PLATO, reinforced by religion, has led mankind to accept the division of the known world into two categories mind and matter. Physics and psychology alike have begun to throw doubt on this dichotomy. It has begun to seem that matter, like the Cheshire Cat, is becoming gradually diaphanous until nothing of it is left but the grin, caused, presumably, by amusement at those who still think it is there. Mind, on the other hand, under the influence of brain surgery and of the fortunate opportunities provided by war for studying the effects of bullets embedded in cerebral tissue, has begun to appear more and more as a trivial by-product of certain kinds of physiological circumstances. This view has been reinforced by the morbid horror of introspection which besets those who fear that a private life, of no matter what kind, may expose them to the attentions of the police. We have thus a curiously paradoxical situation, reminding one of the duel between Hamlet and Laertes, in which students of physics have become idealists, while many psychologists are on the verge of materialism. The truth is, of course, that mind and matter are, alike, illusions. Physicists, who study matter, discover this fact about matter, psychologists, who study mind, discover this fact about mind. But each remains convinced that the other's subject of study must have some solidity. What I wish to do in this essay is to restate the relations of mind and brain in terms not implying the existence of either.
What one may call the conventional view has altered little since the days of the Cartesians. There is the brain, which acts according to the laws of physics; and there is the mind which, though it seems to have some laws of its own, is in many crucial ways subjected to physical conditions in the brain. The Cartesians supposed a parallelism according to which mind and brain were each determined by its own laws, but the two series were so related that, given an event in the one, it was sure to be accompanied by a corresponding event in the other. To take a simple analogy: suppose an Englishman and a Frenchman recite the Apostles' Creed, one in English, the other in French, at exactly the same speed, you can then, from what one of them is saying at a given moment in his language, infer what the other is saying in his. The two series run parallel, though neither causes the other. Few people would now adhere to this theory in its entirety. The denial of interaction between mind and brain contradicts common sense, and never had any but metaphysical arguments in its favor. We all know that a physical stimulus, such as being hit on the nose, may cause a mental reaction in this case of pain. And we all know that this mental reaction of pain may be the cause of a physical movement for example, of the fist. There are, however, two opposing schools, not so much of thought as of practice. One school has as its ideal a complete physical determinism as regards the material universe, combined with a dictionary stating that certain physical occurrences are invariably contemporary with certain mental occurrences. There is another school, of whom the psycho-analysts are the most influential part, which seeks purely psychological laws and does not aim at first establishing a causal skeleton in physics. The difference shows in the interpretation of dreams. If you have a nightmare, the one school will say that it is because you ate too much lobster salad, and the other that it is because you are unconsciously in love with your mother. Far be it from me to take sides in so bitter a debate; my own view would be that each type of explanation is justified where it succeeds. Indeed I should view the whole matter in a way which makes the controversy vanish, but before I can make this clear, there is need of a considerable amount of theoretical clarification.
Descartes, as everybody knows, says "I think, therefore I am," and he goes on at once, as if he had said nothing new, to assert "I am a thing that thinks." It would be difficult to pack so large a number of errors into so few words. To begin with "I think," the word "I" is thrust in to conform with grammar, and grammar embodies the metaphysic of our original Indo-European ancestors as they stammered round their campfires. We must, therefore, cut out the word "I." We will leave the word "think," but without a subject, since the subject embodies a belief in substance which we must shut out of our thoughts. The words "therefore I am" not only repeat the metaphysical sin embodied in the word "I," but commit the further sin, vigorously pilloried throughout the works of Carnap, of confounding a word in inverted commas with a word without inverted commas. When I say "I am," or "Socrates existed," or any similar statement, I am really saying something about the word "I" or the word "Socrates" --roughly speaking, in each case that this word is a name. For it is obvious that, if you think of all the things that there are in the world, they cannot be divided into two classes --namely, those that exist, and those that do not. Non-existence, in fact, is a very rare property. Everybody knows the story of the two German pessimistic philosophers, of whom one exclaimed: "How much happier were it never to have been born." To which the other replied with a sigh: "True! But how few are those who achieve this happy lot." You cannot, in fact, say significantly of anything that it exists. What you can say significantly is that the word denoting it denotes something, which is not true of such a word as "Hamlet." Every statement about Hamlet in the play has implicit the false statement " 'Hamlet' is a name," and that is why you cannot take the play as part of Danish history. So when Descartes says "I am," what he ought to mean is " T is a name" --doubtless a very interesting statement, but not having all the metaphysical consequences which Descartes wishes to draw from it. These, however, are not the mistakes I wish to emphasize in Descartes' philosophy. What I wish to emphasize is the error involved in saying "I am a thing that thinks." Here the substance philosophy is assumed. It is assumed that the world consists of more or less permanent objects with changing states. This view was evolved by the original metaphysicians who invented language, and who were much struck by the difference between their enemy in battle and their enemy after he had been slain, although they were persuaded that it was the same person whom they first feared, and then ate. It is from such origins that common sense derives its tenets. And I regret to say that all too many professors of philosophy consider it their duty to be sycophants of common sense, and thus, doubtless unintentionally, to bow down in homage before the savage superstitions of cannibals.
