THE SCIENTIFIC LITERATURE ON
THE PROBLEMS OF DREAMS
In the following pages I shall provide proof that there is a psychological technique which allows us to interpret dreams, and that when this procedure is applied, every dream turns out to be a meaningful psychical formation which can be given an identifiable place in what goes on within us in our waking life. I shall further try to explain the processes that make the dream so strange and incomprehensible, and infer from them the nature of the psychical forces in their combinations and conflicts, out of which the dream emerges. Having got so far, my account will break off, for it will have reached the point at which the problem of dreaming opens out into more comprehensive problems which will have to be resolved on the basis of different material.
I shall begin with a survey both of what earlier authorities have written on the subject and of the present state of scientific inquiry into the problems of dreams, as I shall not often have occasion to return to it in the course of this treatise. In spite of being concerned with the subject over many thousands of years, scientific understanding of the dream has not got very far. This is admitted by the writers so generally that it seems superfluous to quote individual authors. In the writings I list at the end of my work many stimulating observations and a great deal of interesting material can be found relating to our subject, but little or nothing touching the essential nature of the dream or offering a definitive solution to any of its riddles. And of course, even less has passed into the knowledge of the educated layman.
The first work to treat the dream as an object of psychology seems to be Aristotle's On Dreams and Dream Interpretation . Aristotle concedes that the nature of the dream is indeed daemonic, but not divinewhich might well reveal a profound meaning, if one could hit on the right translation. He recognizes some of the characteristics of the dream-life, for example, that the dream reinterprets slight stimuli intruding upon sleep as strong ones (`we believe we are passing through a fire and growing hot when this or that limb is only being slightly warmed'), and he concludes from this that dreams could very well reveal to the physician the first signs of impending changes in the body not perceptible by day. Lacking the requisite knowledge and teaching and without informed assistance, I have not been in a position to arrive at a deeper understanding of Aristotle's treatise.
As we know, the ancients prior to Aristotle regarded the dream not as a product of the dreaming psyche, but as an inspiration from the realm of the divine, and they already recognized the two contrary trends which we shall find are always present in evaluations of the dream-life. They distinguished valuable, truth-telling dreams, sent to the sleeper to warn him or announce the future to him, from vain, deceptive, and idle dreams intended to lead him astray or plunge him into ruin. This pre-scientific conception of the dream held by the ancients was certainly in full accord with their world-view as a whole, which habitually projected as reality into the outside world what had reality only within the life of the psyche. Their conception also took account of the main impression made on the waking life by the memory of the dream remaining in the morning, for in this memory the dream is opposed to the other contents of the psyche as something alien, coming as it were from another world. It would be wrong, by the way, to think that the theory of the supernatural origin of dreams has no followers in our own day. Quite apart from all the pietistic and mystical writerswho do right to occupy the remains of the once extensive realm of the supernatural, as long as it has not been conquered by scientific explanationwe also encounter clear-sighted men averse to the fantastic who use this very inexplicability of the phenomena of dreams in their endeavours to support their religious belief in the existence and intervention of superhuman powers. The high value accorded to the dream-life by many schools of philosophy, for example, by Schelling's followers, is a distinct echo of the undisputed divinity accorded to dreams in antiquity; and the divinatory, future-predicting power of dreams remains under discussion because the attempts at a psychological explanation are not adequate to cope with all the material gathered, however firmly the feelings of anyone devoted to the scientific mode of thought might be inclined to reject such a notion.
The reason why it is so difficult to write a history of our scientific knowledge of the problems of dreams is that, however valuable our knowledge may have become under single aspects, no progress along a particular line of thought is to be discerned. No foundation of confirmed results has been constructed on which the next researcher might have built further, but every new author tackles the self-same problems afresh as though from the very beginning. If I were to discuss the writers in chronological sequence and summarize what each of them had to say on the problems of dreams, I would have to abandon any clear overall survey of the current state of our knowledge of dreams. That is why I have chosen to construct my account according to topics rather than authors, and in dealing with each dream problem I shall cite whatever material for its solution exists in the literature.
