The Sensory Order

CHAPTER VII

CONFIRMATIONS AND VERIFICATIONS OF THE THEORY

1. OBSERVED FACTS FOR WHICH THE THEORY ACCOUNTS

7.1. The value of a theory of the kind presented here may prove itself by accounting for known facts as consequences of other known phenomena, by enabling us to eliminate phantom problems, by showing that certain earlier theories are special cases of a more general principle, or, finally, by suggesting new questions which can be experimentally investigated. On all these scores our theory appears to have a certain amount of prima facie evidence in its favour.

7.2. The main aim of the theory presented is to show that the range of mental phenomena such as discrimination, equivalence of response to different stimuli, generalization, transfer, abstraction, and conceptual thought, may all be interpreted as different forms of the same process which we have called classification, and that such classifications can be effected by a network of connexions transmitting nervous impulses. From the fact that this classification is determined by the position of the individual impulse or group of impulses in a complex structure of connexions, extending through a hierarchy of levels, follow certain important conclusions concerning the effects which physiological or anatomical changes must be expected to have on mental functions. We shall confine ourselves here to point out a few of the more important consequences of our theory which are in accordance with observed fact.

7.3. Since the qualities of mental events produced by particular impulses or groups of impulses according to this view depend not on any property which these impulses possess by themselves, but on their position in the whole network of connexions, it would follow that the different mental functions need not be localized in any particular part of the cortex.

7.4. While the possibility of a peripheral stimulus producing a sensory quality will in general 1 depend on the preservation of the central endings of the corresponding afferent fibres, there is no reason for expecting that beyond this the capacity of experiencing particular qualities will depend on particular parts of the cortex. We should rather expect to find, as in fact we do find, that a destruction of a limited part of the cortex will lead to some weakening of most or all mental functions, rather than to the extinction of some particular capacities.²

7.6. This may mean either that certain mental processes which are normally based on impulses proceeding in certain fibres may, after these fibres have been destroyed, be relearned by the use of some other fibres, or that certain associations may be effectively brought about through several alternative bundles of connexions, so that, if any one of these paths is severed, the remaining ones will still be able to bring about the result. Such effects have been observed and described under the names of ‘vicarious functioning’ and ‘equipotentiality’.3

7.7. Our account of the translation of the neural impulse into a mental event as a process of classification leads us to expect that we will find that this process not only takes perceptible time but also that it can be observed in different successive stages in which the classification or evaluation is developed to different degrees. This expectation is amply borne out by observation. From the unconscious responses to stimuli and the still unconscious ‘subception’4 through the ‘pre-sensation’⁵ and the various degrees of clarity of the sensation,⁶ perception and ‘apperception’, to judgements and concept formation, there exists clearly a chain of events in which the full evaluation of any mental quality gradually unfolds itself.

7.8. From the account we have given of the determination of sensory qualities it would further follow that the quality of any sense experience attached to certain impulses or group of impulses will not always be the same but will be different in different circumstances. The same individual stimulus, affecting the same receptor organs, must thus be expected to produce different sensory qualities according as different other stimuli operate at the same time.

7.9. As we have already seen (4.45–4.47), this expectation is fully borne out by experimental work. Many stimuli are perceived ‘correctly’ only if received under normal conditions, but lead to different sensations if the setting is not normal.⁷

2. OLDER THEORIES COMPRISED AS SPECIAL CASES

7.10. There is no need here to mention again the various instances where our approach eliminates what now appear to be false questions. We can at once turn to the several instances where our theory embraces as special cases theories which in the past have been advanced in order to explain particular phenomena. Some of these instances have been noticed earlier and now need be mentioned only briefly.

7.11. The first instance of this kind which has been discussed earlier (3.40–3.45) is Berkeley’s theory of spatial vision and the more general theories of space perception which have developed from it. The account of the determination of the spatial order of perception by the co-ordination between the various sense modalities and the kinesthetic sensations is of course merely one particular instance of the theory of the determination of sensory qualities developed here.

