IN the course of our discussion of personal immortality we came upon the seeming dependence of mind on body. Let us consider more closely the relation of these two very different but intimately connected things.
I shall begin by stating the problem as it appears to common sense; or rather to contemporary common sense, for the common sense of one period may be very different from that of another.
When we distinguish between body and mind at all, we normally think of them as two distinct things or substances, each of which takes effect on the other. A human body is thought of as a physical object having shape, solidity, texture, and internal structure. Its parts could quite well exist without the man’s mind; though perhaps they could not be related together in the complex pattern which constitutes a living human body.
By a mind we normally mean one or other of two things. Either we mean simply the continuous but ever-changing sequence of experiences — the thinkings, feelings, imaginings, desirings, and so on, which tumble upon one another's heels throughout our waking hours; or else we mean a vague "something" which is supposed to do or have these experiences.
It seems at first obvious that there are such distinct things as body and mind, and that they do take effect on one another. A physical kick on the physical shin affords the mind a certain experience. Apparently the bruising of the body's tissues takes effect on the course of mental events. Similarly, alcohol sets up changes in the body, and these seemingly produce mental changes. Again, a blow on the head may produce "concussion" and cessation of consciousness.
On the other hand, a mental event, such as the learning of good or bad news, may produce changes in the blood-circulation or general physical vigour. The will to move a limb generally causes the limb to move.
For clarity's sake let us represent the theory of the interaction of body and mind by means of a diagram. Let α, β, γ, δ represent mental events in a certain mind, and let a, b, c, d represent the correlated physical events in the body. The host of physical events which have no observable correlated mental events may be neglected. Let the causal connections be represented by arrows. Then the theory of the interaction of body and mind may be represented thus:
If we accept the general principle of the interaction of body and mind, we are faced with the question whether the two have equal power, or one of them dominates the other. Seemingly the body is greatly influenced by the mind, for voluntary muscular activity is almost continuous. On the other hand, the mind is obviously influenced continuously by the impact of the external world on the body in perception, and less obviously so in changes of mood and intellectual capacity. How far does this influence reach? Is it only an occasional minor factor, or is it at work always? Is it true, as some believe, that the course of mental events is simply an expression of the physical events of the body? Is mind really quite incapable or affecting the physical? Is volition a sheer illusion, an experience caused simply by physical events in the nervous system? Is the movement of the limb (and equally the movement of attention in thinking) really produced by physical, not mental, causes?
(a) Substance and Attribute — The mere statement of the problem of body and mind commonly implies certain assumptions which must be brought into clear consciousness even if they are not yet to be fully discussed.
Common sense, as we have seen, assumes two distinct things or substances, body and mind. Each is thought of as remaining essentially identical from time to time although changes happen to it or in it. Thus, though there are bodily events, such as breathing, eating, digesting, "the body" is thought of as remaining in some sense "the same" body throughout these passing events. Similarly, with the mind there are rapid changes of perception, thought, feeling, and also slow changes of mood, but "the mind" is supposed to remain "the same" mind.
In philosophical language, common sense assumes that body and mind are enduring substances having changeful attributes, and mutually influencing each other.
This mode of thought, in terms of substance and attribute, is open to serious objection. Pressed to say what the enduring substance in each case really is, the plain man would probably be content to reply that the substance is some sort of nucleus which does not change. But it is clear that in the case of the body there is no such constant nucleus. The human body is not much more constant than a candle flame, in which all the material is continually passing in and out of the flame.
Pressed further, common sense would probably say that .the substance is simply that featureless and unknowable something which is "the underlying cause" of all the knowable attributes. But if the substance is unknowable, why introduce it at all? To this, common sense, echoing the thought of the past, might reply that. the unknowable substance is required logically as the unifying and enduring "ground" of the attributes. Some modern philosophers, however, deny that there is any need for such a logical ground. Our craving for it, they say, is due to an accident of our language, which makes use of the grammatical machinery of "subject" and "predicate." We must. outgrow this prejudice, they say, and recognise that (for instance) a body simply is the sum of the events that make up its history, and that a mind simply is the sequence of its mental events.
