Rela tivity. This will be the most convenient place to take note of certain psychological doctrines which, though differing in some material respects, are usually included under the term Law of Relativity. 1. Hobbes's Idem semper sentire et non sentire ad idem recidunt is often cited as one of the first formulations of this law; and if we take it to apply to the whole field of consciousness it becomes at once true and trite: a field of consciousness unaltered either by change of impression or of ideas would certainly be a blank and a contradiction. Understood in this sense the Law of Relativity amounts to what Hamilton called the Law of Variety (Reid's Works, p. 932). But, though consciousness involves change, it is still possible that particular presentations in the field of consciousness may continue unchanged indefinitely. When it is said that "a constant impression is the same as a blank," what is meant turns out to be something not psychological at all, as, e.g., our insensibility to the motion of the earth or to the pressure of the air-cases in which there is obviously no presentation, nor even any evidence of nervous change. Or else this paradox proves to be but an awkward way of expressing what we may call accommodation, whether physiological or psychological. Thus the skin soon adapts itself to certain seasonal alterations of temperature, so that heat or cold ceases to be felt the sensation ceases because the nervous change, its proximate physical counterpart, has ceased. Again, there is what James Mill calls "an acquired incapacity of attention," such that a constant noise, for example, in which we have no interest is soon inaudible. As attention moves away from a presentation its intensity diminishes, and when the presentation is below the threshold of consciousness its intensity is then subliminal, whatever that of the physical stimulus may be. In such a case of psychological accommodation we should expect also to find on the physiological side some form of central reflexion or isolation more or less complete. As a rule, no doubt, impressions do not continue constant for more than a very short time; still there are sad instances enough in the history of disease, bodily and mental, to show that such a thing can quite well happen, and that such constant impressions (and "fixed ideas," which are in effect tantamount to them), instead of becoming blanks, may dominate the entire consciousness, colouring or bewildering everything. 2. From the fact that the field of consciousness is continually changing it has been supposed to follow, not only that a constant presentation is impossible, but as a further consequence that every presentation is essentially nothing but a transition or difference. "All feeling," says Dr Bain, the leading exponent of this view, "is two-sided. . . . We may attend more to one member of the couple than to the other.... We are more conscious of heat when passing to a higher temperature, and of cold when passing to a lower. The state we have passed to is our explicit consciousness, the state we have passed from is our implicit consciousness." But the transition need not be from heat to cold, or vice versa: it can equally well take place from a neutral state, which is indeed the normal state, of neither heat nor cold; a new-born mammal, e.g., must experience cold, having never experienced heat. Again, suppose a Sailor becalmed, gazing for a whole morning upon a stretch of sea and sky, what sensations are implicit here? Shall we say yellow as the greatest contrast to blue, or darkness as the contrary of light, or both? What, again, is the implicit consciousness when the explicit is sweet; is it Litter or sour, and from what is the transition in such a case! It is difficult to avoid suspecting a certain confusion here between the transition of attention from one presentation to another and the qualitative differences among presenta tions themselves. It is strange that the psychologist who has laid such stress on neutral states of surprise as being akin to feeling, and so distinct from special presentations, should in any way confound the two. The mistake, if mistake indeed it be, is perhaps accounted for by the fact that Dr Bain, in common with the rest of his school, nowhere distinguishes between attention and the presentations that are attended to. To be conscious or mentally alive we must have a succession of shocks or surprises, new objects calling off attention from old ones; but, over and above these movements of attention from presentation to presentation, do we find that each presentation is also itself but a transition or difference? "We do not know any one thing of itself but only the difference between it and another thing," says Dr Bain. But it is plain we cannot speak of contrast or difference between two states or things as a contrast or difference if the states or things are not themselves presented, else the so-called contrast or difference would itself be a single presentation, and its supposed "relativity" but an inference. Difference is not more necessary to the presentation of two objects than two objects to the presentation of difference. And, what is more, a difference between presentations is not at all the same thing as the presentation of that difference. The former must precede the latter; the latter, which requires active comparison, need not follow. There is an ambiguity in the words "know," 'knowledge," which Dr Bain seems not to have considered : "to know" may mean either to perceive or apprehend, or it may mean to understand or comprehend.1 Knowledge in the first sense is only what we shall have presently to discuss as the recognition or assimilation of an impression (see below, p. 53); knowledge in the latter sense is the result of intellectual comparison and is embodied in a proposition. Thus a blind man who cannot know light in the first sense can know about light in the second if he studies a treatise on optics. Now in