However, this literature is very scattered and encroaches upon many other subjects, and I have not been able to cope with it all: so I must ask my readers to rest content as long as no fundamental fact or significant point of view has escaped my attention.
Until recently, most writers have felt obliged to deal with sleep and dreams within the same context, as a rule also linking to this their evaluations of analogous states extending into psychopathology, and their assessments of occurrences similar to dreams (such as hallucinations, visions, etc.). The most recent work, on the other hand, endeavours to restrict the subject and take some single question from the field of the dream-life as its object of inquiry. This change of emphasis is, I believe, an expression of the conviction that in such obscure matters enlightenment and agreement may only be reached by a set of detailed investigations. It is a detailed investigation of this kind, specifically of a psychological nature, that I am able to offer here. I have had little occasion to occupy myself with the problem of sleep, for this is essentially a problem of physiology, though a characterization of the sleeping state has to include changes in the conditions under which the psychical apparatus functions. So in my account I have disregarded the literature on sleep.
Scientific interest in the phenomena of dreams as such leads to the following, partly overlapping, questions:
(a) The Relationship of Dreams to Waking Life
The naive judgement of someone who has just woken up assumes that the dreameven if it does not come from another worldhas still transported the sleeper to one. The old physiologist Burdach , to whom we owe a scrupulous and sensitive description of the phenomena of dreams, has expressed this conviction in a celebrated passage (p. 474): `... the life of the day, with its exertions and pleasures, its joys and sorrows, is never repeated: the dream is rather bent on freeing us from it. Even when our entire soul has been engrossed with one object, when our heart has been riven with a deep sorrow or some task has exercised all our mental powers, the dream either gives us something completely alien, or takes for its combinations only individual elements from reality, or only enters into the key of the mood we are in and symbolises reality.'
In his justly acclaimed study of the nature and origin of dreams, L. Strümpell  expresses a similar view (p. 16): `The dreamer has turned away from the world of waking consciousness ...' (p. 17): `In dreams our memory of the ordered content of waking consciousness and its normal behaviour is as good as completely lost ...' (p. 19): `The almost memory-less isolation of the soul in dreams from the routine content and course of waking life ...'
But the great majority of writers have taken the opposite viewpoint, Haffner  writes as follows (p. 19): `At first the dream continues the waking life. Our dreams always follow the ideas present in our consciousness not long before. Meticulous observation will almost always discover a thread in which the dream links up with the experiences of the previous day.' Weygandt  (p. 6) directly contradicts Burdach's assertion quoted above, `for apparently it can often be observed in the great majority of dreams that, rather than freeing us from ordinary life, they lead us right back into it'. Maury  (p. 56) says in a succinct formula: `Nous rêvons de ce que nous avons vu, dit, désiré ou fait.' In his Psychologie of 1855 (p. 530), Jessen  puts it at rather greater length: `The content of dreams is always more or less determined by the individual personality, by age, sex, class, level of education, mode of life and by all the events and experiences of our lives hitherto.'
The ancients shared this idea of the dependence of the content of dreams on life. To cite Radestock  (p. 134): about to begin his campaign against Greece, Xerxes was dissuaded from this decision by sound advice, but was spurred on to it again and again by his dreams; at this, the rational old dream-interpreter of the Persians, Artabanos, told him quite rightly that dream-images usually contain what the dreamer already thinks when awake.
In Lucretius' didactic poem, De rerum natura, we find the passage (iv. v. 959):
Et quo quisque fere studio devinctus adhaeret, aut quibus in rebus multum sumus ante morati atque in ea ratione fuit contenta magis mens. in somnis eadem plerumque videmur obire; causidici causas agere et componere leges, induperatores pugnare ac proelia obire, etc. etc.
Cicero (De Divinatione, ii) says exactly what Maury said so much later: `Maximeque reliquiae earum rerum moventur in animis et agitantur, de quibus vigilantes aut cogitavimus aut egimus.'