The subject-predicate way of thinking suggests that behind the whole world as it appears to us, there lies "reality" itself, which is different from its mere appearances, and is in principle unknowable. Opposed to this view is the view that, however little we know of reality, what we do know is all of a piece with, is of the same order as, what we do not know; that the world is not an unknowable substance, having knowable attributes, but that it is a vast system of "happening," analysable into separate "events" which occur in relation to one another.
With regard to the body-mind problem, even if we give up the substance-attribute way of thinking, the problem still remains. It is no longer a problem about the relations of two substances whose attributes are physical characters and mental characters. But it is a problem about the relation between two sequences of events, namely physical and mental. The course of events in the one sequence is obviously related to the course of events in the other. For instance, when we drink alcohol, certain intestinal events are followed by certain changes of mood. Conversely, certain thoughts and desires are followed by certain bodily movements. In each case, we say, the earlier event "causes" the later.
(b) Causation — This raises another assumption implied in the body-mind problem as it appears to common sense. It is assumed that causation does occur, that one event does have some sort of power in virtue of which the succeeding event has certain characters and not certain others. Lightning "causes" thunder, drugs "cause" mental changes.
Here we come upon one of the great philosophical problems. What sort of thing is this "causation"? What reason have we to believe that it happens? Does it really happen?
It was long ago pointed out by David Hume that we cannot see any necessity in the sequences that we call causal. All that we actually observe is the succession of events. Just because we observe certain recognisable successions of events over and over again, we grow to expect the particular kind of initial event always to be followed by the subsequent events, unless some other influence interferes. This expectation leads to a feeling of necessity, and of some hidden efficacy by which the one event produces the other. Impressed by this criticism, modem science no longer claims to be discovering necessary laws, but merely to be making generalisations from observed sequences, generalisations on which we may base our expectations. Thus stones dropped from high places do not necessarily fall with an acceleration of thirty-two feet per second every second. But experiment discovers that, when irrelevant influences are excluded, they do approximate very closely to this "law." Further, observation reveals that this " law" can be related to other "laws," derived from observation of the movements of the planets. But so far as science is concerned these "laws" might cease to operate to-morrow. Stones might start leaping from the ground. If they did so, scientists would be very surprised, but they would not say that necessity had been violated. They would merely set about re-describing the sequences of events and forming new generalisations, new physical "laws."
In discussing the body-mind problem, then, we are assuming causation, either in the full sense of efficacy or in the modified sense of invariable sequence. We are assuming, that is, that if certain events happen, certain other events will also happen, unless some contrary cause prevents them.
We can now consider some of the difficulties in the theory of the interaction of body and mind.
(a) Inconceivability — It is sometimes said that the interaction of two such different things as body and mind is inconceivable. The causal relation between one bodily event and another is felt to be intelligible in principle, because in this case cause and effect are of the same order. But some regard it as inconceivable that a volition should cause a muscle to contract, that a drug should cause a change of mood, that a physical change in the brain should cause an experience.
This objection seems to be based on the illusion that causal relations between physical events themselves, or between mental events themselves, are conceivable. Actually they are nothing of the sort. That a moving billiard-ball should push another out of its way instead of passing through it or annihilating it or turning it into a fairy coach and six is, after all, an impenetrable mystery.
The objection, however, may be put in a more subtle and plausible manner. Physical causation, it may be said, is credible because all physical events are changes within a single physical system of events. Mental events also are events within a single whole, a single mind, united in virtue of unity of experience. But bodily events and mental events, it may be said, form no such single system together, and so their causal connection is inconceivable.
There is some force in this argument. But we must not be deceived by it. The billiard ball's efficacy within the physical system is really no more intelligible than a drug's effect on the mind. On the other hand, it may well be that if we knew more about body and mind we should see that they are not really two distinct systems but one. In this case, however, we might have to abandon the theory of interaction simply because mind and body no longer appeared as two distinct things interacting, but rather as two aspects of one and the same thing.