What ought we to substitute for Descartes' belief that he was a thing that thought? There were, of course, two Descartes, the distinction between whom is what gives rise to the problem I wish to discuss. There was Descartes to himself, and Descartes to his friends. He is concerned with what he was to himself. What he was to himself is not best described as a single entity with changing states. The single entity is quite otiose. The changing states suffice. Descartes to himself should have appeared as a series of events, each of which might be called a thought, provided that word is liberally interpreted. What he was to others I will, for the moment, ignore. It was this series of "thoughts" which constituted Descartes' "mind," but his mind was no more a separate entity than the population of New York is a separate entity over and above the several inhabitants. Instead of saying "Descartes thinks," we ought to say "Descartes is a series of which the members are thoughts." And instead of "therefore Descartes exists," we ought to say "Since 'Descartes' is the name of this series, it follows that 'Descartes' is a name." But for the statement "Descartes is a thing which thinks" we must substitute nothing whatever, since the statement embodies nothing but faulty syntax.
It is time to inquire what we mean by "thoughts" when we say that Descartes was a series of thoughts. It would be more conventionally correct to say that Descartes' mind was a series of thoughts, since his body is generally supposed to have been something different. His mind, we may say, was what Descartes was to himself and to no one else; whereas his body was public, and appeared to others as well as to himself. Descartes uses the word "thoughts" somewhat more widely than it would be used nowadays, and we shall, perhaps, avoid confusion if we substitute the phrase "mental phenomena." Before we reach what would ordinarily be called "thinking," there are more elementary occurrences, which come under the heads of "sensation" and "perception." Common sense would say that perception always has an object, and that in general the object of perception is not mental. Sensation and perception would, in common parlance, not count as "thoughts." Thoughts would consist of such occurrences as memories, beliefs, and desires. Before considering thoughts in this narrower sense, I should wish to say a few words about
sensation and perception.
Both "sensation" and "perception" are somewhat confused concepts, and, as ordinarily defined, it may be doubted whether either ever occurs. Let us, therefore, in the first instance avoid the use of these words, and try to describe what occurs with as few doubtful assumptions as possible.
It frequently happens that a number of people in the same environment have very similar experiences at approximately the same time. A number of people can hear the same clap of thunder, or the same speech by a politician; and the same people can see the lightning, or the politician thumping the table. We become aware on reflection that there is, in the environment of these people, an event which is not identical with what is heard or seen. There is only one politician, but there is a separate mental occurrence in each of those who see and hear him. In this mental occurrence, psychological analysis distinguishes two elements: one of them is due to those parts of the structure of the individual which he shares with other normal members of his species; the other part embodies results of his past experiences. A certain phrase of the politician evokes in one hearer the reaction "That's put the scoundrels in their place," and in another the quite different reaction, "Never in all my life have I heard such monstrous injustice." Not only such somewhat indirect reactions are different, but often men will actually hear different words because of their prejudices or past experiences. I was present in the House of Lords on an occasion when Keynes felt it necessary to rebuke Lord Beaverbrook for some statistics that the noble journalist had been offering to the House. What Keynes said was: "I have never heard statistics so phony" or "funny." Half the House thought he said "phony," and the other half thought he said "funny." He died almost immediately afterward, leaving the question undecided. No doubt past experience determined which of the two words any given hearer heard. Those who had been much exposed to America heard "phony," while those who had led more sheltered lives heard "funny." But in all ordinary cases past experience is concerned much more intimately than in the above illustration. When you see a solid-looking object, it suggests tactile images. If you are accustomed to pianos, but not to gramophones or radio, you will, when you hear piano music, imagine the hands of the performer on the keys (I have had this experience, but it is one not open to the young). When in the morning you smell bacon, gustatory images inevitably arise. The word "sensation" is supposed to apply to that part of the mental occurrence which is not due to past individual experience, while the word "perception" applies to the sensation together with adjuncts that the past history of the individual has rendered inevitable. It is clear that to disentangle the part of the total experience which is to be called "sensation," is a matter of elaborate psychological theory. What we know without theory is the total occurrence which is a "perception."