7.12. Another similar instance of an anticipation in a particular field which we have already mentioned is the James-Lange theory, of emotions. As has been shown before (4.70–4.72), this theory, carefully restated, might be regarded as a special case of our theory.

7.13. In the case at least of von Helmholtz the emphasis on the effect of experience in determining sensory qualities goes far beyond ascribing to experience the creation of their spatial order, and it probably is due mainly to his influence that it is to-day widely recognized that ‘the manner in which we see things of the external world is sometimes affected by experience to an overwhelming extent’ and that ‘it is often difficult to decide which of our visual experiences are determined immediately by sensation and which, on the contrary, are determined by experience and practice.’8 His conception of the ‘unconscious inference’,⁹ by which stimuli which do not lead to conscious experience are yet utilized in the perception of a complex position, comes very close to the theory developed here. Yet von Helmholtz, like all later writers following on these lines, instead of drawing the conclusion that the factors to which he attributed ‘overwhelming importance’ in determining the sensory qualities might be the sole factors which determine them, in fact insisted that nothing could be recognized as sensation which is demonstrably due to experience¹⁰—thus giving, in fact, support to the conception of a pure core of sensation.

7.14. The same applies to the group of theories which have furthest developed this line of thought, the Reproduktionspsychologie of B. Erdmann, R. Dodge, H. Henning and F. Schumann, which, with its stress on the ‘residua’ which determine sensory qualities, came very close to the position taken here, yet never ceased to distinguish between a ‘stimulus component’ and a ‘residual component’, the former of which still corresponds to the ‘pure core’ of sensation.11

7.15. The relation which exists between our theory and the views of the gestalt school is of a somewhat different character and has already been discussed (3.70–3.79). As was then pointed out, the present approach may be regarded as an attempt to raisc, with regard to all kinds of sensory experiences, the question which the gestalt school raised in connexion with the perception of configurations. And it seems to us, that in some respects at least, our theory may be regarded as a consistent development of the approach of the gestalt school.¹²

7.16. Another instance of a connexion between our theory and a familiar older view has not yet been explicitly mentioned: the obvious relations which exist between it and the basic ideas of the old association psychology. Our view agrees, of course, with associationism in the endeavour to trace all mental processes to connexions established by experience between certain elements. It differs from it by regarding the elements between which such connexions are established as not themselves mental in character but as material events which only through those connexions are arranged in a new order in which they obtain the specific significance characteristic of mental events (5.52).

7.17. This is a step which James Mill very nearly made when he briefly suggested that similarity (‘resemblance’) might be dispensed with as a ‘principle of association’ and be reduced to a ‘particular case’ of the ‘law of frequency’ of co-occurrence.13 This promising beginning was, however, cut short by the somewhat uncomprehending comment added to this passage by his son, who described the brief hint as ‘perhaps the least successful attempt at simplification and generalization of the laws of mental phenomena, to be found in the work.’ The only further development of this idea is to be found in the writings of the last of the old association psychologists, G. H. Lewes, which never seem to have received the attention which they deserve.14

7.18. Finally, we may perhaps once more mention that within the framework of this theory the conception of events which are mental but not conscious receives, for the first time so far as we are aware, a clear meaning. In consequence it provides a systematic place for whatever of the various theories of the unconscious will prove permanent additions to knowledge.

3. NEW EXPERIMENTS SUGGESTED

7.19. The theory developed here is not the kind which one could hope to confirm or refute by a single crucial experiment. Its value ought to show itself rather in suggesting new directions in which experimental work should produce interesting results. The main thesis for which one may hope to find experimental confirmation is that the sensory qualities can be changed by the acquisition of new connexions between sensory impulses. If this central contention is correct it should in principle be possible both, to attach conscious sensory qualities to sensory impulses which before carried no conscious values, and to create discriminations between such impulses which before caused undistinguishable sensations. It should even be possible to create altogether new sensory qualities which have never been experienced before.