The contradiction between these two views of the relationship between dream-life and waking life seems to be indeed irresolvable. So this is a good place to recall F. W. Hildebrandt's  discussion. He takes the view that the peculiarities of the dream cannot be described at all except as a `series of opposites which seem to intensify to the point of becoming contradictions' (p. 8). `The first of these opposites is the strict seclusion or isolation of the dream from real, true life on the one hand, and on the other the continual encroachment of the one on the other, the constant dependence of the one on the other.The dream is something altogether separate from the reality we experience when awake; one might call it an existence hermetically closed within itself, cut off from real life by an unbridgeable chasm. It frees us from reality, extinguishes our normal recollection of it, and places us in another world and in a quite different life-story, which has fundamentally nothing to do with our real one ...' Hildebrandt then explains how, when we fall asleep, our whole being, with the forms of its existence, disappears `as though beneath an invisible trap-door'. Then perhaps we dream we are making a sea-voyage to St Helena in order to offer the imprisoned Napoleon something special by way of Moselle. We are received by the ex-Emperor in the friendliest fashion and are almost sorry to see the interesting illusion disrupted by our awakening. But then we compare the situation in the dream with reality. We have never been a wine-merchant, and have never wanted to be one. We have never been on a sea-voyage, and St Helena is the last place we would choose to take one to. For Napoleon we feel no kind of sympathy, but on the contrary harbour a fierce patriotic hatred towards him. And on top of all that, the dreamer was not yet even among the living when Napoleon died on his island; to enjoy any personal relationship with him was beyond the bounds of possibility. Thus the dream-experience appears as something alien interpolated between two divisions of our life which otherwise fit into each other perfectly and continuously.
`And yet,' Hildebrandt goes on, `the apparent opposite is just as true and correct. In my view, the most intimate relationship and connection nevertheless goes hand in hand with this isolation and seclusion. We may even say: whatever the dream may present, it acquires its material from reality and from the intellectual life which goes along with this reality ... However strangely the dream may employ it, it can never actually get free from the real world: its most sublime and its most farcical phenomena alike must always borrow their raw material either from what has appeared before us in the world of the senses or from what has somehow already found a place in our waking thoughtsin other words, from what we have already experienced, outwardly or inwardly.'
(b) The Dream-MaterialMemory in Dreams
That all the material composing the content of the dream derives in some way from our experience, and so is reproduced, remembered, in the dreamthis at least we may count as undisputed knowledge. But it would be an error to assume that such a connection between the dream-content and waking life is bound to emerge effortlessly as the obvious result of comparing the two. It has rather to be attentively sought, for in a large number of cases it is capable of concealing itself for a long time. The reason for this lies in a number of peculiarities shown by our powers of memory in dreams, and which, although they have been generally remarked, have so far resisted every attempt at an explanation. It will be worth our while to examine these characteristics in detail.
The first thing to emerge is that material appears in the dream-content which, when subsequently awake, we do not recognize as part of our knowledge or experience. We may well remember that we dreamed this or that, but not that we ever experienced it. We are then in a quandary as to what source the dream has drawn on, and may well be tempted to believe that the dream possesses the power of independent production, until, often after a long interval, a new experience restores the memory of the earlier experience and so reveals the source of the dream. We then have to admit that we had known and remembered something in our dream which had escaped our powers of recollection when awake.
A particularly impressive example of this kind is told by Delboeuf  from his own dream-experience. In his dream he saw the yard of his house covered in snow and found two little lizards half-frozen and buried under the snow. Being an animal-lover he took them up, warmed them, and restored them to the little hole in the wall intended for them. In addition he gave them a few leaves from a little fern-plant growing on the wall which he knew they were very fond of. In his dream he knew the name of the plant: Asplenium ruta muralis. The dream then continued, after an interval returned to the lizards, and to Delboeuf's astonishment showed him two new little lizards which had set upon the remains of the ferns. Then he turned and looked into the distance, saw a fifth and a sixth lizard making their way to the hole in the wall, until finally the whole road was covered with a procession of lizards all moving in the same direction, etc.
In his waking life, Delboeuf was familiar with the Latin names of only a few plants, and the Asplenium was not among them. So he was greatly astonished to discover that a fern of this name actually existed: its correct designation was Asplenium ruta muraria, which the dream had slightly distorted. It seemed hardly credible that the name had occurred by coincidence; but where his knowledge of it in his dream had come from, remained a mystery to him.
