So, reductionism has proved fruitful in other disciplines, in elucidating the brain-behaviour relationships of lower animals, and shows signs of doing the same for the human brain. Why then, should it be declared "intellectually bankrupt"? I shall briefly consider the weaker arguments waged against the reductionist, before turning to the more serious ones.
Firstly, some argue that the brain is so complex it will never be able to understand itself. This is more a prediction than an argument, and one that is rash, especially considering the progress in elucidating brain function that has occurred in the last few decades, and the new techniques for investigating the brain that are constantly being made. This "argument" amounts to nothing more than unwarranted pessimism.
Next, there is substance dualism, which argues that mental states are not brain states, but states of a non-physical "soul". Unfortunately there is no explanation of how these two radically different substances interact. Furthermore, excluding theological appeals, where, in evolutionary history, is the "soul-stuff" supposed to have come from? If a chimpanzee's reasoning can be explained in terms of brain functioning, then presumably so can ours. The price of advocating substance dualism is very expensive, especially when the explanatory payoff is meagre.
A more serious argument is advanced by the property dualists who argue that the properties, not the substance, of the mind are irreducible. However, many of the mental properties, that originally so impressed dualists, have now lost much of their impact. Many of artificial intelligence's advances have laid to rest traditional bugbears, such as language and logical thinking. AI still has a long way to go, but recent progress suggests that these properties will not be forever unattainable by a purely physical system. Whether they will be explicable by the physical system is another question, and one I shall return to when I discuss functionalism.
The mental property most persistently thrust in the reductionist's face, by the dualist, is subjective experience. The dualist argues that brain states are causally connected to subjective experience, giving rise to a stream of events in awareness, but the experience itself cannot be identified with any aspect of neuronal activity. Nagel (1974) has argued that feelings resist reduction because introspective access to them has an essentially different character, yielding essentially different information, than does external access via neuroscience. However, as Crick (1979) commented, "we are deceived at every level by our introspection". The red surface of an apple does not look like the matrix of molecules reflecting photons at critical wavelengths, although that is what it is, and so perhaps our introspection, like our sensations, is just not powerful enough to reveal neuroscience's level of detail.
Although this explains why our introspection does not reveal anything of our brain states, it does not explain why our neuroscience fails to reveal the characteristics of our sensations. Jackson (1982) imagines a future neuroscientist who knows everything about the brain but is colour-blind. Thus despite knowing everything about brain states she knows nothing of certain mental states, e.g. the sensation of red. But, as Lewis (1983) noted, the difference between the colour-blind neuroscientist and a normal person, resides, not in what is respectively known, but in the level of representation each has of the same thing, namely brain states. The former has a linguistic representation, the latter pre-linguistic. This means, not that sensations are beyond the reach of the physical sciences, but that the brain uses more modes of representation than the mere storage of sentences. All the reductionist need claim is that these modes of representation will also ultimately yield to neuroscientific explanation. Furthermore, if the neuroscientist knew everything about the brain, it is possible that this would reveal what the sensation of red is, begging the question at issue. Such is the problem with thought experiments!
We now turn to the most sophisticated argument against the reductionist programme. Functionalists (e.g. Putnam, 1967) argue that mental states are defined by the causal relations they bear to the environment, other mental states, and behaviour. For example, pain is defined by bodily damage, distress etc., and crying etc. Any other state, regardless of what structure encodes it, that plays exactly the same functional role, is pain. As Pylyshyn (1980) observes, this means that psychology becomes methodologically autonomous from neuroscience, whose concerns amount to little more than engineering details. Research on neurones is not going to reveal the nature of the functional organisation, but only the embodiment of the functional organisation, and just one instantiation at that. This attack on the reductionist is called the argument from multiple instantiability.
However, as Enc (1983) noted, it is not enough to argue that just because more than one physical system can support mental states, reduction is unfeasible. For example, the reduction of thermodynamics to molecular theory, applied only to the temperature of gases, not say, solids, but it was still a bona fide reduction. Reductions may be reductions relative to a domain of phenomena. The functionalist calls this "multiple instantiability" and uses it as ammunition against the reductionist, but it is normal practice in the rest of science. If human brains and electronic brains both enjoy a certain type of cognitive organisation, we may get two distinct, domain-relative reductions, or after much time and co-evolution, one reductionist account. That there are hardware differences has no implications for whether human psychology can be explained by neuroscience, or whether it can benefit from neuroscientific information, or whether it is autonomous. And when the argument from multiple instantiability falls, the core thesis of functionalism, that mental states are identified by their abstract causal roles in the wider information-processing system, becomes entirely neutral on the question of irreducibility.
A second argument of the functionalists, with which to defend antireductionism, is that logical relations of representations cannot be reduced to the causal relations of physiology. For example, Fodor (1981) argues that a farmer shooting his horse can be explained if he holds the two beliefs, "whenever a horse breaks a leg you must destroy it", and "this horse broke its leg." But it is not the causal relations between physiological realisations of the representations that matter to the explanation, rather the logical relations between the contents of his desires. Furthermore the categories of a psychological theory will map onto an indefinite jumble of neurophysiological categories, and consequently neurophysiological generalisations will miss entirely important relations describable only at the level of representations.
But this is not a substantial justification of why logical relations between representations cannot be explained by causal relations between neurobiological states. A neurobiological theory could address representations and computations in the brain; the representations would not be sentence-like and the computations not inferences, but, say, mathematical, just the sort to be executed by suitably configured neruonal arrays. Why should not such a theory explain logical and meaningful relations between states at the psychological level? And how, a priori, do philosophers know that it cannot?
In conclusion, this essay has shown how the success of reductionism in other areas of science has been replicated to a lesser extent in psychology. Of the two main alternatives to reductionism, dualism is woefully inadequate, and functionalism is more a different approach not a mutually exclusive alternative. Ultimately neuroscience and psychology must co-evolve, information from one level being used to provoke further experiments at the other. Only until we have cogent theories at both levels can we hope to explain the relationship between the two. Meanwhile concepts such as reductionism and functionalism are simply divisive. To understand brain-behaviour relationships requires that we understand phenomena at every level. Physics has never sought to explain the whole of chemistry, but instead has added a deeper level of understanding and detail to chemistry's own explanations. Similarly neuroscience on its own will never "explain" psychology.
Co-evolution of theories is the norm in science, so why has it been so hard to apply to brain and behaviour? I believe it is due to a combination of psychology's struggle to be accepted as a true science, and a fear that neuroscience will yield a mechanistic explanation of the mind, with no room for free-will. The former problem belongs in the past, the latter the future. But at present both disciplines can benefit from each other. Now is the time for psychology and neuroscience to unite, not fight.
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