simple perception or recognition we cannot with any exactness say that two things are perceived: straight is a thing, i.e., a definite object presented; not so not-straight, which may be qualitatively obscure or intensively feeble to any degree. Only when we rise to intellectual knowledge is it true to say, "No one could understand the meaning of a straight line without being shown a line not straight, a bent or crooked line."2 Two distinct presentations are necessary to the comparison that is here implied; but we cannot begin with such definitional differentiation: we must first recognize our objects before we can compare them. We need, then, to distinguish between the comparativity of intellectual knowledge, which we must admit-for it rests at bottom on a purely analytical proposition-and the "differential theory of presentations," which, however plausible at first sight, must be wrong somewhere since it commits us to absurdities. Thus, if we cannot have a presentation but only the presentation of the difference between Y and Z, it would seem that in like manner we cannot have the presentation of Y or Z, nor therefore of their difference X, till we have had the presentation of A and B say, which differ by Y, and of C and D, which we may suppose differ by Z. The lurking error in this doctrine, that all presentations are but differences, may perhaps emerge if we examine more closely what may be meant by difference. We may speak of (a) differences in intensity between sensations supposed 1 Other languages give more prominence to this distinction; compare yvŵvai and eidéval, noscere and scire, kennen and wissen, connaître and savoir. On this subject there are some acute remarks in a littleknown book, the Exploratio Philosophica, of Professor J. Grote. Hobbes too was well awake to this difference, as, c.g., when he says, "There are two kinds of knowledge; the one, sense or knowledge original and remembrance of the same; the other, science or know. ledge of the truth of propositions, derived from understanding." 2 Bain, Logic, vol. i. p. 3. to be qualitatively identical, or of (b) differences in quality regards the qualities of sensations the outlook of the rela- Sensation and Movement. differ ences. One of the first questions to arise concerning our simplest Qualipresentations or sensations 4 is to account for their differ- tative ences of quality. In some respects it may well seem an idle question, for at some stage or other we must acknow3. On the strength of such facts Wundt has formulated ledge final or irresolvable differences. Still, differences can a law of relativity, free, apparently, from the objections be frequently shown to be due to variety in the number, just urged against Dr Bain's doctrine, which runs thus: arrangement, and intensity of parts severally the same,"Our sensations afford no absolute but only a relative these several parts being either simultaneously presented measure of external impressions. The intensities of stimuli, or succeeding each other with varying intervals. It is a the pitch of tones, the qualities of light, we apprehend sound scientific instinct which has led writers like G. H. (empfinden) in general only according to their mutual rela- Lewes and Mr Spencer to look out for evidence of some tion, not according to any unalterably fixed unit given simple primordial presentation-the psychical counterpart, along with or before the impression itself."2 We are not they supposed, of a single nerve-shock or neural tremornow concerned with so much of this statement as relates out of which by various grouping existing sensations have to the physical antecedents of sensation; but that what is arisen. It must, however, be admitted that but little of of psychological account in it requires very substantial such evidence is at present forthcoming; and further, if qualification is evident at once from a single consideration, we look at the question for a moment from the physiological viz., that if true this law would make it quite immaterial standpoint which these writers are too apt to affect, what what the impressions themselves were: provided the rela- we find seems on the whole to make against this assumption continued the same, the sensation would be the same tion. Protoplasm in its simplest state is readily irritated too, just as the ratio of 2 to 1 is the same whether our either by light, heat, electricity, or mechanical shock. Till unit be miles or millimètres. In the case of intensities, the physiological characteristics of these various stimuli e.g., there is a minimum sensibile and a maximum sensibile. are better known, it is fruitless to speculate as to the The existence of such extremes is alone sufficient to turn nature of primitive sensation. But we have certainly no the flank of the thoroughgoing relativists; but there are warrant for supposing that any existing class of sensations instances enough of intermediate intensities that are is entitled to rank as original. Touch, as we experience directly recognized. A letter-sorter, for example, who it now, is probably quite as complex as any of our special identifies an ounce or two ounces with remarkable exact- sensations. If a supposition must be ventured at all, it is ness identifies each for itself and not the first as half the perhaps most in keeping with what we know to suppose second; of an ounce and a half or of three ounces he may that the sensations answering to the five senses in their have a comparatively vague idea. And so generally within earliest form were only slightly differing variations of the certain limits of error, indirectly ascertained, we can more or less massive organic sensation which constituted identify intensities, each for itself, neither referring to a the primitive presentation-continuum. We may suppose, common standard nor to one that varies from time to time in other words, that at the outset these sensations corre-to any intensity, that is to say, that chances