The dream took place in 1862. Sixteen years later the philosopher was visiting a friend when he noticed a little album containing dried flowers of the sort that are sold as souvenirs to visitors in many parts of Switzerland. A memory arose within him, he opened the herbarium, found in it the Asplenium from his dream, and recognized his own handwriting in the Latin name added to it. Now the connection could be made: in 1860two years before the dream of the lizardsa sister of this friend had visited Delboeuf while on her honeymoon. She had had this album with her, intending it for her brother; and Delboeuf had taken the trouble to write its Latin name, at a botanist's dictation, beside each of the little dried plants.
A happy accident, of the sort that make this example so very much worth relating, allowed Delboeuf to trace yet another part of the content of this dream back to its forgotten source. One day in 1877 an old volume of an illustrated magazine came into his hands, and there he saw a picture of the entire procession of lizards as he had dreamed it in 1862. The volume bore the date 1861, and Delboeuf was able to recall that he had been a subscriber to the magazine from the time of its appearance.
The fact that the dream has at its disposal recollections which are inaccessible to the waking person is so remarkable and theoretically significant that I should like to reinforce it by recounting some more `hypermnestic' dreams. Maury  relates that for some time the word Mussidan kept coming into his mind during the day: he knew it was the name of a French town, but nothing more. One night he dreamed of a conversation with a certain person, who told him she came from Mussidan, and when he asked where the town was she replied that Mussidan was the main town in the Département de la Dordogne. When he awoke, Maury gave no credence to the information in the dream. But from the gazetteer he learned that it was perfectly correct. In this case the dream's superior knowledge is confirmed, but the forgotten source of this knowledge has not been traced.
Jessen  relates (p. 551) a very similar event in a dream from older times: `Among these is the dream of the elder Scaliger (Hennings p. 300), who wrote a poem in praise of the famous men of Verona. A man calling himself Brugnolus appeared to him in a dream, complaining that he had been forgotten. Although Scaliger could not recall ever having heard anything of him, he composed lines about him all the same, and his son afterwards learned in Verona that a Brugnolus had indeed once been famous there as a critic.'
In a publication unfortunately not available to me (Proceedings of the Society for Psychical Research), Myers is said to have produced a collection of such hypermnestic dreams. In my view, anyone engaged in the study of dreams is bound to recognize that it is a very common phenomenon for a dream to bear witness to knowledge and recollections which the waking person does not believe he possesses. In my psychoanalytic work with nervous patients, which I shall refer to later, I find myself several times every week in the position of proving to my patients from their dreams that they really do know certain quotations, obscene words, and the like very well, and that they use them in dreams even though they have forgotten them in waking life. I should like to add here a further innocuous case of dream-hypermnesia, for in this instance the source that gave rise to the knowledge accessible only to the dream is very easy to detect.
As part of a lengthy dream a patient dreamed that in a coffee-house he had ordered a `Kontuszowka'. But after relating the dream he asked what it could be: he had never heard the name. I was able to reply that `Kontuszowka' was a Polish schnapps which he could not have invented in his dream, as the name had long been familiar to me from posters. At first the man would not believe me. A few days later, after he had made his dream come true in a real coffee-house, he noticed the name on a poster, and what is more, on one at a street corner which he must have passed at least twice a day for several months.
One of the sources that dreams draw upon for material for reproduction, some of it not remembered or employed in the mental activity of waking life, is the life of childhood. I shall cite only a few of the writers who have noticed this and emphasized it:
Hildebrandt  (p. 23): `It has already been expressly conceded that dreams faithfully bring quite remote and even forgotten events from the distant past back to our minds, sometimes with a marvellous power of reproduction.'
