is analysis, not subsumption under causal law, that is the central explanatory strategy of behaviorism.

Watson conceived of an organism as a transducer the input-output characteristics of which could be altered over time. To characterize the organism psychologically at a moment in time is to specify the current input-output properties-a bundle of S-R connections. The goal of psychological theory is to specify transition laws that subsume successive changes in momentary input-output properties-e.g., the law of stimulus substitution. It is therefore ironic that Watson never introduced a single principle of this type. Instead, his major achievement was the introduction of the analytical strategy into behaviorism in the guise of the response chain. Watson was fond of saying that the point of scientific psychology is the prediction and control of behavior. But Watson's analysis of habit, even had it been sound, would not have increased the power of psychology to predict or control responses at all, though it would have greatly increased its explanatory power. For example, the problem about playing a tune from memory was not that it was unpredictable: whether or not a subject could do this was predictable from the amount of practice. Watson's analysis did not alter this situation at all, for Watson did not isolate a stimulus, or stimulus history, that has as response playing a tune from memory. What Watson did was explain the capacity to play a tune from memory by analyzing it into antecedently understood (or anyway antecedently present) capacities of the organism. This in turn allowed Watson to describe (inaccurately as things turned out) the conditions under which this capacity would be acquired, but these were already known, and, in any case, this is not predicting a response. '

Watson's presentation of his analysis of habit formation in Behaviorism (1924) is introduced as a response to his discussion of the sort of learning Thorndike studied; what we now think of as operant conditioning.


... Let us put in front of the three-year-old child, whose habits of manipulation are well established, a problem box-a box that can be opened only after a certain thing has been done.... Before we hand it to him, we show him the open box containing several small pieces of candy and then we close it and tell him that if he opens it he may have a piece of candy. ... Let us suppose that he has 5 0 learned and unlearned separate responses at his command. At one time or another during his first attempt to open the box, let us assume that he displays, as he will, nearly all of them before he pushes the button hard enough to release the catch. The time the whole process takes, we will say, is about twenty minutes. When he opens it, we give him his bit of candy, close the box and hand it to him again. The next time he makes fewer movements; the third time fewer still. In 10 trials or less he can open the box without making a useless movement and he can open it in two seconds.

Why is the time cut down, and why do movements not necessary to the solution gradually drop out of the series? This has been a hard problem to solve because no one has ever simplified the problem enough really to bring experimental technique to bear upon it. [p. 204]


This is not even prima facie a problem of prediction and control; it is a problem of explanation. It isn't at all clear how Watson's analysis is supposed to help with this particular problem, but it is quite clear that we have a capacity that wants explaining, not a response that wants predicting, and that the explanatory strategy employed is analysis, not subsumption. The learning curves obtained by Thorndike and others specify a capacity of organisms. It was this capacity that Watson sought to explain by analyzing it into the capacity for stimulus substitution and the antecedently available capacities characterized by the organism's pretrial S-R connections.

Behaviorism eventually came to grief because it had no resources to explain the acquisition of novel responses. Pavlovian conditioning attaches new stimuli to old responses, but introduces no new responses. Operant conditioning alters the probability that a given behavior in the repertoire will be emitted, but doesn't add to the repertoire. It is one of the great ironies of the history of science that behaviorism, which identified psychology with learning theory, was ultimately unable to accommodate the principled acquisition of novel behavior--learning, in short.

There is a far more fundamental problem with behaviorism, however. Even if the behaviorist program had succeeded, it would have, at best, specified our psychological capacities; it wouldn't have explained them. To see this, one has only to note that behaviorism has, in principle, no resources to explain why an organism can be conditioned, or why some schedules of reinforcement work better than others. One can say which laws of conditioning characterize which types of organism, but one cannot say why.

This is no accident, of course. Behaviorism seeks to avoid the problem about observing the mind by eliminating the mind from psychology. But it is breathtakingly obvious that we are conditionable, that we can learn language, that we can recognize objects, and so on and so on, because of the way our minds work. Introspection does tell us that much. It just doesn't tell us how our minds work. So, we are back where we started: you cannot have a science of the mind unless you can observe the mental, and the only way to do that appears to be introspection. But introspection cannot be calibrated, and is maddingly passive and hence unsuitable as an analytical tool. Behaviorism got around this by banishing the mind from psychology, but most of the interesting questions were banished along with it.

Inference to unobservables. Suppose we admit that most of the mind is unobservable: why is that a problem? After all, science is full of unobservables. Why cannot the machinery of the mind be inferred from its observable manifestations in the way that genes were inferred from inheritance patterns?

Here, it might seem, the materialist has a real advantage, for, if minds are physical systems, then mental mechanisms are physical mechanisms. Our lately imagined chemist was able to reason cogently about the pros and cons of the burr versus hook-and-eye accounts of chemical bonding because burrs and hooks-and-eyes were antecedently well-understood physical mechanisms. But where is the immaterialist going to find a stock of well-understood mental mechanisms which might be postulated to explain observable psychological phenomena?

