Cartesian Dualism and the Concept of Medical Placebos

This article first appeared in the Journal of Consciousness Studies(1994, vol. 1, 230-233).

Doctors and the mind–body problem

With a few exceptions (Goodman, 1991; Harrison 1991), medical writers generally avoid discussing the mind–body problem directly, but there is little doubt that the implicit assumption in almost any modern textbook or research paper is that materialism is true. This is the case even—or perhaps especially—in psychiatry, which is moving ever closer to becoming a branch of neurology. "With a few dissenters, psychiatry accepts that in principle its disorders are disorders of the brain that can and should be investigated as such". (Lancet, 1994).

Anxiety, depression, and schizophrenia are now thought of as brain diseases, albeit diseases whose symptomatology is predominantly behavioural. One of the main criticisms of conventional medicine by alternative practitioners is precisely that it is materialistic, whereas alternative medicine, in contrast, is said to regard human beings as made up of body, mind, and spirit. So the mind–body problem seems to be settled in medicine; yet I have often been struck by what seems like an inconsistency in the way medical writers who would probably repudiate any suggestion that they were dualists discuss the question of placebos.

What is a placebo?

One widely quoted definition of "placebo" is: "any effect attributable to a pill, potion or procedure, but not to its pharmacodynamic or specific properties" (Wolf, 1959). Many modern treatments, like those of the past, owe a large though unknown part of their effectiveness to the placebo phenomenon, but this is usually inadvertent. The intentional use of placebos as a form of treatment retains an aura of charlatanry; this prejudice is beginning to change, but many doctors still feel uneasy about deliberately giving patients a "medicine" that they know to have no pharmacological effect.

Placebos in clinical trials

There is however a "respectable" use of placebos: in research. The great majority of clinical trials use placebo controls of one kind or another. A tablet is chosen to be as similar as possible to the drug to be investigated and the effect of the placebo is compared with that of the drug.

There are several possible ways of designing placebo-controlled trials, but a commonly used method is to have two groups, one receiving placebo and the other the medicine to be tested. A statistical comparision of the results in the two groups allows, with luck, a valid objective assessment of how far any improvement that may have occurred was due to the drug rather than to chance or suggestion. (The researchers' expectations and hopes are also relevant, which is why most clinical trials are "double blind", with neither the patients nor the researchers knowing which is the active agent.)

A similar principle has been applied to the investigation of non-drug treatments, including surgery but also unconventional or "complementary" therapies such as hypnotism, meditation, exercise, yoga, osteopathy, chiropractic, and acupuncture. However, it is more difficult to design suitable placebos for treatments of this kind than it is for those based on drugs.

There are numerous methodological problems connected with the use of placebos, especially placebos for physical treatments, but they are not what concerns me here; I want to look at the matter from a different standpoint. It seems to me that the placebo idea conceals a philosophical trap.

The placebo concept is based on the idea that a patient's expectations and hopes are liable to influence his or her response to a medication, surgical operation, or some other form of medical intervention. There is ample evidence for the ability of patients' expectations to produce clinical responses independent of any pharmacological effect, but little is known about how such effects occur.

The placebo concept can easily lead people to think dualistically

There is thus a temptation to say, in effect, that there is the body, which is a physiological system on which drugs and other medical treatments are supposed to operate, and there is the mind, which can be affected by suggestion to produce a spurious effect. Between the mind and the body there is an ill-defined no-man's land rather inadequately occupied by something called psychosomatic medicine. It seems to be difficult talk about placebos without implying a dualistic model of this kind.

The philosopher J.R. Searle makes what I take to be essentially the same point in a different context when he writes (Searle, 1994, p.54):

In denying the dualist's claim that there are two kinds of substances in the world or in denying the property dualist's claim that there are two kinds of properties in the world, materialism inadvertently accepts the terms in which Descartes set the debate. It accepts, in short, the idea that the vocabulary of the mental and physical, of material and immaterial, is perfectly adequate as it stands.

Searle seems to be saying here that there is a psychological trap into which philosophers often fall. They intend consciously to reject Cartesian dualism, yet they are still imprisoned in the outmoded terminology that Descartes used. Doctors too, it seems to me, often fall into this trap.

Mind–body dualism seems to be deeply ingrained in consciousness—Western consciousness, anyway. Various explanations have been suggested for this. Perhaps it is simply due to force of habit and the sheer length of time that Western thought has been cast in a dualistic mode, or perhaps it is a consequence of linguistics: perhaps we are channelled into that way of thinking because Indo–European languages, with their subject–verb–object grammatical structure, make it seem natural. Whatever the explanation may be, there seems little doubt that most of us think dualistically, at least outside a formal academic context.

The dualistic mind trap

I want to suggest that many people who write about placebos have fallen into the dualistic trap. Because how placebos work is unknown, it is easy to talk as if they were acting on a non-material mind. But if we accept, as I would wish to do, Searle's claim that "mental phenomena are caused by neurophysiological processes in the brain and are themselves features of the brain" (Searle, 1994, p.1), it follows that a treatment that works via the nervous system cannot but be influenced by whatever is done to the nervous system in question. When you stick an acupuncture needle into any part of the patient, for example, you are inevitably changing the way the nervous system works, even if we cannot yet, or perhaps ever, specify exactly how it happens.

But placebos must act on the brain!