Strümpell  (p. 40): `The issue becomes even more complex when we remark how sometimes dreams draw forth images of individual places, things, and persons quite unscathed and in all their original freshness, as it were out from under the deepest and most massive accumulations deposited by later years on the earliest experiences of our youth. This is not confined merely to those impressions which we were vividly conscious of when they first arose, or were associated with strong psychical values and now recur later in a dream as actual recollections in which the wakened consciousness takes pleasure. Rather, the depths of the dream memory also include those images of people, things, places, and experiences from our earliest years which either impressed themselves only slightly on our consciousness, or possessed no psychical value, or have long since lost both, and so, both in the dream and after we have woken up, they appear quite alien and unfamiliar to us until their early origin is discovered.'
Volkelt  (p. 119): `It is particularly remarkable how easily recollections of childhood and youth enter dreams. The dream never tires of reminding us of things we have long since ceased to think about, things which have long since lost all importance to us.'
The dream's command of childhood material, most of which, as we know, falls through the gaps in our conscious power of recollection, occasions interesting hypermnestic dreams, a few of which I would again like to relate.
Maury  tells us (p. 92) that as a child he often travelled from his home town of Meaux to the nearby town of Trilport, where his father was directing the construction of a bridge. One night a dream transported him to Trilport and set him playing again in the streets of the town. A man approached him wearing a kind of uniform. Maury asked him what his name was; the man introduced himself as C., and said he was a watchman at the bridge. After he woke, Maury still doubted the truth of this recollection, so he asked an old servant who had been with him since childhood whether she could remember a man of this name. Certainly, came her reply, he was the watchman at the bridge your father was constructing at that time.
Maury reports another example, just as satisfyingly confirmed, of the reliability of childhood recollection as it appears in dreams. It came from a Montsieur F., who had grown up as a child in the town of Montbrison. Twenty-five years after he had left it, this man decided to revisit his home town as well as old friends of the family whom he had not seen since. In the night before his departure he dreamed that he had arrived, and in the vicinity of Montbrison encountered a man unknown to him by sight, who told him he was Monsieur T., a friend of his father's. The dreamer knew that as a child he had known a man of this name, though when awake he could no longer remember what he looked like. A few days later, now actually arrived in Montbrison, he found the locality of his dream again, which he had thought was unfamiliar to him, and encountered a man whom he promptly recognized as the T. of his dream. Only that the real person was much older than the dream had represented him.
At this stage I can relate a dream of my own, in which the impression to be recalled is replaced by an association. In a dream I saw a person who I knew in the dream was the doctor of my home town. His face was not clear, but was blended with the impression of one of my teachers at the Gymnasium, whom I occasionally still meet today. After I woke, I could not make out what association linked these two people. But when I asked my mother about this doctor from my earliest years, I learned that he had had only one eyeand the schoolteacher whose figure in the dream had covered that of the doctor also has only one eye. I had not seen the doctor for thirty-eight years, and to my knowledge I have never thought about him in my waking life.
It sounds as if there was an intention to set up a counterweight to the overly great role played by childhood impressions in dreams, when several authorities maintain that most dreams can be shown to contain elements from the days most recently preceding the dream. Robert  (p. 46) even asserts that in general the normal dream is concerned only with the days just past. We shall in any event discover that the theory of dreams Robert has constructed makes it imperative for him to push back the oldest impressions and bring forward the most recent. I can, however, confirm from my own investigations that what Robert says in this respect is a fact. An American writer, Nelson , takes the view that what most often happens is that the dream makes use of impressions from the day before the day of the dream or from the third day before, as though the impressions from the day immediately preceding the dream were toned down, not remote enough.
Several writers who would not doubt the intimate connection between the content of dreams and waking life have observed that impressions that occupy the waking thoughts intensely appear in dreams only when they have to some extent been pushed to one side by the thinking activity of the day. Thus we do not as a rule dream of a dead person dear to us in the time right after their death, while the survivor is still filled with grief (Delage) . Meantime, however, one of the most recent observers, Miss Hallam , has collected examples of the opposite behaviour, and in this regard claims for each of us the right to our psychological individuality.
The third, most remarkable and incomprehensible characteristic of memory in dreams is to be seen in the choice of material reproduced. For what is considered worth remembering is not, as in waking life, only the things that are of most significance, but also on the contrary the most trivial and nondescript things. At this point I shall cite those authors who have expressed their amazement most forcefully.