But, once again, the apparent advantage enjoyed by the materialist intent on a science of the mind is undermined by Leibniz' Gap. For, while there are lots of well-understood physical mechanisms for sticking things together, there are none for producing thoughts, feelings or perceptions or sticking them together. Or rather, there are only two, namely inference and association.

Philosophers and scientists have always followed common sense in explaining the acquisition of certain beliefs and desires by appeal to inference. People are planners, and planning requires inferring what the world will be like when one comes to the time and place of action, inferring what changes one's contemplated actions will make in the world, and inferring what sub-goals one must achieve in order to achieve the goal of the plan. The inferentially mediated interplay of belief, desire and intention that is familiar to common sense seems capable of explaining a vast amount of the kind of behavior that is characteristic of thinking beings. Leibniz, Helmholtz and Freud extended this idea to other phenomena (notably perception and affect) by allowing for unconscious inference as well a conscious inference. But it remained a conceptual mystery how a physical device could be an inference engine, and attributing inferential powers to an immaterial mind simply made the mystery metaphysical. Thus inference, though a powerful explanatory process, was itself left unexplained.

Association fared no better: though a variety of phenomena--especially memory phenomena--could be explained by appeal to principles of association, there was no prospect of explaining association itself.
Computationalism.
Computationalism provides a way around this impasse by proposing that mental processes are computational processes, i.e., by proposing that the mind is a computer, and that inference and association are just two, albeit important, computational processes endogenous to the mind.

Functionalism. Computationalism is a species of functionalism. Functionalism is best seen as a proposed solution the problem posed by Leibniz' Gap. The central idea is that mental concepts specify their instances in terms of what they do--in terms of their functions--rather than in terms of their intrinsic structures. Doorstop, valve-lifter, mouse trap, can opener, pump, and calculator, are all functional concepts. A great variety of different physical structures can be pumps: hearts, propeller and case, vibrator and one-way valve, centrifuges, piston-and-sleve arrangements. What they have in common is a function: to pump. Since they do not have a physical composition in common, you cannot reduce being a pump to having a certain physical structure. Yet certain physical structures are sufficient for pumping; nothing non-physical is required. Functionalism in the philosophy of mind is the proposal that the problem imagined by Leibniz arises because one cannot, in general, read off function from form. Wandering through the mill-sized mind is not enough, as Leibniz pointed out. But, according to functionalism, what is missing is not an immaterial soul but a functional analysis of the mill and its component structures and processes. Wandering through an expanded engine, you would not, simply by looking, realize that the cam-shaft is a valve lifter, or that the things it moves are valves. Of course, most of us know enough simple push and pull mechanics that we could make some shrewd guesses. But a comparable experiment with the micro-chip at the heart of a calculator or computer would leave most of us on the wrong side of a Leibnizian Gap.

Computationalism--the idea that the mind is what Haugeland calls an automatic formal system--is the functionalist proposal that mental capacities can be analyzed and explained as complex computational processes. This idea was given a huge impetus by the dual discovery that inference could be treated as a computational process, and that that process could be instantiated in a machine. Psychology's oldest and most powerful explanatory primitive was finally given a materialistic explanation, though not yet a biological one. Association proved relatively easy to implement computationally and found its most important home in semantic networks.

Computationalism requires some fundamental enabling assumptions to turn it into a serious research program. The first of these is that the mind is fundamentally an engine of thought. Descartes held the essence of mind is thought, and Locke that the essence of mind is the capacity for thought. We think of this as the Mr. Spock assumption: a thinking engine with no other mental characteristics would still count as a mind, but a system with emotions, sensations and other non-cognitive mental processes that did not think (assuming this is even possible) would not count as a mind. Computationalism proposes to follow Descartes in the assumption that the place to start in understanding the mind is thought or cognition. Other aspects of mentality can be added on later, much as terms for friction and air resistance can be added to the basic pendulum equation once it is articulated and understood.

The second fundamental assumption grounding Computationalism is that thought does not require consciousness. This assumption, along with the first one, allows computationalists to put off the explanation of consciousness until some future time.

Given these assumptions, computationalism looked like an attractive research program for psychology. The idea that the mind is essentially a functionally specified computational process running on the brain provides a bridge over Leibniz' Gap (functionalism), a supply of mental mechanisms with precisely specified properties (anything you can program), and medium independence: the possibility that thought can exist in a non-biological computer, and hence can be investigated in the computer lab as well as in the psychological lab. It was a powerful vision. And though it shows signs of fading today, it was, and in some respects continues to be, a hugely prolific vision, fueling the initial birth and development of what came to be called Cognitive Science.

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