The principles laid down in the preceding chapter are equally applicable to the central system. But here greater difficulties await us. We cannot expect traditional views to be easily displaced, when they have taken such hold on the mind, as is the case with regard to Sensibility. To admit that all nerves have a common property, and that their functional relations depend on the organs which they innervate, demands small relinquishment of cherished opinions. But to admit that all nerve-centres have a common property, and that their functional relations depend on their anatomical connections, is to sweep away at once a mass of theoretic interpretations which from long familiarity have acquired an almost axiomatic force. That the brain, and the brain only, is the source and seat of Sensibility is the postulate of modern Physiology. 69. The question is one of extreme complexity, but may be greatly simplified, if we can manage to reduce it to purely physiological terms, and consider the phenomena in their objective aspect. In dealing with nerves and their actions this was comparatively easy; we had for the most part only physiological processes to unravel. It is otherwise in dealing with nerve-centres—the subjective or psychological aspect of the phenomena inevitably thrusts itself on our attention; and all the mysteries of Feeling and Thought cloud our vision of the212 neural process. Do what we will, we cannot altogether divest Sensibility of its psychological connotations, cannot help interpreting it in terms of Consciousness; so that even when treating of sensitive phenomena observed in molluscs and insects, we always imagine these more or less suffused with Feeling, as this is known in our own conscious states.
70. Feeling is recognized as in some way or other bound up with neural processes; but Physiology proper has only to concern itself with the processes; and the question whether these can, and do, go on unaccompanied by Feeling, is, strictly speaking, one which belongs to Psychology. It demands as a preliminary that the term Feeling be defined; and the answer will depend upon that definition, namely, whether Feeling be interpreted as synonymous with Consciousness in the restricted sense, or synonymous with the more general term Sentience. If the former, then since there are unquestionably neural processes of which we are not conscious, we must specify the particular groups which subserve Feeling; as we specify the particular groups which subserve the sensations of Sight, Hearing, Taste, etc.; and localize the separate functions in separate organs. If the latter, then, since all neural processes have a common character, we have only to localize the particular variations of its manifestation, and distinguish sensitive phenomena as we distinguish motor phenomena.
71. It is absolutely certain that the Feeling we attribute to a mollusc is different from that which we attribute to a man; if only because the organisms of the two are so widely different, and have been under such different conditions of excitation. If every feeling is the functional result of special organic activities, varying with the co-operant elements, we can have no more warrant for assuming the existence of the same particular forms213 of Feeling in organisms that are unlike, than for assuming the 47th proposition of Euclid to be presented by any three straight lines. The lines are the necessary basis for the construction, but they are not the triangle, except when in a special configuration. This is not denying that animals feel (in the general sense of that term), it is only asserting that their feelings must be very unlike our own. Even in our own race we see marked differences—some modes of feeling being absolutely denied to individuals only slightly differing from their fellows. If, however, we admit that different animals must have different modes of Feeling, we must also admit that the neuro-muscular activities are generically alike in all, because of the fundamental similarity in the structures. Whether we shall assign Feeling to the mollusc or not will depend on the meaning of the term; but, at all events, we require some term general enough to include the phenomena manifested by the mollusc, and those manifested by all other animals. Sensibility is the least objectionable term. Unless we adopt some such general designation, physiological and psychological interpretations become contradictory and obscure. The current doctrine which assigns Sensibility to the brain, denying it to all other centres, is seriously defective, inasmuch as it implies that tissues similar in kind have utterly diverse properties; in other words, that the same nerve-tissue which manifests Sensibility in the brain has no such property in the spinal cord.
72. How is this tenable? No one acquainted at first hand with the facts denies that the objective phenomena exhibited by the brainless animal have the same general character as those of the animal possessing a brain: the actions of the two are identical in all cases which admit of comparison. That is to say, the objective appearances are the same; differing only in so far as the mechanisms214 are made different by the presence or absence of certain parts. The brain not being a necessary part of the mechanical adjustments in swimming, or pushing aside an irritating object, the brainless frog swims and defends itself in the same way as the normal frog. But no sooner do we pass from the objective interpretation, and introduce the subjective element of Feeling among the series of factors necessary to the product—no sooner do we ask whether the brainless frog feels the irritation against which it struggles, or wills the movements by which it swims—than the question has shifted its ground, and has passed from Physiology to Psychology. The appeal is no longer made to Observation, but to Interpretation. Observation tells us here nothing directly of Feeling. What it does tell us, however, is the identity of the objective phenomena; and Physiology demands that a common term be employed to designate the character common to the varied phenomena. Sensibility is such a term. But most modern physiologists, under the bias of tradition, refuse to extend Sensibility to the spinal cord, in spite of the evidences of the spinal cord possessing that property in common with the brain. They prefer to invoke a new property; they assign spinal action to a Reflex Mechanism which has nothing of the character of Sensibility, because they have identified Sensibility with Consciousness, and have restricted Consciousness to a special group of sensitive phenomena.