But the word "perception," as ordinarily used, is question-begging. Suppose, for example, that I see a chair, or rather that there is an occurrence which would ordinarily be so described. The phrase is taken to imply that there is "I" and there is a chair, and that the perceiving is a relation between the two. I have already dealt with "I," but the chair belongs to the physical world, which, for the moment, I am trying to ignore. For the moment I will say only this: common sense supposes that the chair which I perceive would still be there if I did not perceive it, for example, if I shut my eyes. Physics and physiology between them assure me that what is there independently of my seeing, is something very unlike a visual experience, namely, a mad dance of billions of electrons undergoing billions of quantum transitions. My relation to this object is indirect, and is known only by inference; it is not something that I directly experience whenever there is that occurrence which I call "seeing a chair." In fact the whole of what occurs when I have the experience which I call "seeing a chair" is to be counted as belonging to my mental world. If there is a chair which is outside my mental world, as I firmly believe, this is something which is not a direct object of experience, but is arrived at by a process of inference. This conclusion has odd consequences. We must distinguish between the physical world of physics, and the physical world of our everyday experience. The physical world of physics, supposing physics to be correct, exists independently of my mental life. From a metaphysical point of view, it is solid and self-subsistent, always assuming that there is such a world. Per contra, the physical world of my everyday experience is a part of my mental life. Unlike the physical world of physics, it is not solid, and is no more substantial than the world that I see in dreams. On the other hand it is indubitable, in a way in which the physical world of physics is not. The experience of seeing a chair is one that I cannot explain away. I certainly have this experience, even if I am dreaming. But the chair of physics, though certainly solid, perhaps does not exist. It does not exist if I am dreaming. And even if I am awake it may not exist, if there are fallacies in certain kinds of inference to which I am prone, but which are not demonstrative. In short, as Mr. Micawber would say, the physical world of physics is solid but not indubitable, while the physical world of daily experience is indubitable but not solid. In this statement I am using the word "solid" to mean "existing independently of my mental life."
Let us ask ourselves a very elementary question: What is the difference between things that happen to sentient beings and things that happen to lifeless matter? Obviously all sorts of things happen to lifeless objects. They move and undergo various transformations, but they do not "experience" these occurrences whereas we do "experience" things that happen to us. Most philosophers have treated "experience" as something indefinable, of which the meaning is obvious. I regard this as a mistake. I do not think the meaning is obvious, but I also do not think that it is indefinable. What characterizes experience is the influence of past occurrences on present reactions. When you offer a coin to an automatic machine, it reacts precisely as it has done on former occasions. It does not get to know that the offer of a coin means a desire for a ticket, or whatever it may be, and it reacts no more promptly than it did before. The man at the ticket office, on the contrary, learns from experience to react more quickly and to less direct stimuli. This is what leads us to call him intelligent. It is this sort of thing which is the essence of memory. You see a certain person, and he makes a certain remark. The next time you see him you remember the remark. This is essentially analogous to the fact that when you see an object which looks hard, you expect a certain kind of tactile sensation if you touch it. It is this sort of thing that distinguishes an experience from a mere happening. The automatic machine has no experience; the man at the ticket office has experience. This means that a given stimulus produces always the same reaction in the machine, but different reactions in the man. You tell an anedote, and your hearer replies: "You should have heard how I laughed the first time I heard the story." If, however, you had constructed an automatic machine that would laugh at a joke, it could be relied upon to laugh every time, however often it had heard the joke before. You may, perhaps, find this thought comforting if you are tempted to adopt a materialistic philosophy.
