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SENSATION] collection of pools because it is fed by separate streams. If it could be shown that in the brain as a whole there is no functional continuity a formidable psychophysical problem would no doubt arise.

As regards the quality of sensations—the primitive sensation of sight appears to consist only of the single quality we call

“light,” a quality which ranges in intensity from a dazzling brightness that becomes painful and blinding down to a zero of complete darkness; a limit which, however, is never completely attained, since the retina is always more or less internally stimulated—hence what is called the eye's own light (Eigenlicht). The first responses to light-stimulation seem to be very much on a par with our own to diffused heat or cold; some organisms seek the light and others shun it. As little as our temperature-sense yields us a perception of form does the light-sense at this level yield any. Not until the stage of visual spatial perception is reached and some discrimination of form is possible, do black and white attain the meaning they now have for us. An object can be visually perceived only when its colour or shade differs from that of the surrounding field; so far black as a “secondary quality” is on a par with colour, that is to say, when we are talking of things it may be called a quality. But there is still an important difference; in a light field many colours or shades may be distinguished, but in a dark field none. Though it is correct to speak of perceiving a black object, must we not then maintain that—so far as it is really black —the object yields us directly no sensation? Similarly, the piper is said to “feel” the holes in his whistle when actually he only touches the solid metal in which they are pierced; or the soldier is said to hear the tattoo, though he has no auditory sensation of the silence intervening between successive taps on the drum. And it has yet to be shown that there is any more justification for speaking of visual sensations without luminosity. Meanwhile we must maintain that in absolute darkness we do not see black, since we do not see at all. No doubt we are prone to identify the two concepts darkness and blackness, for what we may call their sensory content is the same, viz. the absence of visual sensation.

Whereas in nature the only diffused light we need consider is that emitted by the sun, the rays transmitted by the things about us vary in physical quality and in their effects upon protoplasm. As soon, therefore, as visual forms can be distinguished, a differentiation among light-sensations becomes obviously advantageous. The first colours to be differentiated were probably yellow and blue, or perhaps it would be truer to say “warm” colour and “cold” colour, upon which there followed a further differentiation of the warm colour into red and green. It is interesting to note that all possible sensations of colour constitute a specific continuum. We may represent it by 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 located 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.

In dealing with the quality of auditory sensations we have to distinguish between the simple sensations called tones and the sensation-complexes, either clangs or noises, which result from their combination. Simple tones also constitute a qualitative continuum, but it has only one dimension, their so-called “pitch”; this may be represented by a straight line ranging between two more or less indefinite extremes. If intensity, that is to say loudness, is taken into account, we have of course a continuum of two dimensions. The tone-continuum is also universally regarded as steadily diminishing in massiveness or extensity as the pitch rises. And, in fact, as we approach the lower

limit, the so-called deep or grave tones become less “even,” till at length distinct, more or less pervasive, tremors are felt rather than heard as distinct impulses on the ear-drum. The so-called high or acute tones again, as we approach their limit, are accompanied by tactual, often more or less painful, sensations, as if the ear were pierced by a fine needle. This connexion of auditory with tactual sensations confirms the independent evidence of biology pointing to an original differentiation of sound from touch. The special characteristics of tone-complexes, whether clangs or noises, are due to the remarkable analytic power which belongs to the sense of hearing. Two colours cannot be simultaneously presented unless they are differently localized, but several tones may form one complex whole within which they, as “partial” tones, are distinguishable, though spatially undifferentiated.

Unlike the higher senses of sight and hearing, the lower senses of touch, taste, smell, &c., do not constitute qualitative continua. Temperatures may indeed be represented as ranging in opposite directions, i.e. through heat or through cold, between a zero of no sensation and the organic sensations due to the destructive action of both extremes, heat and cold alike. But the continuity in this case is intensive rather than qualitative. Tastes fall into the four isolated qualities known as sweet, sour, bitter, saline; but smells hardly admit of classification at all. Sensations of touch and sight have in a pre-eminent degree a certain peculiar continuity which differentiations of extensity entail, and which we shall have presently to consider further under the title of local signs. The various sensations classed together as organic, hunger, thirst, physical pain, &c., are left to the physiologist to describe.

Our motor presentations contrast with the sensory by their want of striking qualitative differences. They are divided

into two groups: (a) motor presentations proper and (b) auxilio-motor of kinaesthetic presentations. The former answer to our “feelings of muscular effort” or “feelings of innervation.” The latter are those presentations due to the straining of tendons, stretching and flexing of the skin, and the like, by which the healthy man knows that his efforts to move are followed by movement, and so knows the position of his body and limbs. It is owing to the absence of these presentations that the anaesthetic patient cannot directly tell whether his efforts are effectual or not, nor in what position his limbs have been placed by movements from without. Thus under normal circumstances motor presentations are always accompanied by auxilio-motor; but in disease and in passive movements they are separated and their distinctness thus made manifest. Originally we may suppose kinaesthetic presentations to have formed one imperfectly differentiated continuum, but now, as with sensations, they have become a collection of special continua, viz. the groups of movements possible to each limb and certain combinations of these movements.

But whereas kinaesthetic presentations were commonly allowed to be purely sensory, the concomitants of centripetal excitations—hence the older name of “muscular or sixth sense” applied to them by Sir Charles Bell, Weber, Sir William Hamilton and others—concerning motor presentations proper, a very different view, first tentatively advanced by the great physiologist Johannes Müller, and adopted by Helmholtz, Wundt, and especially by Bain, long prevailed. It is, however, now generally discredited, if not completely overthrown. According to this view, “the characteristic feeling of exerted force” must be regarded, Bain maintained, “not as arising from an inward transmission. . . but as the concomitant of the outgoing current by which the muscles are stimulated to act” (op. cit. p. 79). The necessity for this assumption has certainly not been established on physiological grounds, nor apparently did Bain rely primarily on these; for at the very outset of his discussion we find him saying “that action is a more intimate and inseparable property of our constitution than any of our sensations, and enters as a component part into every one of our senses” (op. cit. p. 59). But this important psychological truth is affirmed as strenuously by some, at any rate (e.g. Professor James) of Bain's opponents as it was by Bain himself. Unhappily many, under the same psychophysical