7.20. There exists a great deal of evidence that the capacities for sensory discrimination can be greatly developed by practice. The greatly heightened capacities for tactual, auditory and olfactory discrimination often acquired by the blind,15 the development of taste, smell, vision and touch by the professional tasters and samplers of wine,¹⁶ spirits, tobacco, chocolate, perfumes, wool,¹⁷ cheese,¹⁸ and the like, the development of the sense of smell by some doctors and chemists,19 of the auditory sense of musicians²⁰, and of the colour sense of artists and dyers²¹ are familiar, although quite inadequately studied, examples.

7.21. In more recent times, largely under the influence of the gestalt school, the effect of experience and practice on what has come to be known as ‘perceptual organization’ has received a good deal of attention. It appears to have been established beyond doubt that the perception of the various configurations and complexes can be profoundly altered by experience.22 But although this fact is closely connected with our problem, and (if the belief, held both by the gestalt school and ourselves, is correct, that there is no real difference between sensation and perception) goes far to make the variability of even the most elementary sensory qualities probable, it does not directly confirm that the latter is the case.

7.22. Most of this discussion of sensory organization—not excluding much of the work of the gestalt school, in spite of its fight against the ‘constancy assumption’—however, still suffers from an underlying belief that this problem is one of how given sensations become ‘organized’, as if there could be unorganized sensory data, something like W. James’s ‘blooming buzzing confusion’ in the mind of the newly born, and that it is these initial fixed sense data which perception organizes in a pattern.23 These remnants of the old ‘mosaic theory’ which still pervade the discussion cannot be finally eliminated until it is realized that sensory organization and the determination of the individual qualities are one and the same problem.

7.23. Connected with the studies of the effect of experience on sensory organization are the known facts about the manner in which congenitally blind who by an operation have become able to see (and animals reared in darkness)24, learn to perceive visual objects. The ample material collected on this problem by Senden 25 shows clearly that at least the ordering of the individual sensations has to be gradually learnt, but also that apparently such persons are able from the first moment to distinguish colours. But as it appears that no completely blind person has ever gained vision in this manner²⁶ and that all those operated persons whose vision had been obstructed by cataract were, before the operation, able to distinguish shades of light and probably also colours, this information is of little direct use for our purpose.

7.24. Perhaps the most significant experimental findings in this field are the extensive investigations of Stratton, Ewert and, more recently, Erismann on the effect of the prolonged wearing of various kinds of spectacles which either invert or distort vision,27 and the corresponding experiments by P. T. Young with the ‘pseudo-phone’, an apparatus which effects an acoustical transposition of sound between the two ears.²⁸ All these experiments show that the significance or position of different stimuli of one modality relative to stimuli of another modality can be altered if they are regularly made to occur in a new combination.

7.25. The older treatises on psychology contain a good deal of discussion on the effect of practice on sensory discrimination. William James, e.g., in a section headed ‘the improvement of discrimination by practice’ even explicitly mentions as the first cause ‘which we can see at work whenever experience improves discrimination’ the fact that ‘the terms whose difference comes to be felt contract disparate associates and these help to draw them apart’.29

7.26. Little systematic work, however, has been done on this problem and even the meaning of the concept of practice as applied to sensory discrimination, and of the conception of new or improved discriminations, has been left somewhat obscure. Indeed the older psychologists who paid at least some attention to the effect of practice in this connexion, were inclined to regard it mainly as a nuisance, an effect which had to be eliminated before serious experimental work could start, rather than as a phenomenon which deserved investigation for its own sake.