Hildebrandt  (p. 11): `For the remarkable thing is that as a rule the dream takes its elements not from the important and far-reaching events, not from the great and motivating interests of the previous day, but from the trivial extras, from the worthless scraps, as it were, of the recent or more remote past. A distressing death in the family, whose impressions send us late to sleep, is blotted from our memory until our first waking moment brings it back to us vividly in all its sadness. On the other hand, a wart on the forehead of a stranger we encountered and did not give a moment's thought to after we had passed him bythis plays a part in our dream ...'
Strümpell  (p. 39): `... cases where the analysis of a dream discovers elements which, though it is true they derive from the experiences of the previous day or the day before that, were so worthless and insignificant to the waking consciousness, that they sank into oblivion soon after they were experienced. Experiences of this kind are for instance words heard by accident, someone else's actions scarcely noticed, fleeting perceptions of things or persons, brief passages from our reading, and the like.'
Havelock Ellis  (p. 727): `The profound emotions of waking life, the questions and problems on which we spread our chief voluntary mental energy, are not those which usually present themselves to dream consciousness. It is so far as the immediate past is concerned mostly the trifling, the incidental, the "forgotten" impressions of daily life which reappear in our dreams. The psychic activities that are awake most intensely are those that sleep most profoundly.'
Binz  (p. 45) takes these very peculiarities of memory in the dream as the occasion for expressing his dissatisfaction with the explanations of the dream he himself supports: `And the natural dream raises similar questions. Why is it that we do not always dream the memory-impressions of the most recent day we have spent, but often plunge without any recognizable motive into a past that lies far behind us, almost obliterated? In dreams why does the consciousness so often receive the impression of trivial recollections, while the brain-cells, the very place where the most sensitive register of our experience is lodged, mostly lie still and silent, unless they have been freshly aroused in waking hours shortly before?'
It is easy to see how the strange preference of the dream-memory for trifling and hence unnoticed things among the experiences of the day was largely bound to lead to a general failure to understand the dream's dependence on the life of the day, or at least to making this very difficult to demonstrate in individual cases. Consequently, in Miss Whiton Calkins's  statistical analysis of her (and her companion's) dreams, it was possible for her to be left with 11 per cent of the total in which no relation to the life of the day was perceptible. Hildebrandt is certainly right in maintaining that all dream-images could be explained genetically, if we spent sufficient time and concentration on tracing their origin. True, he calls this `an extremely laborious and thankless task. For it would mostly amount to tracking down all sorts of things with no psychical value in the remotest corners of the chamber of memory, and bringing to light all sorts of utterly trivial impulses from times long past from depths where they had been buried perhaps the very hour after they had occurred.' But I must express my regret that this penetrating writer failed to pursue this path from this modest beginning; it would have led him straight to the centre of the explanation of dreams.
The way the memory behaves in dreams is certainly most important for any theory of memory in general. It teaches us that `nothing that is once mentally our own can ever be entirely lost' (Scholz  p. 34). Or, as Delboeuf  puts it, `que tout impression même la plus insignifiante, laisse une trace inaltérable, indéfiniment susceptible de reparaître au jour', a conclusion which so many otherpathologicalphenomena of the psychical life likewise force us to make. Now if we keep this extraordinary capability of the dream-memory in mind, we will be keenly aware of the contradiction which certain dream-theories, referred to later, are bound to set up, as they try to explain the absurdity and incoherence of dreams by the partial forgetting of what is known to us by day.
