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Radical materialism or radical physicalism is certainly a selfconsistent position. For it is a view of the universe which, as far as we know, was adequate once; that is, before the emergence of life and consciousness.
There is a slight awkwardness felt by most of those who hold and defend this theory now: the very fact that they propose a theory (qua theory), their own belief, their own words, their own arguments, all seem to contradict it. In order to get over this difficulty, the radical physicalist must adopt radical behaviourism and apply it to himself: his theory, his belief in it, is nothing; only the physical expression in words, and perhaps in arguments — his verbal behaviour and the dispositional states that lead to it — is something.
What speaks in favour of radical materialism or radical physicalism is, of course, that it offers us a simple vision of a simple universe, and this looks attractive just because, in science, we search for simple theories. However, I think that it is important that we note that there are two different ways by which we can search for simplicity. They may be called, briefly, philosophical reduction and scientific reduction.6 The former is characterized by an attempt to simplify our view of the world; the second by an attempt to provide bold and testable theories of high explanatory power.71 believe that the latter is an extremely valuable and worthwhile method; while the former is of value only if we have good reasons to assume that it corresponds to the facts about the universe.
Indeed, the demand for simplicity in the sense of philosophical rather than scientific reduction may actually be damaging. For even in order to attempt a scientific reduction, it is necessary for us first to get a full grasp of the problem to be solved, and it is therefore vitally important that interesting problems are not "explained away" by philosophical analysis. If, say, more than one factor is responsible for some effect, it is important that we do not pre-empt the scientific judgement: there is always the danger that we might refuse to admit any ideas other than the ones we happen to have at hand; explaining away, or belittling the problem. The danger is increased if we try to settle the matter in advance by philosophical reduction. Philosophical reduction also makes us blind to the significance of scientific reduction.8
It is in this light that I think we should consider the radical physicalist's approach to the problem of consciousness. Not only do we have, in the phenomena of consciousness, something that seems radically different from what, on our current view, is to be found in the physical world. We also have the dramatic and, from a physical point of view, strange changes that have taken place in the physical environment of man, due, it appears, to conscious and purposeful action. This should not be ignored, or dogmatically explained away.
I would even suggest that the greatest riddle of cosmology may well be neither the original big bang, nor the problem why there is something rather than nothing (it is quite possible that these problems may turn out to be pseudoproblems), but that the universe is, in a sense, creative: that it created life, and from it mind — our consciousness — which illuminates the universe, and which is creative in its turn. It is one of the high points in Herbert Feigl's Postscript  to his essay The 'Mental' and the 'Physical' when he relates how, in a conversation, Einstein said something like this: "If there were not this internal illumination, the universe would merely be a rubbish heap."9This, Feigl tells us, is one of the reasons why he does not accept radical physicalism (as I call it) but the identity theory, which recognizes the reality of mental and especially of conscious processes.
It might also be worth bearing in mind that, while in science, our quest is for simplicity, it is a real problem whether the world is itself quite so simple as some philosophers think. The simplicity of the old theory of matter (that of Descartes or that of Newton or even that of Boscovich) is gone: it clashed with the facts. The same happened to the electrical theory of matter which, for twenty or thirty years, seemed to offer a hope of an even greater simplicity. Our present theory of matter, quantum mechanics, turns out (especially in the light of the thought experiment of Einstein, Podolsky and Rosen, and the results of J. Bell, and of S.J. Freedman and R. A. Holt ) to be even less simple than one might have hoped. It is also clearly incomplete: in spite of Dirac's result which may be interpreted as the prediction of anti-particles, quantum theory cannot be said to have led to the prediction or explanation of the many new elementary particles which have been found in recent years. Thus appeals to simplicity can hardly be accepted as decisive, not even within physics. In particular, we should not deprive ourselves of interesting and challenging problems — problems that seem to indicate that our best theories are incorrect and incomplete — by persuading ourselves that the world would be simpler if they were not there. But it seems to me that modern materialists are doing just this.10
I may say here perhaps that I should regard radical physicalism, if it were compatible with the facts, as an intellectually satisfying theory. But it is not compatible with the facts. And the facts, difficult as they are to absorb, are intellectually challenging. So to me the decision seems to be between intellectual ease (or let us call it smugness) and unease.