7.27. The earliest and for a long time the only systematic experiments in these fields were those performed nearly a hundred years ago by A. W. Volkmann 30 on the effect of practice on the threshold for discrimination between two neighbouring points on the skin. Later experiments³¹ have amply confirmed his findings that not only these thresholds could be decreased by short practice by as much as from 50 mm. to 0.5 mm., but also that practice with such tactual stimuli on a part of the skin on one side of the body would similarly decrease the threshold for discrimination between symmetrically corresponding points on the other side of the body.

7.28. Almost the only systematic work done in this field in more recent times are a number of somewhat inconclusive studies on the effect of practice on pitch discrimination in hearing, conducted by various students interested mainly in musical education.32 These studies are not very helpful for our purpose because they addressed themselves in the main to the question whether practice would improve discrimination, rather than to the problem of the conditions under which it would do so. The one significant point which emerges is that it seems to be generally true that no mere repetition but only knowledge of results of the attempts to discriminate will lead to an improvement of discrimination.

7.29. This unsatisfactory state of knowledge of the whole subject is probably in a great measure due to the uncertain meaning of the concept of practice when applied to these problems. Although this meaning is usually taken (and sometimes explicitly said 33) to be obvious, it is by no means clear that the sense of improving an existing ‘capacity’ by repeated exercise, which is probably roughly what is meant by the effect of practice in other fields, fits the case of sensory discrimination.

7.30. There is little difficulty about understanding why the repetition of any particular series of movements should enable us to perform them afterwards more quickly, surely, smoothly or otherwise more efficiently. But there seems to be no similar obvious reason why any number of attempts to distinguish between two stimuli which we have not been able to distinguish before should teach us to do so. The whole approach to the problem seems still to be determined by the rather meaningless conception that these different sensations are always ‘there’ in some concealed sense, and that the problem is merely to learn to notice these ‘unnoticed’ sensations which are assumed to be necessarily and invariably coupled with the sensory impulse.

7.31. With regard to any kind of movements, practice clearly means some effect of memory and, as we have seen (5.10–5.12), it is difficult to see what other meaning ‘memory’ can have but the retention of connexions or relations. But while this conception applies directly to the acquisition of new series of movements which can become coupled with each other, and makes it easy to see why, e.g., such a series of movements which at first could be performed only by conscious effort, comes later to be performed automatically, at least the traditional view of the character of sensations does not fit into this pattern.

7.32. To acquire the capacity for new sensory discrimination is not merely to learn to do better what we have done before; it means doing something altogether new. It means not merely to discriminate better or more efficiently between two stimuli or groups of stimuli: it means discriminating between stimuli which before were not discriminated at all. If qualities are, as we have maintained, subjective, then, if new discriminations appear for the first time, this means the appearance of a new quality. There is no sense in saying that, if a chemist learns to distinguish between two smells which nobody has ever distinguished before, he has learnt to distinguish between given qualities: these qualities just did not exist before he learnt to distinguish between them.

7.33. Of course, such a ‘new’ quality can never be unlike any quality ever experienced before: to be recognizable as a distinct quality it must, in certain ways, be related to already familiar qualities, be in various respects similar to, or different from them. It will be a quality only by occupying a certain position in the order of all qualities, an order which can only be gradually extended and more finely subdivided. But although thus most ‘new’ qualities will constitute merely a new step in a pre-existing gradation or scale, and share their various attributes with different other qualities, they will nevertheless be new qualities which did not exist before.

7.34. The prevalent uncritical attitude towards the whole problem probably has been much assisted by the fact that the very term ‘discrimination’ suggests something like a ‘recognition’ of objective differences between the stimuli (2.32) and belongs thus to an earlier stage of theoretical development.³⁴ To this idea probably is also due the still widely held view that what is affected by practice is merely the ‘interpretation’ of a ‘given’ sensory quality or datum. The whole problem is still largely approached as if the differences between sensory qualities could be accounted for by a different physiological sensitivity of the sense organs—a physiological ‘capacity’ which needs merely to be ‘developed’ and which at the same time sets a ‘physiological limit’ to the extent to which discrimination can be improved. These concepts of the ‘capacity’ and the ‘physiological limit’ are as obscure and need as much clearing up as the concept of practice itself.

