Chapter 7
Development of the proof drawn from the opposition existing
between extrasubjective phenomena accompanying sensation,
and sensation itself
450. We come now to the second of the proofs I have indicated. It could receive great support from the work of students of anatomy and physiology if we were prepared to compare in an orderly fashion the extrasubjective phenomena (of matter) with the corresponding phenomena of feeling, and note their contrasts. Let me give a little example of such comparison.
1. The nerves, to whose movements the sensation responds, are composed of very fine filaments, called nerve-fibres, which now and again communicate with one another in the form of a plexus. It is also asserted that every nerve-fibre has a delicate, transparent covering called a neurolemma. The extrasubjective phenomenon which immediately precedes or accompanies a sensation is not, therefore, the movement of a single fibre, but of a bundle of innumerable fibres. If the sensation were a mechanical or material effect of movement, it would either be or represent a multitude of different movements.
But this is not the case; the sensation is single, and a simple principle is needed in which and in virtue of which the sensation originates. It cannot arise in multiple little fibres disturbed contemporaneously by many distinct movements. Consequently, the statement, 'The impressions of external things received at the nerve-extremities are carried to the brain' is altogether inexact, although repeated endlessly by many authors. What are these impressions? The idols of Epicurus? No one would dream of such a thing today. They can only be movements which are not carried to the brain, but communicate with it. In other words, they extend along the nerve-fibre as far as the brain. Once and for all, therefore, we have to substitute the quoted statement by another: 'The whole nerve-fibre, or nerve-substance of the fibre, moves but unless the motion continues to the brain there is no sensation.'
There is no doubt that the impression itself cannot be carried to the brain because it is not something capable of being carried. It remains where it began, in the extremities; it is only the beginning, the impetus of motion that has been received. Granted this, there is nothing in the extrasubjective phenomena parallel to the sensation except longitudinal motion (whether this comes about mechanically or dynamically through solid filaments or through liquids is not important here) to the brain. But the sensation, that is, the corresponding subjective phenomenon, has no length, nor is it felt in the brain. It is felt in the extremity to which external force was applied. The extrasubjective phenomenon, therefore, provides extension, the subjective none. The former requires different movements in different parts where no subjective phenomenon is manifested. This subjective phenomenon, therefore, is not the extrasubjective phenomenon nor a merely material or an immediate effect of what is extrasubjective. If it were, it would have to manifest the image and nature of the extrasubjective. But motion, without the presence of a principle of a totally different nature, cannot produce anything except motion, nor can extension offer anything except extension.
451. According to physiologists, extrasubjective phenomena are even more complicated. The sensible nerves are bound together and in certain ways depend upon one another. If these mutual dependencies are removed, the phenomenon of sensation is no longer manifest. Magendie discovered through repeated experiments that the sensitivity of the head, and particularly of the face and its cavities, depends on the fifth pair of nerves. If this nerve is cut before it leaves the cranium, the face feels nothing.(197) Moreover, he also believed he had demonstrated that the principal seat of the sensorium and of the special sensories is not properly speaking in the brain or the cerebellum. He offers the following experiment.
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Remove the lobes of the brain and of the cerebellum of a mammal, then see if it feels. You will easily note that it is still sensitive to strong smells, to tastes, to sounds and to taste impressions. |
He concludes:
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It is obvious, therefore, that the seat of these sensations is not in the lobes of either the brain or the cerebellum. |
An even more extended and complicated mechanism is shown in the extrasubjective phenomena that precede or accompany the subjective phenomenon of sight. Magendie reports:
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The experiments carried out by Rolando and Flourens show that sight is terminated if the cerebral lobes are removed. If the right lobe is removed, the left eye no longer sees, and vice-versa [App., no. 6]. — Damage to the optic thalamus of mammals is also followed by loss of sight in the opposite eye. Although I have never seen damage of the anterior optic or quadrigeminal tubercle alter the sight of mammals, this effect is very clear in birds. — Thus, the parts of the nervous system necessary for the exercise of sight are manifold. Exercise of this sense requires integrity of the hemispheres, of the optic thalami, perhaps of the anterior, quadrigeminal tubercles and finally of the fifth pair. Note that the influence of the hemispheres and of the optic thalami cross over, while that of the fifth pair is direct.(198) |
It is clear that, if various organs concur simultaneously to bring about a single sensation, such as sight, these organs must have a single, simple principle in which the sensation has its existence. It is also clear that this simple principle cannot be any single organ because a single organ does not produce the sensation. Nor can it be all the organs together because the sensation is single, not multiple. A single, subjective phenomenon corresponds to many extrasubjective phenomena which inhere as modifications to different organs. The subjective phenomenon must, therefore, have a single, simple principle which receives a single, simple modification parallel to the multiple, distinct and extended movements.
452. Finally, there are many sensitive organs to which special classes of sensations correspond. If one or other of these organs is destroyed, one or other class of sensations is also destroyed, but not all; the organs serving to arouse sensation show a certain mutual independence. But the principle which feels is always the same; all sensations of different classes arise equally in it. Consequently, it cannot be a special organ, nor a modification of an organ. It must be such that it responds equally to all the organs. This is the subjective principle, to which unicity and simplicity pertain. As such, it is essentially different from the extrasubjective principle, to which the contrary properties of multiplicity and extension pertain.
Notes
(197) Précis Elémentaire de Physiologie, De la sensibilité.
(198) Ibid.