Radical behaviourism, on which the radical physicalist must depend in order to explain to himself his theoretical activities as "verbal behaviour", derives most of its appeal from a misunderstanding of a problem of method. The behaviourist demands, rightly, that any scientific theory, and therefore also the theories of psychology, must be testable by reproducible experiments, or at least by intersubjectively testable observation statements: by statements about observable behaviour, which in the case of human psychology includes verbal behaviour.
But this important principle refers only to the test statements of a science. Just as in physics we introduce theoretical entities — electrons and other particles, or fields of forces, etc. — in order to explain our observation statements (about photographs of the events in bubble chambers, for example), so we can introduce, in psychology, conscious and unconscious mental events and processes, if these are helpful in explaining human behaviour, such as verbal behaviour. In this case, the attribution of a mind and of subjective conscious experiences to every normal human person is an explanatory theory of psychology of about the same character as the existence of relatively stable material bodies in physics. In both cases the theoretical entities are not introduced as something ultimate — as substances in the traditional sense; both create vast regions of unsolved problems, and so does their interaction. But in both cases our theories are well testable: in physics, by the experiments of mechanics; in psychology, by certain experiments
which lead to reproducible verbal reports (and thus to reproducible "verbal behaviour"). Since all, or almost all, experimental subjects react in these experiments with recognizably the same reports — reports about what they subjectively experience in the experimental situation — the theory of their having these subjective experiences is well tested.
I will here describe a simple experiment which every reader can carry out himself, and check with any of his friends. It is taken from the work of the great Danish experimental psychologist, Edgar Rubin (, pp. 366 f.). I use optical illusions because here the character of subjective experiences becomes very clear.
The following two figures are taken from Rubin, with very slight changes.
It will be seen from Figure 1 that, since AB, CD, and EF are parallel and equidistant, the inclined line AF is cut in half at G; so that AG = GF.
We explain all this to our experimental subject, who thus does not need to measure the distances AG and GF in order to make sure that they are equal.
We now put to him the following questions.
(1) Look at Figure 2. You know that AG = GF, in view of the proof indicated by Figure 1. Do you agree?
We wait for the reply.
(2) Does AG look to you equal to GF?
We wait again for the reply.
Question (2) is the decisive question. The reply ("No") which is obtained from every (or almost every) experimental subject can be explained most directly by the conjecture that the subjective visual experience of every subject deviates systematically from what we all know (and can prove) to be objectively the case. This establishes an easily repeatable objective and behavioural test of the existence of subjective experience. (Of course, only as long as we take the reports of our experimental subjects seriously; but the radical behaviourist can still reinterpret ad hoc their verbal responses: every falsification can be evaded by one who is not prepared to learn from experience.)
We could have confined ourselves to Figure 3 (the so-called "Sanders Illusion"), and measured AG and GB, which is perhaps even more dramatic.
Yet measurements may leave some doubt: small errors may matter and may not be easily detectable. On the other hand, it is clear that the three vertical lines in Figure 1 and Figure 2 are parallel and equi-distant. A further question is:
(3) Does your theoretical knowledge with respect to Figure 2 help you to see the distances AG and GF as equal?
A related but slightly different experiment may convince us that our mental processes are often mental activities. It operates with an ambiguous figure. (Such figures are used in Wittgenstein's Philosophical Investigations, yet, it seems, with very different aims or purposes.) The figure used (the "Winson Figure") is taken from a paper by Ernst Gombrich (, p. 239).
Figure 4. Taken from R. L. Gregory and E. H. Gombrich (eds.)  with the kind permission of the author, the publisher, and of Alphabet and Image.