7.35. Discrimination in the relevant sense (better described as classification) involves not only the learning to respond differently to different physical stimuli, but also the learning to respond similarly to stimuli which physically may be different or similar, and to respond differently to the same stimulus in different contexts. In order that a problem of discrimination should arise, it is necessary, of course, that the different stimuli should cause impulses in different sensory fibres (or, though this does not seem to be the case, different kinds of impulses in the same fibre). But this condition would appear to be the only ‘physiological limit’: different impulses which affect the same receptor organs in the same manner must under the same conditions produce undistinguishable effects.

7.36. Unless we assume the theory of the specific energy of the nerves to be true in its illegitimate interpretation (1.33), there is indeed no reason why it should not be possible to learn to attach different qualities to impulses caused by stimuli which are physically identical, and proceeding in fibres which belong to the same sense modality. Cases are, of course, known where identical physical stimuli, acting on receptors belonging to different modalities (‘paradoxical cold’, vibration and sound, and the different sensory qualities produced by the same physical stimulus acting on the mucous membranes of the eye and the mouth—1.40) produce different sensations, but the same should in principle also be possible where otherwise identical receptors at different points of the body are involved.

7.37. From the whole approach followed in the present inquiry it would follow that learning to distinguish between different individual stimuli can only mean that we come to attach to these stimuli different effects irrespective of the manner in which these stimuli differ objectively. Learning to discriminate does not necessarily produce a better reproduction of the physical order of the-stimuli; it merely means the creation of a new distinction in the phenomenal order which, if it were the result of a non-recurring, accidental or artificial combination of stimuli during a particular period, might indeed prove later not a help but an obstacle to orientation and appropriate behaviour.

7.38. The only sense in which the improvement of sense discrimination by practice can be said to be a ‘development’ of preexisting capacities is that, in order that such discrimination at the higher levels should become possible, the occurrence of distinct processes at some lower level (at least the receptor level) must be presupposed. That is, the organism must initially respond in some way differently to the different stimuli (even if it only be that impulses are set up in the first instance in different fibres) if it is to be possible that these stimuli should acquire different significance for the higher nervous centres. It is at least likely that in most instances different responses to the impulses in the different fibres will already have taken place on a reflex or spinal level before the higher centres learn to discriminate between those impulses, since the development of distinct receptors for different physical stimuli probably goes hand in hand with the development of different responses to those stimuli.

7.39. There appear to exist three principal ways in which the attaching of new connexions to sensory impulses which arrive at the higher centres might lead to the appearance of new sensory qualities: 1. impulses which before did not produce a distinct sensation might come to do so; 2. different impulses produced by different physical stimuli which formerly produced the same sensory quality might be made to be perceived as different sensory qualities; and 3. impulses produced by the action of physically identical stimuli on similar receptor organs at different points of the body might also acquire different sensory qualities.

7.40. The task of experimentation in all these instances would be to ascertain whether we could either become aware of particular sensory impulses of which we were before not conscious, or whether sensory impulses could be given distinct qualitative significance different from that of other such impulses from which they were formerly indistinguishable; this might be done by attaching to them a distinct set of connexions which are different from those attached to other such impulses which before were perceived as identical.

7.41. It would seem that in any such experiments we must be able to rely on verbal reports of the subject and that therefore animal experiments cannot be used for our purpose. It would be necessary to ascertain before experiments start that the subject is either unaware of the stimulus, or unaware of any qualitative difference between the effects of different stimuli. And although we can teach animals to discriminate between stimuli with respect to certain responses, it would be impossible to decide whether an animal has merely learnt to attach a new response to distinct sensations which it perceived before, or whether it has acquired a new capacity for discrimination. Considering the difficulty of merely ascertaining, e.g., whether particular animals possess colour vision or not,35 it would seem that animal experiments must be ruled out in this connexion.