It might perhaps occur to us to reduce the phenomenon of dreaming entirely to that of remembering and regard the dream as the expression of a reproductive activity, unresting even at night, which is an end in itself. Accounts such as that given by Pilcz  would accord with this, for he claims that firm connections can be demonstrated between the time of dreaming and the content of the dream, such that in deep sleep the dream reproduces impressions from the earliest times, but reproduces recent ones towards morning. But such a conception is made improbable from the outset by the way the dream deals with the material to be remembered. Strümpell  rightly points out that in the dream repetitions of experiences do not occur. True, the dream makes a start at it, but the next link is missing; it appears in altered form, or something entirely strange appears in its place. The dream presents only fragmentary reproductions. Certainly, this is so often the rule that we can make theoretical use of it. However, exceptions do occur in which a dream will repeat an experience just as completely as our waking recollection can. Delboeuf tells of how in a dream one of his university colleagues (who teaches at present in Vienna) repeated a dangerous coach journey in all its detail, where he escaped an accident only as if by a miracle. Miss Calkins  mentions two dreams whose content consisted of the exact reproduction of an experience from the previous day, and I myself will later take the opportunity of relating an instance known to me of the return in a dream, unaltered, of a childhood experience.
(c) Dream-Stimuli and Dream-Sources
What we are to understand by dream-stimuli and dream-sources can bc clarified by referring to the popular saying, `dreams come from the stomach'. Behind these concepts there is concealed a theory which regards the dream as the consequence of a disturbance of sleep; we would not have dreamed if something to disturb us had not stirred in our sleepand the dream is the reaction to this disturbance.
Discussion of the causes that generate dreams occupies more space than any other topic in the writings about them. It goes without saying that the problem could only arise after the dream had become an object of biological inquiry. The ancients, who regarded the dream as divine inspiration, had no need to seek for its source; the dream proceeded from the will of the divine or daemonic power, its content from their knowledge or intention. For science, the question arose straight away whether the incentive to dreaming was always the same, or whether there might be several incentiveswhich brought with it the need to consider whether the explanation for the causes of dreams fell to psychology or to physiology. Most authorities seem to assume that there can be many kinds of cause for disturbances of sleep, that is, for the sources of dreaming, and that both somatic stimuli and psychical excitations can act as causative agents of dreams. But there is wide disagreement in privileging the one or the other among the sources of dreams, and in establishing a hierarchy among them according to their importance for the genesis of the dream.
With a complete enumeration of the sources of dreams it emerges that there are ultimately four kinds, which have also been used for the classification of dreams:
1. External (objective) sensory excitation; 2. Internal (organic) sensory excitation; 3. Internal (organic) somatic stimulus; 4. Purely psychical sources of stimulus.
1. External sensory stimuli
Strümpell the younger, son of the philosopher whose work on dreams has already served us several times as a guide to the problems of dreams, has, as we know, reported on his observation of an invalid afflicted with general anaesthesia of the skin and paralysis of several of the higher sensory organs. When this man's few remaining sensory gateways were cut off from the outside world, he fell asleep. When we want to go to sleep, we all try to attain a situation similar to the one in Strümpell's experiment. We close the most important sensory gateways, our eyes, and try to protect our other senses from any stimulus or change in the stimuli acting on them. We then go to sleep, though we are never completely successful in carrying out our aim. We can neither isolate our sensory organs entirely from the stimuli, nor wholly neutralize their excitability. The fact that we can always be woken by stronger stimuli may be taken as proof `that even in sleep the psyche has remained in constant contact with the world external to the body'. The sensory stimuli that reach us in sleep could well become sources for dreams.
Now among such stimuli there are a great number, ranging from the unavoidable ones entailed in the sleeping state itself or occasionally admitted to it, to the chance stimulus to waking which is likely or intended to put an end to sleep. A rather strong light can penetrate the eyes, a sound become audible, or an odour agitate the nasal membrane. While we are asleep, involuntary movements may uncover parts of our body and in this way expose them to the sensation of getting colder, or by changing our position we may produce sensations of pressure or contact. A fly may sting us, or a slight nocturnal accident assail several of our senses at once. Attentive observers have collected a large number of dreams in which the stimulus established on waking and a part of the dream-content coincide so closely that the stimulus could be recognized as the source of the dream.