to be simul- sponded more completely with what we might call the taneously presented; just as an enlisting sergeant will general physiological action of light, heat, &c., as distinct recognize a man fit for the Guards without a yard measure from the action of these stimulants on specially differenand whether the man's comrades are tall or short. Of tiated end-organs. But, short of resolving such sensations course such identification is only possible through the re- into combinations of one primordial modification of conproduction of past impressions, but then such reproduction sciousness, if we could conceive such, there are many itself is only possible because the several impressions con- interesting facts which point clearly to a complexity that cerned have all along had a certain independence of related we can seldom directly detect. Many of our supposed impressions, and a certain identity among themselves. As sensations of taste, e.g., are complicated with sensations of 1 Common language seems to recognize some connexion even here, or we should not speak of harsh tastes and harsh sounds, or of dull sounds and dull colours, and so forth. All this is, however, superadded to the sensation, probably on the ground of similarities in the accompanying organic sensations. 2 Physiologische Psychologie, 1st ed., p. 421; the doctrine reappears in the 2d ed., but no equally general statement of it is given. 3 Those who, like Helmholtz, explain the phenomena of contrast and the like as illusions of judgment, must class them as cases of comparativity; those who, like Hering, explain them physiologically, would see in them nothing but physiological adaptation. 4 For a detailed account of the various sensations and perceptions pertaining to the several senses the reader is referred to the articles EYE, EAR, TOUCH, TASTE, SMELL, &c. Quali. tative con touch and smell: thus the pungency of pepper and the dryness of wine are tactual sensations, and their spicy flavours are really smells. How largely smells mingle with what we ordinarily take to be simply tastes is best brought home to us by a severe cold in the head, as this temporarily prevents the access of exhalations to the olfactory surfaces. The difference between the smooth feel of a polished surface and the roughness of one that is unpolished, though to direct introspection an irresolvable difference of quality, is probably due to the fact that several nerve-terminations are excited in each case: where the sensation is one of smoothness all are stimulated equally; where it is one of roughness the ridges compress the nerve-ends more, and the hollows compress them less, than the level parts do. The most striking instance in point, however, is furnished by musical timbre (see EAR, vol. vii. p. 593). continuous variations in quality, as in looking at the rain- The groups of sensations known as touches, smells, tastes, We find other evidence of the complexity of our existing It is interesting to note that all possible sensations of colour, of tone, and of temperature constitute as many tinuity, groups of qualitative continua. By continuum is here meant a series of presentations changing gradually in quality, ie, so that any two differ less the more they approximate in the series. We may represent this relation among presentations spatially, so long as the differences do not exceed three. In this way our normal colour-sensations have been compared to a sphere, in which (a) the maximum of luminosity is at one pole and the minimum at the other; (b) the series of colours proper (red to violet and through purple back to red), constituting a closed line, are placed round the equator or in zones parallel to it, according to shade; and (c) the amount of saturation (or absence of white) for any given zone of illumination increases with distance from the axis. The several musical tones, again, have been compared to an ascending spiral, a given tone and its octaves lying in the same perpendicular. Temperatures similarly might be represented as ranging in opposite directions, i.e., through heat or through cold, between a zero of no sensation and the organic sensations that accompany the destructive action of heat and cold alike. As we frequently experience a continuous range of intensity of varying amount, so we may experience Perception. In treating apart of the differentiation of our sensory Mental and motor continua, as resulting merely in a number of synthesis distinguishable sensations and movements, we have been or intecompelled by the exigencies of exposition to leave out of gration, sight another process which really advances pari passu with this differentiation, viz., the integration or synthesis of these proximately elementary presentations into those complex presentations which are called perceptions, intuitions, sensori-motor reactions, and the like. It is, of course, not to be supposed that in the evolution of mind any creature attained to such variety of distinct sensations and movements as a human being possesses without making even the first step towards building up this material into the most rudimentary knowledge and action. On the contrary, there is every reason to think, as has been said already incidentally, that further differentiation was helped by previous integration, that perception prepared the way for distincter sensations, and purposive action for more various movements. This process of synthesis, which is in the truest sense a psychical process, deserves some general consideration before we proceed to the several complexes that result from it. Most complexes, certainly the most important, are consequences of that principle of subjective selection whereby interesting sensations lead through the intervention of feeling to movements; and the movements that turn out to subserve such interest come to have a share in it. In this way-which we need not stay to examine more closely now-it happens that, in the alternation of sensory