50 PSYCHOLOGY to be qualitatively identical, or of (6) differences in quality in the same continuum or class of presentations, or of (c) differences between sensations of different classes or con- tinua. Now, as regards (a) and (6), it will be found that the difference between two intensities of the same quality, or between two qualities of the same continuum, may be itself a distinct presentation. But nothing of this kind holds of (c). 1 In passing from a load of 10 ft to one of 20 Ib, or from the sound of a note to that of its octave, it is possible to make the change continuously, and to esti- mate it as one might the distance between two places on the same road. But in passing from the scent of a rose to the sound of a gong or a sting from a bee we have no such means of bringing the two into relation scarcely more than we might have of measuring the length of a journey made partly on the common earth and partly through the looking-glass. In (c), then, we have only a change, a difference of presentation, but not a presentation of difference ; and we only have more than this in (a) or (6) provided the selected presentations occur together. If red follows green we may be aware of a greater differ- ence than we could if red followed orange ; and we should ordinarily call a 10-Bb load heavy after one of 5 R> and light after one of 20 Bb. Facts like these it is which make the differential theory of presentations plausible. 3. On the strength of such facts Wundt has formulated a law of relativity, free, apparently, from the objections just urged against Dr Bain's doctrine, which runs thus : " Our sensations afford no absolute but only a relative measure of external impressions. The intensities of stimuli, the pitch of tones, the qualities of light, we apprehend (empfinden) in general only according to their mutual rela- tion, not according to any unalterably fixed unit given along with or before the impression itself." 2 We are not now concerned with so much of this statement as relates to the physical antecedents of sensation ; but that what is of psychological account in it requires very substantial qualification is evident at once from a single consideration, viz., that if true this law would make it quite immaterial what the impressions themselves were : provided the rela- tion continued the same, the sensation would be the same too, just as the ratio of 2 to 1 is the same whether our unit be miles or millimetres. In the case of intensities, e.g., there is a minimum sensibile and a maximum sensibile. The existence of such extremes is alone sufficient to turn the flank of the thoroughgoing relativists ; but there are instances enough of intermediate intensities that are directly recognized. A letter -sorter, for example, who identifies an ounce or two ounces with remarkable exact- ness identifies each for itself and not the first as half the second ; of an ounce and a half or of three ounces he may have a comparatively vague idea. And so generally within certain limits of error, indirectly ascertained, we can identify intensities, each for itself, neither referring to a common standard nor to one that varies from time to time to any intensity, that is to say, that chances to be simul- taneously presented; just as an enlisting sergeant will recognize a man fit for the Guards without a yard measure and whether the man's comrades are tall or short. Of course such identification is only possible through the re- production of past impressions, but then such reproduction itself is only possible because the several impressions con- cerned have all along had a certain independence of related impressions, and a certain identity among themselves. As 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, super- added to the sensation, probably on the ground of similarities in the accompanying organic sensations. 3 Physiologische Psychologic, 1st ed., p. 421 ; the doctrine re- appears in the 2d ed., but no equally general statement of it is given. regards the qualities of sensations the outlook of the rela- tivists is, if anything, worse. In what is called Meyer's experiment, e.g. (described under EYE, vol. viii. p. 825), what appears greenish on a red ground will appear of an orange tint on a ground of blue ; but this contrast is only possible within certain very narrow limits. In fact, the phenomena of colour -contrast, so far from proving, dis- tinctly disprove that we apprehend the qualities of light only according to their mutual relation. In the case of tones it is very questionable whether such contrasts exist at all. Summing up on the particular doctrine of relativity of which Wundt is the most distinguished adherent, the truth seems to be that, in some cases where two presenta- tions whose difference is itself presentable occur in close connexion, this difference as we indirectly learn exerts a certain bias on the assimilation or identification of one or both of the presentations. There is no " unalterably fixed unit " certainly, but, on the other hand, " the mutual relations of impressions" are not everything. 3 Sensation and Movement. One of the first questions to arise concerning our simplest Qu; presentations or sensations 4 is to account for their differ- t^ti ences of quality. In some respects it may well seem an idle question, for at some stage or other we must acknow- ledge final or irresolvable differences. Still, differences can be frequently shown to be due to variety in the number, arrangement, and intensity of parts severally the same, these several parts being either simultaneously presented or succeeding each other with varying intervals. It is a sound scientific instinct which has led writers like G. H. Lewes and Mr Spencer to look out for evidence of some simple primordial presentation the psychical counterpart, they supposed, of a single nerve-shock or neural tremor out of which by various grouping existing sensations have arisen. It must, however, be admitted that but little of such evidence is at present forthcoming ; and further, if we look at the question for a moment from the physiological standpoint which these writers are too apt to affect, what we find seems on the whole to make against this assump- tion. Protoplasm in its simplest state is readily irritated either by light, heat, electricity, or mechanical shock. Till the physiological characteristics of these various stimuli are better known, it is fruitless to speculate as to the nature of primitive sensation. But we have certainly no warrant for supposing that any existing class of sensations is entitled to rank as original. Touch, as we experience it now, is probably quite as complex as any of our special sensations. If a supposition must be ventured at all, it is perhaps most in keeping with what we know to suppose that the sensations answering to the five senses in their earliest form were only slightly differing variations of the more or less massive organic sensation which constituted the primitive presentation-continuum. We may suppose, in other words, that at the outset these sensations corre- sponded more completely with what we might call the general physiological action of light, heat, &c., as distinct from the action of these stimulants on specially differen- tiated end-organs. But, short of resolving such sensations into combinations of one primordial modification of con- sciousness, if we could conceive such, there are many interesting facts which point clearly to a complexity that we can seldom directly detect. Many of our supposed sensations of taste, e.g., are complicated with sensations of 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.
