Before going any further, let us invite another philosophical system to take a place within the circle of our discussion; for the same answer will suffice for it as well as for the preceding one, and it will be as well to deal with both at once. This new system, parallelism, in great favour at the present day, appears to me to be a materialism perfected especially in the direction of caution. To escape the mystery of the genesis of the mind from matter, this new system places them parallel to each other and side by side, we might almost say experimentally, so much do parallelists try to avoid talking metaphysics. But their position is untenable, and they likewise are the victims of the mirage of concepts; for they consider the mental as capable of being parallel to the physical without mingling with it, and of subsisting by itself and with a life of its own. Such a hypothesis is only possible by reason of the insufficient definition given to the mind. If it be recognised that the mind has an incomplete existence and is only realised by its incarnation in matter, the figure which is the basis of parallelism becomes indefensible. There is no longer on the one hand the physical, and on the other the mental, but on one side the physical and the mental combined, and on the other the same combination; which amounts to saying that the two faces to a reality, which it was thought had been made out to be so distinct, are identical. There are not two faces, but one face; and the monism, which certain metaphysicians struggle to arrive at by a mysterious reconciliation of the phenomenal duality within the unity of the noumenon, need not be sought so far afield, since we already discover it in the phenomenon itself.
The criticisms I have just pointed out to you, only too briefly, are to be found in several philosophers, confusedly in Berkeley, and with more precision in M. Bergson's book on Matière et Mémoire. The latter author, remarking that our brain and the outer world are to us images of the same order, refuses to admit that the brain, which is only a very small part of these images, can explain and contain the other and much larger part, which comprises the vast universe. This would amount to saying that the whole is comprised in the part. I believe that this objection is analogous to the one just stated with less ingenuity.
It is interesting to see how M. Bergson gets out of the difficulty which he himself raised. Being unwilling to bring forth from the molecular movement of the brain the representation of the world, or to superpose the representation on this movement as in the parallelist hypothesis, he has arrived at a theory, very ingenious but rather obscure, which consists in placing the image of the world outside the brain, this latter being reduced to a motor organ which executes the orders of the mind.
We thus have four philosophical theories, which, while trying to reconcile mind with matter, give to the representation a different position in regard to cerebral action. The spiritualist asserts the complete independence of the representation in relation to cerebral movement; the materialist places it after, the parallelist by the side of, the cerebral movement; M. Bergson puts it in front.
I must confess that the last of these systems, that of M. Bergson, presents many difficulties. As he does not localise the mind in the body, he is obliged to place our perception—that is to say, a part of ourselves—in the objects perceived; for example, in the stars when we are looking at them. The memory is lodged in distant planes of consciousness which are not otherwise defined. We understand with difficulty these emigrations, these crumblings into morsels of our mind. This would not matter if our author did not go so far as to maintain that the sensory nerves of the brain are not sensory nerves, and that the severance of them does not suppress sensations, but simply the motor efforts of these sensations. All the physiologist in me protests against the rashness of these interpretations.
The principal difficulties of the problem of the union between the mind and the body proceed from the two following facts, which seem incompatible. On the one hand, our thought is conditioned by a certain intra-cerebral movement of molecules and atoms; and, on the other hand, this same thought has no consciousness of this molecular movement. It does not know the path of the wave in our nerves; it does not suspect, for example, that the image of the objects is reversed in the retina, or that the excitements of the right eye for the most part go into the left hemisphere. In a word, it is no anatomist. It is a very curious thing that our consciousness enters into relation only with the extra-cerebral, the external objects, and the superficies of our bodies.
From this, this exact question suggests itself: a molecular wave must come as far as our visual cerebral centre for us to have the perception of the object before our eyes; how is it that our consciousness is unaware of this physiological event from which it depends, and is borne towards the distant object as if it sprang forth outside our nervous system?
Let us first remark, that if we do not perceive this wave, yet it must contain all we know of the external object, for it is evident that we only know of it that part of its properties which it transmits to our nerves and our nerve centres. All the known substance of the external object is, then, implied in this vibration; it is there, but it is not there by itself. The vibration is the work of two collaborators; it expresses at once the nature of the object which provokes it, and the nature of the nerve apparatus which transports it, as the furrow traced in the wax of the phonograph implies the joint action of an aërial vibration with a stylus, a cylinder, and, a clock-work apparatus.
I therefore suppose—and this is, I say it plainly, but an hypothesis—that if the nervous vibration so little resembles the external excitant which generates it, it is because the factor nervous system superadds its effect to the factor excitant. Let us imagine, now, that we have managed to separate these two effects, and we shall understand that then the nervous event so analysed might resemble only the object, or only the nervous system. Now, of these two effects, one is constant, that one which represents the action of the nervous system; there is another which varies with each new perception, and even with every moment of the same perception—that is to say, the object. It is not impossible to understand that the consciousness remains deaf to the constant and sensitive to the variable element. There is a law of consciousness which has often been described, and fresh applications of which are met with daily: this is, that the consciousness only maintains itself by change, whether this change results from the exterior by impressions received, or is produced from the interior by movements of the attention. Let us here apply this empirical law, and admit that it contains a first principle. It will then be possible for us to understand that the consciousness formed into a dialyser of the undulation may reject that constant element which expresses the contribution of the nervous system, and may lay bare the variable element which corresponds to the object: so that an intestinal movement of the cerebral substance, brought to light by this analytical consciousness, may become the perception of an object. By accepting this hypothesis, we restore to the sensory nerves and to the encephalic centres their property of being the substrata of representation, and avoid the objection made above against materialism and parallelism, that they did not explain how a cerebral movement, which is material, can engender the perception of an object which differs greatly from it and is yet as material as the movement itself. There is not here, properly speaking, either generation, transformation, or metamorphosis. The object to be perceived is contained in the nerve current. It is, as it were, rolled up in it; and it must be made to go forth from the wave to be seen. This last is the work of the consciousness.