I think it would be just to say that the most essential characteristic of mind is memory, using this word in its broadest sense to include every influence of past experience on present reactions. Memory includes the sort of knowledge which is commonly called knowledge of perception. When you merely see something it can hardly count as knowledge. It becomes knowledge when you say to yourself that you see it, or that there it is. This is a reflection upon the mere seeing. This reflection is knowledge, and because it is possible, the seeing counts as experience and not as a mere occurrence, such as might happen to a stone. The influence of past experience is embodied in the principle of the conditioned reflex, which says that, in suitable circumstances, if A originally produces a certain reaction, and A frequently occurs in conjunction with B, B alone will ultimately produce the reaction that A originally produced. For example: if you wish to teach bears to dance, you place them upon a platform so hot that they cannot bear to leave a foot on it for more than a moment, and meanwhile you play "Rule Britannia" on the orchestra. After a time "Rule Britannia" alone will make them dance. Our intellectual life, even in its highest flights, is based upon this principle.
Like all other distinctions, the distinction between what is living and what is dead is not absolute. There are viruses concerning which specialists cannot make up their minds whether to call them living or dead, and the principle of the conditioned reflex, though characteristic of what is living, finds some exemplification in other spheres. For example: if you unroll a roll of paper, it will roll itself up again as soon as it can. But in spite of such cases, we may take the conditioned reflex as characteristic of life, especially in its higher forms, and above all as characteristic of human intelligence. The relation between mind and matter comes to a head at this point. If the brain is to have any characteristic corresponding to memory, it must be in some way affected by what happens to it, in such a manner as to suggest reproduction on occasion of a suitable stimulus. This also can be illustrated in a lesser degree by the behavior of inorganic matter. A watercourse which at most times is dry gradually wears a channel down a gully at the times when it flows, and subsequent rains follow the course which is reminiscent of earlier torrents. You may say, if you like, that the river bed "remembers" previous occasions when it experienced cooling streams. This would be considered a flight of fancy. You would say it was a flight of fancy because you are of the opinion that rivers and river beds do not "think." But if thinking consists of certain modifications of behavior owing to former occurrences, then we shall have to say that the river bed thinks, though its thinking is somewhat rudimentary. You cannot teach it the multiplication table, however wet the climate may be.
At this point I fear you will be becoming indignant. You will be inclined to say: "But, my dear Sir, how can you be so dense? Surely even you must know that thoughts and pleasures and pains cannot be pushed about like billiard balls, whereas matter can. Matter occupies space. It is impenetrable; it is hard (unless it is soft); thoughts are not like this. You cannot play billiards with your thoughts. When you banish a thought, the process is quite different from that of being ejected by the police. You, of course, as a philosopher" (so, no doubt, you will continue) "are superior to all human passions. But the rest of us experience pleasures and pains, and sticks and stones do not. In view of all this I cannot understand how you can be so stupid as to make a mystery of the difference between mind and matter."