(b) The Conservation of Energy — Against Interactionism it is often argued that if mind interferes with the course of physical events in the body, the physical Law of the Conservation of energy must be violated. Energy may be roughly defined as the capacity for doing work. This capacity can be measured according to recognised standards. And though the measurement of one form of work against another introduces serious difficulties, we may take it as fairly well established that, within a closed physical system, the sum of the potential and the kinetic energy, or of the possible and the actual work, is the same at every moment.
The living body is a physical system. Therefore, we are told, it must keep the Law of Conservation. Of course, the body cannot be completely isolated so as to become a closed physical system, since if it were it would die. But the amount of energy entering and; leaving it can be fairly accurately measured and; accounted for. On the whole, the evidence suggests that a living body does function according to established physical laws. Experiment and observation have led to a steady advance in our knowledge of the physiological mechanisms of the body, and have made it seem to some scientists increasingly probable that in time we shall have a complete account of the body's behaviour in terms of biochemical laws. Such an account would refer to nerve-tracks and glandular secretions, and would allow no room for any influence of mind on body. For, in this view, the purely automatic behaviour of the system could not be interfered with by the mind without either introducing additional energy into the system from some occult source, or by withdrawing energy from the system. The body is like a moving motor-car. To alter its purely mechanical course the driver must at least apply energy to the steering-wheel or the accelerator pedal.
It is sometimes argued in defence of Interactionism that the mind might alter the direction or the timing of energy-changes in the body without infringing the Law of Conservation. This is clearly a mistake; for, according to the law, the direction and timing of energy-changes is quite strictly determined by the preceding physical conditions. There is no room for interference of any kind without adding to or subtracting from the sum of energy.
All the same the theory of Interactionism need not necessarily be false. In the first place, the physical observations on which the Law of Conservation is based are far from accurate enough to justify a confident assertion that the law applies to the living body as a system closed against mind's interference. It may be, for all we yet know, that the mind, or that the course of mental events, does actually create or annihilate very minute quantities of physical energy at critical points of the nervous system, and so control the course of the nerve current, and therefore determine behaviour. Some believe that it does this in raising or lowering the resistance of the "synapses," the junctions of the nerve fibres, and so directing or blocking the nerve current. This is not inconceivable. But the Law of Conservation within the physical sphere has become so familiar and useful that this possibility has come to seem very unplausible.
Some scientific workers, however, and some philosophers, have been forced to the conviction that purely physical laws cannot possibly give a complete account of the body's working. So intricately purposeful, they say, is the structure and function of the body, so subtly self-regulative, that some purposive or teleological principle must be supposed to control the physical functioning of the body's organs. They have not been able to tell us anything at all clear about this non- physical influence.
A more radical defence of Interactionism may be derived from recent criticisms of the Law of Conservation itself. It has been pointed out that the Law works just because we have so stated it that it must work. We have affirmed that so much energy in one form shall be equivalent to so much energy in another form, so that we may produce a workable Law of Conservation. I am, not competent to criticise this contention. But it is necessary to point out that these established equivalences do hold good systematically in the physical world, and that they leave (apparently) no room for interference on the part of mind.
On the whole, then, the difficulty over Conservation remains a serious one. But if strong reasons were forthcoming to make us believe in Interactionism, this difficulty should not stand in the way. For, after all, we cannot yet be sure that the Law of Conservation really does apply to living bodies.
Let us, however, suppose for the moment that Conservation is true of the human body, and that human behaviour can be fully described in terms of physical laws. The relation of mind and body has then to be stated in terms of the theory of Epiphenomenalism.
According to this theory causation occurs only in the physical sphere. There is no causal relation between one mental event and another, or between a mental event and a succeeding physical event. The desire to solve an intellectual problem does not cause the subsequent mental operations. The desire to move a limb does not cause muscle fibres to contract. In both cases the real cause is a physiological event in the body. All experiencing is a sort of by-product of physiological machinery, like the noise of a factory. Mind is only an epiphenomenon, an ineffective "appearance upon" the physical causal sequence.
Epiphenomenalism may be represented diagrammatically. As before, let α, β, γ, δ represent mental events, and a, b, c, d physical events, and let the causal relations be represented by arrows.