7.42. With human subjects the chances of successful experiments on these lines probably differ greatly between the different sense modalities. With a sense as highly developed and as fully used in humans as sight, practice in most instances probably has been carried to a point where a definite order has been so deeply engrained that it would at least take very long to obtain any results. It should be noted, however, that as von Kries has pointed out,36 in another sense this most highly developed of human sensory capacities is the most imperfect of the senses: the correspondence between physical differences between the stimuli and the differences between the sensory qualities is probably less close here than it is with other senses. Every colour can be produced by a great variety of mixtures of wave lengths in addition to (in most instances) a monochromatic (homogeneous) light. We do not know whether this equivalence of various combinations of stimuli is determined by a peripheral (i.e. receptor) or by a central mechanism. If the latter were the case, it should not be impossible to learn to see as different colours different mixtures of light waves which initially appear to be indistinguishable.

7.43. Better chances of experimental results exist probably in the less practised sense modalities, particularly those, such as the human sense of smell, of which in an earlier state of development man made greater use than he does in civilized life, and where the physiological capacities of distinguishing between different stimuli is probably much greater than that which we use. It has been pointed out by a competent observer that in this field ‘the influence of practice is so enormous, particularly in the beginning, that some people require on the second day of experimentation only small fractions of the threshold values necessary on the first day, and that they then easily solve qualitative analyses which on the previous day seemed impossible to them.’37

7.44. As against this advantage of the relative unpractised state of olfaction in civilized men, and the consequent high degree of educability of this sense, stands our ignorance of the nature of the proximal stimuli 38 and of the differential sensitivity of the receptor organs for these stimuli. We shall nevertheless outline the kind of experiments which might be attempted with respect to this sense as the one which seems on the whole to be the most promising one for our purposes.

7.45. The task of the experiment would be to attempt to attach to originally undiscriminated stimuli as many distinct connexions with other sensory stimuli and emotional states as possible. That such intersensory associations can be created has, of course, been demonstrated by the recent work on sensory conditioning.39 The problem is whether by attaching such distinct associations to initially indistinguishable stimuli new discriminations can be created.

7.46. Experiments had probably best start with stimuli which highly practised persons are known to distinguish, but which to an unpractised person are undistinguishable. The points to be ascertained would be not only whether by repeated exposure to the stimuli people can be taught to discriminate between them, but whether this process is considerably speeded up if the different stimuli are made to act under completely different accompanying circumstances. This implies of course the necessity of parallel control experiments in which the conditions under which the two stimuli act are the same.

7.47. For such experiments it would be desirable to alter the whole surroundings and the state of the organism in which the different stimuli were made to act: one of two stimuli might, e.g., be made to act regularly at a particular time of the day (say on awakening in the morning) so that it always coincided with the same phase of the rhythm of the body, in a state of restedness, warmth and inactivity, immediately preceding food and in combinations with a constant combination of colours, tones, etc.; while the other stimulus should as regularly be made to act in circumstances which were in all respects different from those just described: say in the late afternoon, out of doors, in a state of considerable activity and exhilaration, nervous excitement, cold and hunger and in combination with an altogether different set of visual and auditory perceptions.

7.48. In using sensory associations to assist the discrimination between stimuli, care would have to be taken not to run counter to well-established synesthetic relations between the qualities of the different senses. The existence of such synesthetic relations between two scales or dimensions of different modalities might, however, well be used to transfer to the other the finer distinctions which the scale of the one modality possesses. Our inadequate knowledge of the character of the stimuli at present probably makes it impossible to use the technique of differential thresholds with regard to olfaction. But as between colours and tones, for instance, persons who have the capacity of colour hearing might well be tested on whether, by deliberately making connexions even closer, the greater capacity of discrimination which they possess in one sense can be transferred to the other.