My answer to this consists in saying that I know very much less than you do about matter. All that I know about matter is what I can infer by means of certain abstract postulates about the purely logical attributes of its space-time distribution, Prima facie, these tell me nothing whatever about its other characteristics. Moreover there are the same reasons for not admitting the concept of substance in the case of matter, as there are in the case of mind. We reduced Descartes' mind to a series of occurrences, and we must do the same for his body. A piece of matter is a series of occurrences bound together by means of certain of the laws of physics. The laws that bind these occurrences together are only approximate and macroscopic. In proper quantum physics, the identity which physical particles preserve in old-fashioned physics disappears. Suppose I want to say: "This is the same chair as it was yesterday." You cannot expect me to tell you accurately what I mean, because it would take volumes to state this correctly. What I mean may be put roughly as follows: classical physics a system now abandoned worked with the assumption of particles that persist through time. While this conception lasted, I could maintain that when I said "This is the same chair" I meant "this is composed of the same particles." Before the coming of quantum physics, particles were already out of date, because they involved the concept of substance. But that did not matter so much because it was still possible to define a particle as a certain series of physical occurrences, connected with one another by the law of inertia and other similar principles. Even in the days of the Rutherford-Bohr atom, this point of view could still be maintained. The Rutherford-Bohr atom consisted of a certain number of electrons and protons. The electrons behaved like fleas. They crawled for a while, and then hopped. But an electron was still recognizable after the hops as being the same one that had previously crawled. Now, alas, the atom has suffered atomic disintegration. All that we know about it, even on the most optimistic hypothesis, is that it is a distribution of energy which undergoes various sudden transitions. It is only the transitions of which it is possible to have evidence, for it is only in a transition that energy is radiated, it is only when energy is radiated that our senses are affected, and it is only when our senses are affected that we have evidence as to what has occurred. In the happy days when Bohr was young, we were supposed to know what was going on in the atom in quiet times: there were electrons going round and round the nucleus as planets go round the sun. Now we have to confess to a complete and absolute and eternally ineradicable ignorance as to what the atom does in quiet times. It is as if it were inhabited by newspaper reporters who think nothing worth mentioning except revolutions, so that what happens when no revolution is going on remains wrapped in mystery. On this basis, sameness at different times has completely disappeared. If you want to explain what you mean in physics when you say "This is the same chair as it was yesterday," you must go back to classical physics. You must say: when temperatures are not too high, and chemical circumstances are ordinary, the results obtained by old-fashioned classical physics are more or less right. And when I say that "this is the same chair", I shall mean that old-fashioned physics would say it was the same chair. But I am well aware that this is no more than a convenient and inaccurate way of speaking, and that, in fact, every smallest piece of the chair loses its identity in about one hundred thousandth part of a second. To say that it is the same chair is like saying that the English are the same nation as they were in the time of Queen Elizabeth I, or rather, like what this would be, if many millions of generations had passed since the death of Good Queen Bess.
We have not yet learned to talk about the human brain in the accurate language of quantum physics. Indeed we know too little about it for this language to be necessary. The chief relevance, to our problem, of the mysteries of quantum physics consists in their showing us how very little we know about matter, and, in particular, about human brains. Some physiologists still imagine that they can look through a microscope and see brain tissues. This, of course, is an optimistic delusion. When you think that you look at a chair, you do not see quantum transitions. You have an experience which has a very lengthy and elaborate causal connection with the physical chair, a connection proceeding through photons, rods and cones, and optic nerve to the brain. All these stages are necessary if you are to have the visual experience which is called "seeing the chair." You may stop the photons by closing your eyes, the optic nerve may be severed, or the appropriate part of the brain may be destroyed by a bullet. If any of these things has happened you will not "see the chair." Similar considerations apply to the brain that the physiologist thinks he is examining. There is an experience in him which has a remote causal connection with the brain that he thinks he is seeing. He can only know concerning that brain such elements of structure as will be reproduced in his visual sensation. Concerning properties that are not structural, he can know nothing whatever. He has no right to say that the contents of a brain are different from those of the mind that goes with it. If it is a living brain, he has evidence through testimony and analogy that there is a mind that goes with it. If it is a dead brain, evidence is lacking either way.
I wish to suggest, as a hypothesis which is simple and unifying though not demonstrable, a theory which I prefer to that of correspondence advanced by the Cartesians. We have agreed that mind and matter alike consist of series of events. We have also agreed that we know nothing about the events that make matter, except their space-time structure. What I suggest is that the events that make a living brain are actually identical with those that make the corresponding mind. All the reasons that will naturally occur to you for rejecting this view depend upon confusing material objects with those that you experience in sight and touch. These latter are parts of your mind. I can see, at the moment, if I allow myself to talk the language of common sense, the furniture of my room, the trees waving in the wind, houses, clouds, blue sky, and sun. All these common sense imagines to be outside me. All these I believe to be causally connected with physical objects which are outside me, but as soon as I realize that the physical objects must differ in important ways from what I directly experience, and as soon as I take account of the causal trains that proceed from the physical object to my brain before my sensations occur, I see that from the point of view of physical causation the immediately experienced objects of sense are in my brain and not in the outer world. Kant was right to put the starry heavens and the moral law together, since both were figments of his brain.