It is sometimes objected against Epiphenomenalism that in volition we have actual experience of the necessary causal efficacy of mind on body. Volition certainly does feel as though it caused the desired action. In this respect it is different from the experience of a mere reflex act, such as hiccoughing, which we do not feel to be mentally caused at all. But the feeling of causal efficacy in volition is easily explained by Epiphenomenalism. The volition, it may be said, consists of a desire followed by a muscular movement. Both are physically caused. But the often repeated experience of the sequence "desire-movement" generates a strong expectation that desire will again be followed by movement. Or rather, putting the matter more accurately from the Epiphenomenalist's point of view, the physiological events corresponding to the experience of desire-followed-by-movement cause the physiological events corresponding to the expectation desire-will-be-followed-by-movement.
But though we must reject the view that in volition we actually experience a necessary causal connection between a mental event and a physical event, we may reasonably hold that the conviction of the efficacy of volition should not be abandoned unless Epiphenomenalism is supported by very strong evidence. And no such strong evidence has yet appeared.
Another objection to Epiphenomenalism is based on the nature of rational thinking. When we think, the sequence of our thoughts is apparently determined by the logical implications of our thoughts. To argue that the sequence is really controlled not in this manner but by mere physiological events in the brain is to undermine thought itself, and therewith even the theory of Epiphenomenalism. Any theory which denies the validity of thinking cuts the ground from under its own feet.
To this the Epiphenomenalist may perhaps reasonably reply that the neural tracks in the brain are themselves in the first instance determined by the impact of the environment, and that the experience of logical implication in thinking, though only an epiphenomenon, is none the less a true reflection of the logical structure of the objective world. Thus, though the intuition of logical implication does not actually cause the sequence of thoughts, it is the conscious aspect of the physiological connections which do cause the sequence of thoughts, and which, moreover, are themselves determined in the first instance by the logic of the objective world.
But once more we must suspend judgment. Though reasoning can thus be accounted for by Epiphenomenalism, we should not lightly pretend to abandon the belief, inescapable in practice, that in reasoning the course of thought is directly controlled by intuitions of logical implication. Nothing short of overwhelming evidence should destroy this conviction. And the evidence for Epiphenomenalism is far from overwhelming.
It is sometimes said that Epiphenomenalism is incredible because, if consciousness were ineffective, its occurrence would be inexplicable. It is affirmed that consciousness must be explained in terms of survival-value. It occurs and has reached a high stage of development just because it has proved biologically useful, because it has made for survival. Now this may be true. But perhaps what had survival-value was not actually consciousness but a highly integrated nervous system; and perhaps a highly developed consciousness is just the mental epiphenomenon of this.
A more general objection to Epiphenomenalism is this. If consciousness throughout the universe is ineffective,. the universe is meaningless, futile, unintelligible. To this we must answer that, after all, the universe is very far from intelligible anyhow, and we have no right to expect it to be intelligible.
A moral argument is sometimes brought against Epiphenomenalism. If men come to believe that volition is ineffective, all moral striving will cease. To this the answer is that the moral consequences of a belief in the theory are irrelevant to the question of the theory's truth.
There is some tentative physiological evidence against all theories based on an exact correspondence of a physical and a mental series of events. I refer to the supposed "vicarious functioning" of brain-tracts. It is admittedly true that damage to specific brain-tracts is often followed by specific mental disabilities, such as disorders of speech or sensation. But we are told that after a while a neighbouring tract can take over the office of the damaged tract. If this is the case, it makes nonsense of Epiphenomenalism, since, if Epiphenomenalism is true, each tract should have its inalienable function. However, the evidence for vicarious functioning is far from conclusive, and is seriously obscured by the probability that neighbouring undamaged tracts which were temporarily thrown out of gear by the lesion may subsequently recover their powers. When they begin to function again we may be tempted to suppose that they are recovering not merely their own powers but the powers of the damaged tract.