7.49. It is evident that such a technique for the education of the senses might prove to be of considerable practical importance and should thus be studied even apart from its theoretical significance. It is, of course, more than likely that in such attempts it will be found that the crude approach suggested here is inadequate and that, before much can be accomplished in this direction, much more knowledge about the nature of the sensory order, that is about the interrelations between the dimensions of the various sense modalities, will have to be acquired.

7.50. In addition to such attempts to teach new discriminations between stimuli which were already consciously perceived but not distinguished, the possibility should not be overlooked of attaching conscious values to impulses which did not possess them before. In this connexion perhaps stimuli acting inside the body might offer the most interesting field, and the new techniques of deep heating would seem to open possibilities which ought to be explored. Also the possibility of extending the range of the more familiar senses in this manner should not be disregarded. Although the upper and lower limits of the visible spectrum and the range of audible sounds may well be true physiological limits determined by the nature of the receptor organs, they may in part be centrally determined, and in this case be alterable by training. The considerable inter-individual differences between these limits rather suggest that this may be so, and even such reports as that a blind person has acquired the capacity of smelling colours 40 should not be dismissed as altogether impossible.

7.51. It is not at all improbable that man possesses a considerable number of ‘reflex senses’, as the action of the semicircular canals in controlling balance has been aptly described,41 a sensitivity of the body for certain specific stimuli to which a specific response is effected at lower levels, but which have not occurred with sufficient regularity in the company of particular other stimuli to give them a distinct conscious quality. In all such instances it might be possible to raise these impulses to a conscious level by deliberately attaching to them that characteristic following which they did not have occasion to acquire in natural surroundings.

4. POSSIBILITIES OF EXPERIMENTAL REFUTATION

7.52. It will assist further to define the content of our main thesis if we state briefly the main alternative theories whose confirmation would at the same time disprove the theory here developed.

7.53. Disregarding all those theories which, like parallelism, assume the existence of some mind-substance and which are unverifiable almost by definition, the first of the alternative theories which might be mentioned is that of a cell-memory or of the ‘storage’ of impressions in the individual cell, such as underlies R. Semon’s conception of the ‘engram’.42 This conception implies of course, the assumption that whatever it is that is thus stored possesses by itself the different attributes by which different sensory qualities are distinguished. Although it is difficult to see how this assumption could ever be experimentally verified, its confirmation would, of course, refute our theory and in fact eliminate the problem which the latter is intended to solve.

7.54. A more direct refutation of our theory would be obtained by the discovery of such differences in the physical properties transmitted by the different nerve fibres that these could be said to correspond to the differences in the sensory qualities produced by those impulses—that is, if the theory of the specific energy of nerves in what we have called its illegitimate interpretation (1.33) should prove to be correct. It was by suggesting the search for such physiological differences between the individual impulses that the theoretical views widely held in the past have posed a problem to physiological research to which, if our view is correct, no answer can be found.

7.55. A special modern form of that theory is the resonance theory developed (for efferent nervous impulses) by P. Weiss 43 which suggests that it is not the fact of a transmission of impulses through special pathways but rather the character of the impulses in some fibres which determines that similar impulses are being set up in other distant fibres. This view, if proved correct for afferent impulses, would also disprove most of the present theory. The same would be true if the views of some modern gestaltists were confirmed, who seem to suggest that it is not the topological position of the group of impulses in the whole structure of connexions but the spatial configuration 44 of these impulses, irrespective of the particular fibres in which they occur, which counts.

7.56. Finally we might mention as a conceivable alternative theory, although it seems doubtful whether it has ever been carried to its ultimate consequences, the view that sensory discrimination is determined entirely by peripheral motor events. Although we do certainly not wish to minimize the importance of motor responses at all the various levels of the hierarchy of the central nervous system, it is difficult to see how they should ever make those central ‘symbolic’ or classificatory processes unnecessary with whose functions we were mainly concerned.