If materialsm is true there must be some part of the nervous system corresponding to a particular set of beliefs or perceptions. We tend, I suppose, to picture this state as mainly affecting the neocortex of the brain—the phylogenetically newest part of the nervous system on which our higher mental functions are thought to depend. But there is no reason to suppose that changes occurring at this level do not also influence other areas of the brain, including those, such as the hypothalamus and brain stem, which are concerned with regulation of the endocrine glands, the immune system, and so on. Thus, the information that is fed into the brain, in the form of verbal and non-verbal suggestion, may be supposed to have an important effect on how the body functions.

New ideas about pain

This idea should not come as a surprise to anyone who takes an interest in modern ideas of how the central nervous system processes pain. Modern therapeutic approaches to pain are increasingly based on a view of the nervous system as a dynamic structure, whose mode of functioning is constantly shifting and adapting.

This is a new way of understanding brain function. Models of the nervous system tend to reflect the technology of the time in which they are conceived. In the sixteenth and seventeenth centuries people were familiar with mechanical and hydraulic mechanisms and these were the inspiration for models of nervous system activity. Pain was conceived of as an essentially simple input–output process: you burn your hand, you pull it away; Descartes in 1645 compared the transmission of pain from skin to brain to a man pulling a bell rope and ringing a bell in the tower.

With the invention of the telephone and the telephone exchange a new technological metaphor became available: one that allowed for more complex activities. In our own day, of course, the digital computer has proved seductively powerful as a metaphor; and whatever reservations one may have about equating the brain with a computer and the mind with a computer program, the metaphor has at least enabled us to think of the nervous system in a much more complex and flexible way than hitherto.

The gate theoory of pain

The first shift in the way modern physiologists thought about pain occurred when Melzack and Wall proposed their gate theory (Melzack and Wall, 1982, 1988). According to this, there were supposed to be gates or filters in the spinal cord which could facilitate or inhibit the transmission of pain. The gates were supposed to be closed by impulses arriving via the large-diameter (A-beta) nerve fibres from the periphery. This theory was able to explain various pain phenomena, such as relief of pain produced by rubbing the painful area or even a different area, phantom limb pain, and so forth.

Over the years the theory has been refined and extended in various ways, both by its originators and by numerous others. The spinal cord gates are now seen as just one of a large number of areas within the central nervous system whose activity can modulate pain perception. Among these areas are "higher" structures in the brain stem, thalamus, and cerebral cortex. Note that these are structures that are also concerned in the generation or modification of consciousness.

In practical terms this means that physicians and surgeons involved in the day-to-day treatment of pain are increasingly coming to think that pain is not just a local phenomenon. In the past, patients who suffered from backache, for example, would undergo investigations and perhaps surgery designed to put right whatever anatomical damage was supposed to underlie their pain. Sometimes these measures worked; but if they did not, or if the investigations failed to show any structural abnormality to account for the pain, the conclusion usually was that the pain was "functional", which was simply a polite way of saying that it was psychological and therefore not "real".

Information from brain imaging

Today the development of imaging systems such as magnetic resonance imaging (MRI) and positron emission tomography (PET) allows detailed examination of the living brain in a way that was never previously possible, and these techniques have revealed that there are actual measurable changes in certain brain areas, such as the cingulate cortex, thalamus, and lenticular nucleus, in patients who are suffering from chronic backache (Jayson, 1994).

Light is also being thrown on the central nervous system pathways involved in angina pectoris (Rosen, 1994). It seems very likely that similar changes will be found in the case of other types of pain, including those, such as atypical facial pain, which are regarded as largely psychological. Hence the distinction between "real physical" pain and "psychological" pain is becoming increasingly blurred.

Can we legitimately talk of 'unreal' pain?

This way of looking at pain mechanisms, although widely accepted among specialists in pain relief, has not been fully taken into acount by all doctors, quite a number of whom still distinguish between "functional" pain and "real" pain. Perhaps that is hardly surprising. There are many patients who suffer pain due to somatization—that is, they experience their unhappiness or depression not as a psychological state but as a physical pain of some kind. Many doctors find such patients irritating and difficult to cope with. No doubt they should not, but they do, and they tend to think that pain of this kind is somehow less deserving of sympathy than pain that is obviously due to a physical disease.

On the materialist assumption it is illogical to make a distinction between "functional" and "real" pain. All pain is real; only the mechanism by which it is produced differs. But doctors perceive the two types of pain differently. A similar difference in perception, I believe, affects how they talk about the placebo phenomenon. Pharmacological effects are "real" whereas placebo effects are "merely psychological". The modifier "mere" frequently hovers invisibly in front of "placebo" in medical texts. The ghost has not yet been fully exorcised from the machine; a shadowy vestige of it, at least, still haunts the clinics and laboratories in which clinical trials are conducted.

Finding a more satisfactory way of talking about the philosophical implications of the placebo concept will not be easy, for it will require the attaining of a generally accepted theory of consciousness—something that still seems a long way off. But we can at least say that it will have to be a biological theory. It will need to focus on the fact that we are creatures with a long evolutionary history who have evolved to make sense not just of the outer world but of the facts of pain and diseease. An interesting attempt to construct such a theory has been made recently by Nicholas Humphrey (1992), whose ideas, I believe, may well be applicable to the question I have discussed in this article.