and motor phases which is common few times might be perceived as familiar. Such percept | being a "presentative-representative" complex, and wholly sensory, we might symbolize it, details apart, as S+s, using S for the present sensation, and s for a former S represented. According to the latter usage, an entirely new sensation, provided it were complicated with motor experiences in the way required for its localization or projection, would become a perception. Such a perception might be roughly symbolized as X + (M+m), or as X+m simply, M standing for actual movements, as in ocular adjustment, which in some cases might be only former movements represented or m. But as a matter of fact actual perception probably invariably includes both cases: impressions which we recognize we also localize or project, and impressions which are localized or projected are never entirely new,— they are, at least, perceived as sounds or colours or aches, &c. It will, however, frequently happen that we are specially concerned with only one side of the whole process, as is the case with a tea-taster or a colour-mixer on the one hand, or, on the other, with the patient who is perplexed to decide whether what he sees and hears is " subjective," or whether it is "real." Usually we have more trouble to discriminate the quality of an impression than to fix it spatially; indeed this latter process was taken for granted by most psychologists till recently. But, however little the two sides are actually separated, it is important to mark their logical distinctness, and it would be well if we had a precise name for each. In any other science save psychology such names would be at once forthcoming; but it seems the fate of this science to be restricted in its terminology to the ill-defined and well-worn currency of common speech, with which every psychologist feels at liberty to do what is right in his own eyes, at least within the wide range which a loose connotation allows. If there were any hope of their general acceptance we might propose to call the first-mentioned process the assimilation or recognition of an impression, and might apply the term localization to its spatial fixation, without distinguishing between the body and space beyond, a matter of the less importance as projection hardly enters into primitive spatial experience. But there is still a distinction called for: perception as we now know it involves not only localization, or "spatial reference," as it is not very happily termed, but "objective reference" as well. We may perceive sound or light without any presentation of that which sounds or shines; but none the less we do not regard such sound or light as merely the object of our attention, as having only immanent existence, but as the quality or change or state of a thing, an object distinct not only from the subject attending but from all presentations whatever to which it attends. Here again the actual separation is impossible, because this factor in perception has been so intertwined throughout our mental development with the other two. Still a careful psychological analysis will show that such "reification," as we might almost call it, has depended on special circumstances, which we can Meaning Perception as a psychological term has received various, at any rate conceive absent. These special circumstances of per- though related, meanings for different writers. It is are briefly the constant conjunctions and successions of ception. sometimes used for the recognition of a sensation or move-impressions, for which psychology can give no reason, and ment as distinct from its mere presentation, and thus is said to imply the more or less definite revival of certain residua or re-presentations of past experience which resembled the present. More frequently it is used as the equivalent of what has been otherwise called the "localization and projection" of sensations, that is to say, a sensation presented either as an affection of some part of our own body regarded as extended or as a state of some foreign body beyond it. According to the former usage, strictly taken, there might be perception without any spatial presentation at all: a sensation that had been attended to a The mere process of "association"-whereby we may suppose the synthesis of presentations to be effected so that presentations originally in no way connected tend to move in consciousness together-will confront us with its own problems later on. We need for the present only to bear in mind that the conjunction or continuity upon which the association primarily depends is one determined by the movements of attention, which movements in turn depend very largely upon the pleasure or pain that presentations occasion. To some extent, however, there is no doubt that attention may pass non-voluntarily from one indifferent presentation to another, each being sufficiently intense to give what has been called a "shock of surprise," but not so intense as to awaken feeling to move for their detention or dismissal. But throughout the process of mental development, where we are concerned with what is new, the range of such indifference is probably small: indifferent presentations there will be, but that does not matter while there are others that are interesting to take the lead. the constant movements to which they prompt. Thus we sions. Assimi- impressions is wide: between the simplest mental process | feels he can clear by a jump and the other feels he cannot. lation of they may be supposed to denote and the most complex In the concrete "up" is much more than a different direcimpres there is a great difference. The penguin that watched tion from "along." The hen-harrier, which cannot soar, is unmoved the first landing of man upon its lonely rock indifferent to a quarry a hundred feet above it—to which becomes as wild and wary as more civilized fowl after the peregrine, built for soaring, would at once give chasetwo or three visits