The Figure shows ambiguously the profile of an American Indian and a view, from the back, of an Eskimo. What I wish to draw attention to is that we can voluntarily switch from the one interpretation to the other, although perhaps not easily. It seems that most people can easily see the American Indian and have difficulties in switching over to the Eskimo. (However, it is the other way round for some people.)
Now the point is that we can voluntarily and actively build up the profile of the Indian by looking at his nose, mouth, and chin, and then proceeding to his eye. As to the Eskimo, we can start to build him up from his right boot. (And of course, we can formulate experimental questions about these activities which lead to intersubjectively repeatable answers.)
There are also other sorts of intersubjectively testable experiments which are most successful and convincing tests of the theory that men have conscions experiences. For example, there are the experiments conducted by the great brain surgeon Wilder Penfield. Penfield  repeatedly stimulated, with the help of an electrode, the exposed brain of patients who were being operated on while fully conscious. When certain areas of the cortex were thus stimulated, the patients reported re-living very vivid visual and auditive experiences while being, at the same time, fully aware of their actual surroundings. "A young South African patient lying on the operating table . . . was laughing with his cousins on a farm in South Africa, while he was also fully conscious of being in the operating room in Montreal." (Penfield , p. 55.) Such reports, clearly reproducible, and repeated in many cases, can be only explained so far as I can see by admitting conscious subjective experiences. Penfield's experiments have sometimes been criticized for having been conducted only with epileptic patients. However, this does not affect the problem of the existence of subjective, conscious experiences.
These experiments of Penfield's may be compatible with an identity theory. They do not seem to be compatible with radical physicalism — with the denial of the existence of subjective states of consciousness. There are many similar experiments.11 They test and establish, by behaviourist methods, the conjecture — if it is to be called a conjecture rather than a fact — that we have subjective experiences; conscious processes. Admittedly, there is every reason to think that these go hand in hand with brain processes. It is, it appears, the brain rather than the self which "insists", as it were, on the inequality of distances we know to be equal. (A corresponding remark holds for the Gestalt switch.) Yet my main point here is merely that we can establish empirically, by behaviourist methods, that subjective, conscious experience exists.
A word may be added on the unusual or paradoxical character of both types of experiment here mentioned — optical illusion and Penfield's stimulation of the cortex. Our mechanism of perception is normally not reflexively directed onto itself, but directed towards the outside world. Thus we can forget about ourselves in normal perception. In order to become quite clear about our subjective experience it is therefore useful to choose experiments in which something is out of the ordinary and clashes with the normal perceptual mechanism.
6 See my [1972 (a)], chapter 8, where these ideas are discussed in more detail.
7 See, for example, my [1972 (a)], chapter 5.
8 Consider, for example, what a dogmatic philosophical reductionist of a mechanistic disposition (or even a quantum-mechanistic disposition) might have done in the face of the problem of the chemical bond. The actual reduction, so far as it goes, of the theory of the hydrogen bond to quantum mechanics is far more interesting than the philosophical assertion that such a reduction will one day be achieved.
9 Cp. Feigl , p. 138. Feigl translates a German conversation; I have slightly changed the wording of the translation (as Feigl also did, according to his report).
10 5 It might be mentioned that the conflict described in the text could also be seen as the conflict between conventionalism and realism in the philosophy of science. Perhaps Charles S. Sherrington (, p. xxiv) may be quoted here: "That our being should consist of two fundamental elements offers I suppose no greater inherent improbability than that it should rest on one only."
11 One important type of experiment is due to modern sleep research: rapid eye movements have been shown to indicate dreaming; and dreaming is clearly a (low-level) conscious experience. (The radical behaviourist or materialist would have to say, in order to avoid refutation, that rapid eye movements signify a manifestation of a disposition which would lead people if woken up to say that they had been dreaming (while in reality no such things as dreams exist). But this would obviously be an ad hoc way of evading refutation.)
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