I shall cite here from Jessen  (p. 527) a collection of dreams of this kind which derive from objectivemore or less accidentalsensory stimulation. Each indistinctly heard sound arouses corresponding dream-images: the rumble of thunder transports us to the midst of a battle; the crowing of a cock can be transformed into a human cry of fear; the creaking of a door call forth dreams of burglars breaking in. If we lose our bedclothes at night, we might dream that we are walking about naked or that we have fallen into the water. If we are lying at an angle in bed and our feet project over the edge, we might dream that we are standing at the edge of a terrible abyss, or that we are falling from a steep cliff. If by accident our head gets under the pillow, there is a great rock hanging over us about to bury us beneath its weight. Accumulations of semen produce lascivious dreams, localized pains the idea of abuses we have suffered, hostile attacks or the infliction of bodily injuries ...
`Meier (Versuch einer Erklärung des Nachtwandelns, Halle: 1758, p. 33) once dreamed that he was set upon by a number of people who stretched him out on the ground on his back and drove a stake into the ground between his big toe and the one next to it. As he was imagining this in his dream, he woke and felt a straw sticking between his toes. On another occasion, according to Hennings (Von den Träumen und Nachtwandlern, Weimar: 1784, p. 258), Meier had fastened his shirt rather tightly around his neck, and dreamed he was being hanged. Hoffbauer dreamed as a young man that he was falling from a high wall, and discovered when he woke that the bedstead had come apart and that he really had fallen ... Gregory reports that he once laid a hot-water bottle at his feet when he went to bed and then dreamed he had made a journey to the top of Mount Etna, where he found the heat of the ground almost unbearable. Another dreamed after he had had a poultice applied to his head that he was being scalped by Red Indians. A third, who went to sleep in a damp shirt, dreamed he was being dragged through a river. An attack of gout while he was asleep led an invalid to believe he was in the hands of the Inquisition and was being tortured (Macnish).'
The argument from the similarity between stimulus and dream-content is strengthened if by this systematic application of sensory stimuli it is possible to produce in the sleeper dreams corresponding to the stimulus. According to Macnish, Giron de Buzareingues had already tried such experiments. `He left his knee uncovered, and dreamed that he was travelling by post-chaise at night. He observes in his account that travellers would certainly know how cold the knees would become in a coach at night. At another time he left the back of his head uncovered and dreamed that he was attending a religious ceremony in the open air. For in the country where he lived, it was the custom to keep the head covered, except on such occasions as the one just mentioned.'
Maury  reports a number of new observations of dreams he himself had produced. (A further series of experiments proved unsuccessful.)
1. He is tickled on the lips and tip of the nose with a feather.Dreams of a terrible torture; a mask of pitch is placed on his face, then torn away, taking the skin with it.
2. A pair of scissors is sharpened against a pair of tweezers.He hears bells pealing, then warning-bells, and is transported to the June days of 1848.
3. He smells eau de Cologne.He is in Cairo in the shop of Johann Maria Farina. There follow wild adventures which he is unable to reproduce.
4. He is squeezed gently in the neck.He dreams that he is having a poultice applied, and thinks of a doctor who treated him in childhood.
5. A hot iron is held to his face.He dreams of the `stokers' who have crept into the house and are forcing the inhabitants to hand over their money by thrusting their feet into a brazier. Then the Duchess of Abrantés, whose secretary he is in the dream, appears on the scene.
8. A drop of water is poured on to his forehead.He is in Italy, perspiring heavily and drinking the white wine of Orvieto.
9. The light from a candle, filtered through a red paper, is made to fall on him repeatedly.He dreams of storms, of heat, and finds himself once again in a storm at sea which he once experienced in the English Channel.
Other attempts at producing dreams experimentally have been made by d'Hervey , Weygandt , and others.
Many have noticed the `striking skill of the dream in weaving sudden impressions from the sensory world into its creations in such a way that they come to form a catastrophe whose onset has already been gradually prepared and ushered in' (Hildebrandt ). `When I was younger,' this author relates, `in order to get up regularly at a certain hour in the morning, I sometimes used the familiar alarm usually attached to clocks. It must have happened a hundred times that the sound of this alarm fitted into an apparently very long and coherent dream, as if the whole dream were directed towards it as its logically indispensable point and natural goal.'
I shall refer to three more of these alarm-clock dreams in another connection.
Excerpted from The Interpretation of Dreams by Sigmund Freud Copyright © 2000 by Sigmund Freud. Excerpted by permission.
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