However this may be, there can be no doubt that a good deal of correspondence does exist between brain areas and mental functions. And it is certainly possible, some would say probable, that the correspondence is, in fact, exact. Many kinds of experience which formerly seemed independent of physical causation are now known to be physiologically determined. But Epiphenomenalism cannot be established till such physical dependence is shown to be universal, so that there is no room for mental causation to insert itself anywhere, and also no general "looseness" in the physical causal system, such that mental causation might be enabled to insert itself unobtrusively everywhere. In fact, a strong objection to the theory is the intellectual objection that it is based on insufficient evidence. It is a case of the all-too-common "fallacy of the specialist," who is so impressed with the success of his particular technique that he assumes it to be universally valid.
We may reasonably hold that the theory treats the physical much too seriously, or uncritically. It is too credulous that, while the objects studied by natural science, the drugs, molecules, atoms, electrons, are substantial, the events which we call mental are merely phantasmal. But this criticism we cannot develop till we have raised the question of the status of the external world.
Some philosophers, impressed by the importance of treating the mental as seriously as the physical, and anxious at the same time to do full justice to the claims of physiology, have adopted a theory known as Psychophysical Parallelism. According to this theory there are in the living brain two entirely independent causal sequences, the one physical, the other mental (or, as some say, "psychical"); yet the two sequences run parallel, in the sense that for every event in the one sequence there is a corresponding event in the other. For example, in writing these lines I experience certain mental events which are causally connected with each other in a purely mental manner; but at the same time a series of physical events occurs in my brain, and these are causally connected in a physical manner. The total mental state at any moment and the total physical state at the same moment are very complex, and, of course, qualitatively different. But the elements that make up the mental state are related together in a pattern which I corresponds, point by point, with the pattern of the physical state.
We may represent the theory diagrammatically, using the same symbols as in the other cases, thus:
One objection can be brought against both Parallelism and Epiphenomenalism. Both, it may be said, render consciousness useless in evolution. This argument, as I have already said, has some force, but it could not carry weight against any strong positive reason for believing in Parallelism. However, there do not seem to be any strong reasons for Parallelism; and there are strong reasons against it.
Perhaps the chief reason against it is the extreme improbability that two complex causal sequences should continue indefinitely with strict correspondence and no connection with each other. This improbability is vastly increased by the fact that the bodily sequence is being constantly influenced by contact with the external physical world, while the mental sequence, by hypothesis, is not so influenced, and is presumably wholly insulated.
This difficulty is particularly obvious in the case of sudden violent collisions between the body and other physical objects. A man is knocked over by a motor-car, and his brain is damaged. Henceforth, perhaps, he suffers from specific disorders of speech, or perhaps he goes blind. Such cases overwhelmingly suggest that physical events take effect on the course of mental events. If the man's aphasia or blindness was not caused by the physical damage to his brain but by some purely mental cause, what was it? And how strange that a catastrophic change in the one sequence of events should occur just when a similarly catastrophic change occurs in the other!
It may reasonably be objected against Parallelism that it implies the theory that every physical event, whether in a living brain or not, has a mental correlate. In this view there is a mental universe, no less complex than the physical universe, and correlated with it in every detail. Certainly this supposition would help the Parallelist out of the difficulty about the motor accident; for he could say that the patient's catastrophic mental change was due to the mental influences of the mental events correlated with the physical events of the on-coming car.
Now it is not wholly inconceivable that every physical unit (say, every electron and proton) is the body of a very simple mind. But if this is so, where does the mind of a man come in? For in this view his body is a host of bodies of very simple electronic and protonic minds. Perhaps we shall be told that his mind is in some strange manner just all these simple minds merged into one complex mind. It is easy to use such language, but what does it really mean? I am not a host of atomic minds. I am a single mind.
The truth is that the theory of a parallel mental universe is too cumbersome a support for the Parallelism of human body and mind. There may be such a universe, but we have no evidence for it.
It would seem, then, that there are no cogent reasons for accepting Parallelism, and some strong objections to it.