73. Nor is it to be denied that on this ground they have a firm basis. Every one could testify to the fact that many processes normally go on without being accompanied by consciousness, in the special meaning of the term. Reflex actions,—such as winking, breathing, swallowing,—notoriously produced by stimulation of sensitive surfaces, take place without our “feeling” them, or being “conscious” of them. Hence it is concluded that215 the Reflex mechanism suffices without the intervention of Sensibility. I altogether dispute the conclusion; and in a future Problem will endeavor to show that Sensibility is necessary to Reflex Action. But without awaiting that exposition we may at once confront the evidence, by adducing the familiar fact that “unconscious” processes go on in the brain as well as in the spinal cord; and this not simply in the sphere of Volition, but also in the sphere of Thought.110 We act and think “automatically” at times, and are quite “unconscious” of what we are doing, or of the data we are logically grouping. We often think as unconsciously as we breathe; although from time to time we become conscious of both processes. Yet who will assert that these unconscious processes were independent of Sensibility? Who will maintain that because cerebral processes are sometimes unaccompanied by that peculiar state named Consciousness, therefore all its processes are unaccompanied by Feeling? And if here we admit that the Reflex mechanism in the brain is a sensitive mechanism, surely we must equally admit that the similar Reflex mechanism in the spinal cord is sensitive?
74. Let it be understood that Sensibility is the common property of nerve-centres, and physiological interpretations will become clear and consistent. Consciousness, as understood by psychologists, is not a property of tissue, it is a function of the organism, dependent indeed on Sensibility, but not convertible with it. There is a greater distinction between the two than between Sensation, the reaction of a sensory organ, and Perception, the216 combined result of sensory and cerebral reactions; or than that between Contractility, the property of the muscles, and Flying, the function of a particular group of muscles. It is not possible to have Consciousness without Sensibility; but perfectly possible to have Sensations without Consciousness. This will perhaps seem as inconceivable to the reader as it seemed to Schr?der van der Kolk.111
75. Let us illustrate it by the analogy of Pain. There is a vast amount of sensation normally excited which is totally unaccompanied by the feelings classed as painful. The action of the special senses may be exaggerated to an intolerable degree, but the exaggeration never passes into pain: the retina may be blinded with excess of light, and the ear stunned with sound—the optic nerve may be pricked or cut—but no pain results. The systemic sensations also are habitually painless, though they pass into pain in abnormal states. Clearly, then, Pain is not the necessary consequence of Sensibility; and this is true not only of certain sensitive parts, but of all; as is proved in the well-known facts of Analgesia, in which complete insensibility of the skin as regards Pain co-exists with vivid sensibility as regards Touch and Temperature. Hence the majority of physiologists refuse to acknowledge that the struggles and cries of an animal, after removal of the brain, are evidences of pain; maintaining that they are “simply reflex actions.” This is probable; the more so as we know the struggles and cries which tickling will produce, yet no pain accompanies tickling. But if the struggles and cries are not evidence of pain, they are surely evidence of Sensibility.
76. Now for the term Pain in the foregoing paragraph217 substitute the term Consciousness, and you will perhaps allow that while it may be justifiable to interpret the actions of a brainless animal as due to a mechanism which is unaccompanied by the specially conditioned forms of Sensibility classed under Consciousness—just as it is unaccompanied by the specially conditioned forms of Perception and Emotion—there is no justification for assuming the mechanism not to have been a sensitive mechanism. The wingless bird cannot manifest any Of the phenomena of flight; but we do not therefore deny that its other movements depend on Contractility.
77. Difficult as it must be to keep the physiological question apart from the psychological when treating of Sensibility, we shall never succeed in our analysis unless the two questions are separately treated. The physiologist considers organisms and their actions from their objective side, and tries to detect the mechanism of the observed phenomena. These he has to interpret in terms of Matter and Motion. The psychologist interprets them in terms of Feeling. The actions which we see in others we cannot feel, except as visual sensations; the changes which we feel in ourselves we cannot see in others, except as bodily movements. The reaction of a sensory organ is by the physiologist called a sensation,—borrowing the term from the psychologist; he explains it as due to the stimulus which changes the molecular condition of the organ; and this changed condition, besides being seen to be followed by a muscular movement, is inferred to be accompanied by a change of Feeling. The psychologist has direct knowledge only of the change of Feeling which follows on some other change; he infers that it is originated by the action of some external cause, and infers that a neural process precedes, or accompanies, the feeling. Obviously there are two distinct questions here, involving distinct methods. The physiologist is compelled to complete218 his objective observations by subjective suggestions; compelled to add Feeling to the terms of Matter and Motion, in spite of the radical diversity of their aspects. The psychologist also is compelled to complete his subjective observations by objective interpretations, linking the internal changes to the external changes. A complete theory must harmonize the two procedures.
78. In a subsequent Problem we shall have to examine the nature of Sensation in its psychological aspect; here we have first to describe its physiological aspect. To the psychologist, a sensation is simply a fact of Consciousness; he has nothing whatever to do with the neural process, which the physiologist considers to be the physical basis of this fact; and he therefore regards the physiologists as talking nonsense when they talk of “unconscious sensations,” the phrase being to him equivalent to “unfelt feelings,” or “invisible light.” It is quite otherwise with the physiologist, who viewing a sensation solely as a neural process, the reaction of a sensory organ, can lawfully speak of unconscious sensations, as the physicist can speak of invisible rays of light,—meaning those rays which are of a different order of undulation from the visible rays, and which may become visible when the susceptibility of the retina is exa............