If what I am saying is correct, the difference between mind and brain does not consist in the raw material of which they are composed, but in the manner of grouping. A mind and a piece of matter alike are to be considered as groups of events, or rather series of groups of events. The events that are grouped to make a given mind are, according to my theory, the very same events that are grouped to make its brain. Or perhaps it would be more correct to say that they are some of the events that make the brain. The important point is, that the difference between mind and brain is not a difference of quality, but a difference of arrangement. It is like the difference between arranging people in geographical order or in alphabetical order, both of which are done in the post office directory. The same people are arranged in both cases, but in quite different contexts. In like manner the context of a visual sensation for physics is physical, and outside the brain. Going backward, it takes you to the eye, and thence to a photon and thence to a quantum transition in some distant object. The context of the visual sensation for psychology is quite different. Suppose, for example, the visual sensation is that of a telegram saying that you are ruined. A number of events will take place in your mind in accordance with the laws of psychological causation, and it may be quite a long time before there is any purely physical effect, such as tearing your hair, or exclaiming "Woe is me!"
If this theory is right, certain kinds of connection between mind and brain are inescapable. Corresponding to memory, for example, there must be some physical modifying of the brain, and mental life must be connected with physical properties of the brain tissue. In fact, if we had more knowledge, the physical and psychological statements would be seen to be merely different ways of saying the same thing. The ancient question of the dependence of mind on brain, or brain on mind, is thus reduced to linguistic convenience. In cases where we know more about the brain it will be convenient to regard the mind as dependent, but in cases where we know more about the mind it will be convenient to regard the brain as dependent. In either case, the substantial facts are the same, and the difference is only as to the degree of our knowledge.
I do not think it can be laid down absolutely, if the above is right, that there can be no such thing as disembodied mind. There would be disembodied mind if there were groups of events connected according to the laws of psychology, but not according to the laws of physics. We readily believe that dead matter consists of groups of events arranged according to the laws of physics, but not according to the laws of psychology. And there seems no a priori reason why the opposite should not occur. We can say we have no empirical evidence of it, but more than this we cannot say.
Experience has shown me that the theory which I have been trying to set forth is one which people are very apt to misunderstand, and, as misunderstood, it becomes absurd. I will therefore recapitulate its main points in the hope that by means of new wording they may become less obscure.
First: the world is composed of events, not of things with changing states, or rather, everything that we have a right to say about the world can be said on the assumption that there are only events and not things. Things, as opposed to events, are an unnecessary hypothesis. This part of what I have to say is not exactly new, since it was said by Heraclitus. His view, however, annoyed Plato and has therefore ever since been considered not quite gentlemanly. In these democratic days this consideration need not frighten us. Two kinds of supposed entities are dissolved if we adopt the view of Heraclitus: on the one hand, persons, and on the other hand, material objects. Grammar suggests that you and I are more or less permanent entities with changing states, but the permanent entities are unnecessary, and the changing states suffice for saying all that we know on the matter. Exactly the same sort of thing applies to physical objects. If you go into a shop and buy a loaf of bread, you think that you have bought a "thing" which you can bring home with you. What you have in fact bought is a series of occurrences linked together by certain causal laws.
Second: sensible objects, as immediately experienced, that is to say, what we see when we see chairs and tables and the sun and the moon and so on, are parts of our minds and are not either the whole or part of the physical objects that we think we are seeing. This part of what I am saying is also not new. It comes from Berkeley, as reinforced by Hume. The arguments that I should use for it, however, are not exactly Berkeley's. I should point out that if a number of people look at a single object from different points of view, their visual impressions differ according to the laws of perspective and according to the way the light falls. Therefore no one of the visual impressions is that neutral "thing" which all think they are seeing. I should point out also that physics leads us to believe in causal chains, starting from objects and reaching our sense organs, and that it would be very odd if the last link in this causal chain were exactly like the first.