An attempt has been made to overcome the difficulties of Interactionism by supposing that body and mind are complementary aspects of one and the same substance, like the inside and outside surfaces of a sphere. The mind-process and the body-process, it is said, are really one and the same process of events; but in the one case the process is observed externally, and in the other it is lived through internally. Events in this psycho-physical sequence cause succeeding psychophysical events. They also causally influence and are influenced by the environment, which, of course, we know only externally, as physical. Whether the physical environment also has an internal, mental aspect need not be decided.
The theory may be roughly represented thus:
As before, the mental is represented by Greek letters, the bodily by Roman. Arrows of causation connect successive states of the body-mind. Other arrows of causation impinge upon the body-mind from the external physical world, and in turn issue from the body-mind to the external physical world. In both cases the body-aspect is the medium of intercourse with the external world; but internal causation is as truly mental as physical.
It may turn out that this way of stating the mind-body relation is more accurate than any which regards body and mind as two distinct substances, or, on the other hand; regards one as substantial and the other,as a mere phantasm. But in so far as the Double Aspect theory depends on the substance-attribute distinction it is to be suspected. According to the theory, body and mind are two attributes of one substance. What is the relation between these attributes, or between each of them and the substance which comprises both? Clearly, in the present state of our knowledge the theory is not very helpful, because, instead of solving the difficulties, it merely conceals them. For it is not self-evident that the body and mind imply one another, as do the inside and the outside of a sphere. Consequently we must still enquire how it is that their changes correspond. And in particular we must still enquire which of the two aspects of the psycho-physical substance is the significant one for understanding the causal sequence. Inevitably the theory resolves itself into either a disguised Interactionism or a disguised Epiphenomenalism.
Some philosophers, impressed with the seeming purposefulness of much in the behaviour and structure of living things, have adopted a far-reaching theory of the "Emergence" of life and consciousness from the physical. When physical units are organised in certain very complex patterns, it is said, new capacities emerge in the wholes thus formed. The most striking of these capacities are the capacity for purposeful, or teleological, behaviour, directed toward the survival of the individual organism or the species, and (on a still higher plane of organisation) consciousness. In passing we may note that the concept of teleological behaviour does not necessarily involve consciousness. Behaviour is said to be teleological, whether conscious or not, if it cannot be adequately described without reference to an end or goal, if it observably infringes mechanical laws in order to reach a goal.
The behaviour of a purely physical system can always at least in theory be predicted in terms of purely physical laws. This is said by some philosophers to be impossible, even in theory, in the case of the living organism. However thoroughly we study the behaviour of physical units in purely physical situations, we cannot (it is said) conceivably discover solely by such physical study all the laws of their behaviour in the essentially different biological kind of situation. The behaviour of the emergent whole is not accountable simply in terms of the laws descriptive of the behaviour of the parts as revealed in non-organic situations. Merged in the unified whole of the organism they are able to manifest potentialities which elsewhere they cannot manifest at all. From the physical point of view there is nothing in the organism but electrons, protons, electromagnetic undulations, etc. But in the organism these together produce the teleological and conscious behaviour of the organism. Of course, much that goes on in the organism is purely physical. And there is constant conflict between the purely physical and the emergent behaviour, which is always teleological and in some respects conscious.
Let us consider the bearing of this theory on the body-mind problem. Mind is regarded as emergent. Its behaviour cannot be fully described (even in theory) in terms of the laws of physical science. In some respects, of course, mental events are controlled by the physical events of the body; but in other respects these physical events are controlled by emergent mental events. For the understanding of the relation between mind and body, then, although we must, of course, study the effects of drugs and nerve currents on mental events, we must also study psychology on its own emergent plane. Fundamentally, however, the relation between body and mind must, in this view, necessarily remain unintelligible.
