(D) Sensation and Movement
Qualitative Differences in Sensations
One of the first questions to arise concerning our simplest presentations or sensations [Footnote 50-4] is to account for their differences of quality. In some respects it may well seem an idle question, for at some stage or other we must acknowledge 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 presentationthe psychical counterpart, they supposed, of a single nerve-shock or neural tremorout 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 assumption. 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 of less massive organic sensation which constituted the primitive presentation-continuum. We may suppose, in other words, that at the outset these sensations corresponded 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 differentiated end-organs. But, short of resolving such sensations into combinations of one primodial modification of consciousness, 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 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 severe cold in the head, as this temporarily prevents the access of exhalations tot he olfactory surfaces. The difference between the smooth 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 instances in point, however, is furnished by musical timbre (see EAR, vol. vii. p. 593).
We find other evidence of the complexity of our existing sensations in the variations in quality that accompany variations in intensity, extensity, and duration. What the exception of special red all colours give place, sooner or later, to a mere colourless grey as the intensity of the light diminishes, all in like manner become indistinguishably white after a certain increase of increase of intensity. A longer time is also inmost cases necessary to produce a sensation of colour than to produce a sensation merely of light or brightness : the solar spectrum seen for a moment appears not of seven colours but of two onlyfaintly red towards the left side and blue towards the right. Very small objects, again, such as coloured specks on a white ground, though still distinctly seen, appear as colourless if of less than a certain sized, the relation between their intensity and extensity being such that within certain limits the brighter they are the smaller they may be without losing colour, and the larger they are fainter in like manner. Similar facts are observable in the case of other senses, so that generally we seem justified in regarding what we now distinguish as a sensation as probably complicated in several respects. In other words, if physical magnification were possible, we might be directly aware that sensations which we now suppose to be both single and simple were both compound and complexthat they consisted, that is, of two or more sensational elements or changes, alike or different in quality, of uniform or variable intensity, and occurring either simultaneously or in regular or irregular succession.
50-4 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.
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