Third: I should admit that there may be no such thing as a physical world distinct from my experiences, but I should point out that if the inferences which lead to matter are rejected, I ought also to reject the inferences which lead me to believe in my own mental past. I should point out further that no one sincerely rejects beliefs which only such inferences can justify. I therefore take it that there are events which I do not experience, although some things about some of these can be inferred from what I do experience. Except where mental phenomena are concerned, the inferences that I can make as to the external causes of my experiences are only as to structure, not as to quality. The inferences that are warranted are those to be found in theoretical physics; they are abstract and mathematical and give no indication whatever as to the intrinsic character of physical objects.
Fourth: if the foregoing is accepted there must be two sorts of space, one the sort of space which is known through experience, especially in my visual field, the other the sort of space that occurs in physics, which is known only by inference and is bound up with causal laws. Failure to distinguish these two kinds of space is a source of much confusion. I will take again the case of a physiologist who is examining someone else's brain. Common sense supposes that he sees that brain and that what he sees is matter. Since what he sees is obviously quite different from what is being thought by the patient whom he is examining, people conclude that mind and matter are quite different things. Matter is what the physiologist sees, mind is what the patient is thinking. But this whole order of ideas, if I am right, is a mass of confusions. What the physiologist sees, if we mean by this something that he experiences, is an event in his own mind and has only an elaborate causal connection with the brain that he imagines himself to be seeing. This is obvious as soon as we think of physics. In the brain that he thinks he is seeing there are quantum transitions. These lead to emission of photons, the photons travel across the intervening space and hit the eye of the physiologist. They then cause complicated occurrences in the rods and cones, and a disturbance which travels along the optic nerve to the brain. When this disturbance reaches the brain, the physiologist has the experience which is called "seeing the other man's brain." If anything interferes with the causal chain, e.g. because the other man's brain is in darkness, because the physiologist has closed his eyes, because the physiologist is blind, or because he has a bullet in the brain at the optic center, he does not have the experience called "seeing the other man's brain." Nor does the event occur at the same time as what he thinks he sees. In the case of terrestrial objects, the difference of time is negligible, but in the case of celestial objects it may be very large, even as much as millions of years. The relation of a visual experience to the physical object that common sense thinks it is seeing is thus indirect and causal, and there is no reason to suppose that close similarity between them that common sense imagines to exist. All this is connected with the two kinds of space that I wrote of a moment ago. I horrified all the philosophers by saying that their thoughts were in their heads. With one voice they assured me that they had no thoughts in their heads whatever, but politeness forbids me to accept this assurance. Perhaps, however, it might be well to explain exactly what I mean, since the remark is elliptical. Stated accurately, what I mean is as follows: physical space, unlike the space of perception, is based upon causal contiguity. The causal contiguities of sense perceptions are with the physical stimuli immediately preceding them and with the physical reactions immediately following them. Precise location in physical space belongs not to single events but to such groups of events as physics would regard as a momentary state of a piece of matter, if it indulged in such old-fashioned language. A thought is one of a group of events, such as will count for purposes of physics as a region in the brain. To say that a thought is in the brain is an abbreviation for the following: a thought is one of a group of compresent events, which group is a region in the brain. I am not suggesting that thoughts are in psychological space, except in the case of sense impressions
(if these are to be called "thoughts")
Fifth: a piece of matter is a group of events connected by causal laws, namely, the causal laws of physics. A mind is a group of events connected by causal laws, namely, the causal laws of psychology. An event is not rendered either mental or material by any intrinsic quality, but only by its causal relations. It is perfectly possible for an event to have both the causal relations characteristic of physics and those characteristic of psychology. In that case, the event is both mental and material at once. There is no more difficulty about this than there is about a man being at once a baker and a father. Since we know nothing about the intrinsic quality of physical events except when these are mental events that we directly experience, we cannot say either that the physical world outside our heads is different from the mental world or that it is not. The supposed problem of the relations of mind and matter arises only through mistakenly treating both as "things" and not as groups of events. With the theory that I have been suggesting, the whole problem vanishes.
In favor of the theory that I have been advocating, the most important thing to be said is that it removes a mystery. Mystery is always annoying, and is usually due to lack of clear analysis. The relations of mind and matter have puzzled people for a long time, but if I am right they need puzzle people no longer.