It is difficult to reach any clear conclusions about the value of the Emergence theory itself. There is obviously a sense in which mental events, such as thinkings and perceivings and desirings, cannot be described or accounted for in terms of the laws of any purely physical science. Those laws simply have no direct bearing on the mental. All the same it might turn out that (as the mechanists claim) the sequence of mental events was strictly related to physical events in the body, in such a manner that with nothing more than a full knowledge of the physical events we could predict the mental events. In the present state of our knowledge we cannot say whether this is so or not. Similarly, if it is true that all seemingly teleological behaviour studied by biologists can be explained away in terms of non-teleological physical laws, then biology can be reduced to physics. But if this cannot be done, if the concept of teleology is finally needed for the understanding of some biological facts, biology cannot even in theory be reduced to physics, which has no room for teleology. These are questions which cannot yet be answered. We must remember, however, that bio-chemistry, which claims to be a purely physical science, has recently made great progress. Very much that has seemed mysterious in growth and in behaviour has been shown to depend on chemical factors in the body. On the other hand, perhaps our biochemical knowledge of the relation between chemical reactions and mental states may turn out to be concerned, not solely with physical causal laws, but partly with the systematic reactions of the emergent whole itself to purely physical stimuli.
We started by considering the problem of the relation of body and mind from the point of view of common sense. We assumed that a body and a mind were different things or substances, or made of different "stuffs," the one physical, the other mental. The problem was to explain the relationship between them. We have examined several different theories, but we have found none that is satisfactory. Nevertheless we may, I think, draw certain important conclusions, and raise certain further questions.
In the first place we must beware of the substance-attribute way of thinking, which distinguishes between a "thing" and its characters. All that we can profitably think about is the actually observed, or at least in principle observable, characters that make up the tissue of our experience. Of "substances" behind these characters we know nothing.
We have also seen that in respect of causation all that we can hope to discover is, not an inner necessity uniting cause and effect, but regular sequences of events.
The mind-body problem, then, consists in the need to state clearly the relation between the sequences of the physical characters that make up a human body and the sequences of mental characters that make up a human mind.
We have seen that it is not yet possible to describe this relationship at all satisfactorily. Throughout this discussion we have assumed that we do at any rate know what we mean by "body" and what by "mind." It is now time to recognise that this assumption is unjustifiable. Let us try to form a clearer view of what, in a man's actual experience, constitutes his body, and what his mind.
A man's body, as we perceive it, is a system of sensory characters, such as colour, shape, softness. This system, in spite of large fluctuations due to the voluntary movement of limbs, remains on the whole constant in form, and lies permanently at the centre of his perceived world. In fact, his body is made up of visual appearances, tactual "appearances" (as when he strokes or pushes his head with his hand), sensations of warmth, cold, pressure, pain, on the surface of his perceived body's shape or within its interior. The changeful three-dimensional shape of his body is really an abstraction, a formula derived from the spatial relations of this host of sensory characters, which constitute his body, and the relations of this sensory system to the other host of sensory characters, which constitute external physical objects.
Now all these sensory characters, both those of his body and those of the external world (such as the coloured shape of a tree or a house that he is seeing) are also, in some sense, characters of his mind. They are all bits of his experience. In some way the physical world and the mind overlap. Of course, there is much in the physical world that is not in any sense part of his mind; for instance, all the objects that he is not perceiving or even thinking about. And there is apparently much "in" his mind that is not part of the physical world; for instance, his admiration or dislike of the perceived tree or house. His thinking, desiring, fearing, and his actual perceiving (as distinct from what he perceives) belong only to his mind. The phrase "in the mind" is misleading. Things are not in the mind as marbles may be in a box. There is, of course, a sense in which all that I experience is "in" my mind, within my mental horizon; but, more accurately, I reach out to, apprehend, have mental contact with, the objects of my experience. When John knows Jane, Jane herself does not become part of John's mind.
Evidently we have opened up some new and very obscure problems, which we may express in the following questions: What precisely do we mean by "a physical object"? What precisely do we mean by "a mind"? When a mind perceives a physical object, what precisely perceives what? And how should this relationship of perceiving be described?
These questions lead at once into a very formidable philosophical jungle. An immense amount of careful, subtle, hair-splitting work has been done upon them; and yet the upshot is far from conclusive. In a book like this it is impossible to attempt a detailed discussion. But we cannot leave the subject untouched. Some realisation of the problems, and some grasp of the possible tentative conclusions are necessary before we can go on to explore fields which have a more direct bearing on our central theme.
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