from its molester it then recognizes but is on the alert as soon as it descries prey of the same that featherless biped. His friends at home also recognize apparent magnitude, but upon the ground. Similarly, in the him though altered by years of peril and exposure. In the concrete, the body is the origin or datum to which all posilatter case some trick of his voice or manner, some "strik- tions are referred, and such positions differ not merely ing" feature, calls up and sustains a crowd of memories quantitatively but qualitatively. Moreover, our various of the traveller in the past,-events leading on to the bodily movements and their combinations constitute a netpresent scene. The two recognitions are widely different, work of co-ordinates, qualitatively distinguishable but geoand it is from states of mind more like the latter than the metrically, so to put it, both redundant and incomplete. former that psychologists have usually drawn their descrip- It is a long way from these facts of perception, which the tion of perception. At the outset, they say, we have a brutes share with us, to that scientific conception of space primary presentation or impression P, and after sundry as having three dimensions and no qualitative differences repetitions there remains a mass or a series of P residua, which we have elaborated by the aid of thought and lanP1P2P3 ; perception ensues when, sooner or later, P, guage, and which reason may see to be the logical presup"calls up" and associates itself with these re-presentations position of what in the order of mental development has or ideas. Much of our later perception, and especially chronologically preceded it. That the experience of space when we are at all interested, awakens, no doubt, both is not psychologically original seems obvious quite apart distinct memories and distinct expectations; but, since from any successful explanation of its origin from the these imply previous perceptions, it is obvious that the mere consideration of its complexity. Thus we must have earliest form of recognition, or, as we might better call it, a plurality of objects-4 out of B, B beside C, distant from assimilation, must be free from such complications, can D, and so on; and these relations of externality, juxtapohave nothing in it answering to the overt judgment, P, is sition, and size or distance imply further specialization; a P. Assimilation involves retentiveness and differentia- for with a mere plurality of objects we have not straighttion, as we have seen, and prepares the way for re-presenta- way spatial differences. Juxtaposition, e.g., is only possible tion; but in itself there is no confronting the new with when the related objects form a continuum; but, again, the old, no determination of likeness, and no subsequent not any continuity is extensive. Now how has this comclassification. The pure sensation we may regard as a plexity come about? psychological myth; and the simple image, or such sensation revived, seems equally mythical, as we may see later on. The nth sensation is not like the first: it is a change in a presentation-continuum that has itself been changed by those preceding; and it cannot with any propriety be said to reproduce these past sensations, for they never had the individuality which such reproduction implies. Nor does it associate with images like itself, since where there is association there must first have been distinctness, and what can be associated can also, for some good time at least, be dissociated. tion of sions Localiza. To treat of the localization of impressions is really to give The first condition of spatial experience seems to lie Extenin what has been noted above (p. 46) as the extensity of sity. sensation. This much we may allow is original; for the longer we reflect the more clearly we see that no combination or association of sensations varying only in intensity and quality, not even if motor presentations are added, will account for the space-element in our perceptions. A series of touches a, b, c, d may be combined with a series of movements m, m2, m, m; both series may be reversed; and finally the touches may be presented simultaneously. In this way we can attain the knowledge of the coexistence of objects that have a certain quasi-distance between them, and such experience is an important element in our perception of space; but it is not the whole of it. For, as has been already remarked by critics of the associationist psychology, we have an experience very similar to this in singing and hearing musical notes or the chromatic scale. The most elaborate attempt to get extensity out of succession and coexistence is that of Mr Herbert Spencer. He has done, perhaps, all that can be done, and only to make it the more plain that the entire procedure is a pоTEрov. We do not first experience a succession of touches or of retinal excitations by means of movements, and then, when these impressions are simultaneously presented, regard them as extensive, because they are associated with or symbolize the original series of movements; but, before and apart from movement altogether, we experience that massiveness or extensity of impressions in which movements enable us to find positions, and also to measure. But it will be objected, perhaps not without 1 We are ever in danger of exaggerating the competence of a new mistake, not only in the use they have made of the conception of assodiscovery; and the associationists seem to have fallen into this ciation in psychology in general, but in the stress they have laid upon the fact of movement when explaining our space-perceptions in particular. Indeed, both ideas have here conspired against them,association in keeping up the notion that we have only to deal with a plurality of discrete impressions, and movement in keeping to the front the idea of sequence. Mill's Examination of Hamilton (3d ed., p. 266 sq.) surely ought to convince us that, unless we are prepared to |