Let us see what becomes of it, when my hypothesis is adopted. It explains how certain excitements proceeding from the objects—that is to say, forming part of the variable element—cease to be perceived when they are repeated and tend to become constant. A fortiori, it seems to me, should the same law explain how the constant element par excellence, the one which never varies from the first hour, is never perceived. There is, in the concert of the sounds of nature, an accompaniment so monotonous that it is no longer perceived; and the melody alone continues to be heard.
It is in this precisely that my hypothesis consists. We will suppose a nerve current starting from one of the organs of the senses, when it is excited by some object or other, and arriving at the centre of the brain. This current contains all the properties of the object, its colour, its form, its size, its thousand details of structure, its weight, its sonorous qualities, &c., &c., properties combined with and connected by the properties of the nerve-organ in which the current is propagated. The consciousness remains insensible to those nervous properties of the current which are so often repeated that they are annulled; it perceives, on the contrary, its variable and accidental properties which express the nature of the excitant. By this partial sensibility, the consciousness lays bare that which, in the nerve current, represents the object—that is to say, a cognition; and this operation is equivalent to a transformation of the current into a perception, image, or idea. There is not, strictly speaking, a transformation, but an analysis; only, the practical result is the same as that of a transformation, and is obtained without its being necessary to suppose the transmutation of a physical into a mental phenomenon.
Let us place ourselves now at the moment when the analysis I am supposing to be possible has just been effected. Our consciousness then assists at the unrolling of representations which correspond to the outer world. These representations are not, or do not appear to be, lodged in the brain; and it is not necessary to suppose a special operation which, taking them in the brain, should project them to the periphery of our nerves. This transport would be useless, since for the consciousness the brain does not exist: the brain, with its fibres and cells, is not felt; it therefore supplies no datum to enable us to judge whether the representation is external or internal with regard to it. In other words, the representation is only localised in relation to itself; there is no determinate position other than that of one representation in relation to another. We may therefore reject as inexact the pretended law of eccentricity of the physiologists, who suppose that sensation is first perceived as it were centrally, and then, by an added act, is localised at the peripheric extremity of the nerve. This argument would only be correct if we admitted that the brain is perceived by the consciousness of the brain. I have already said that the consciousness is not an anatomist, and that therefore this problem does not present itself.
Such as it is, this hypothesis appears to me to present the advantage of explaining the reason why our consciousness coincides, in certain circumstances, with the actions of the brain, and, in others, does not come near them. In other words, it contains an explanation of the unconscious. I can show this by quoting certain exact facts, of which the explanation has been hitherto thought to present difficulties, but which become very easy to understand on the present hypothesis. The first of these facts relates to the psychology of the motor current. This current has been a great feature in the studies which have been made on the feeling of effort and on the physical basis of the will. The motor current is that which, starting from the cerebral cells of the motor region, travels by way of the fibres of the pyramidal tract into the muscles of the body; and it is centrifugal in direction. Researches have been made as to whether we are or may be conscious of this current; or rather, the question has been put in somewhat different terms. It has been asked whether a psychological state can be the counterpart of this motor current,—if, for example, the feeling of mental effort produced in us at the moment of executing a difficult act or of taking a grave resolution, might not have this motor current for a basis.
The opinion which has prevailed is in the negative. We have recognised—a good deal on the faith of experiment, and a little also for theoretical reasons—that no sensation is awakened by the centrifugal current. As to the sensation of effort, it has been agreed to place it elsewhere. We put it among the centripetal sensations which, are produced as the movement outlines itself, and which proceed from the contracted muscles, the stretched ligaments, and the frictional movements of the articulations. Effort would therefore form part of all the psychical phenomenology, which is the duplicate of those sensory currents which are centripetal in direction.
In the long run, I can see no sort of theoretical reason for subordinating the consciousness to the direction of the nerve current, and for supposing that the consciousness is aroused when this current is centripetal, and that it cannot follow the centrifugal current. But this point matters little. My hypothesis would fairly well explain why the motor current remains unconscious; it explains the affair by taking into consideration the nature of this current and not its direction. This current is a motor one because it is born in the central cells, because it is a discharge from these cells, and is of entirely nervous origin. Since it does not correspond with the perception of an object—the ever varying object—it is always the same by nature. It does not carry with it in its monotonous course the débris of an object, as does the sensory current. Thus it can flow without consciousness.
This same kind of hypothesis supplies us with the reasons why a given sensory current may be, according to circumstances, either conscious or unconscious. The consciousness resulting from the analysis of the molecular wave is, as it were, a supplementary work which may be subsequently added to the realised wave. The propagation of the wave is the essential fact—there is always time to become conscious of it afterwards. It is thus that we happen, in moments of abstraction, to remain insensible to certain even very powerful excitements. Our nervous system registers them, nevertheless, and we can find them again, later on, within the memory. This is the effect of a belated analysis.
The converse phenomenon occurs much more frequently. We remark many actions and perceptions which occur the first time with consciousness, emotion, and effort. Then, when they are repeated, as coordination becomes stronger and easier, the reflex consciousness of the operation becomes feebler. This is the law of habit, which slowly carries us towards automatism. These observations have even been extended, and the endeavour made to apply them to the explanation of the origin of reflex actions and of instincts which have all started with consciousness. This is a rather bold attempt, for it meets with many serious difficulties in execution; but the idea seems fairly correct, and is acceptable if we may limit it. It is certain that the consciousness accompanies the effort towards the untried, and perishes as soon as it is realised. Whence comes this singular dilemma propounded to it by nature: to create something new or perish? It really seems that my hypothesis explains this. Every new act is produced by nerve currents, which contain many of those variable elements which the consciousness perceives; but, in proportion as the action of the brain repeats itself and becomes more precise and more exact, this variable element becomes attenuated, falls to its lowest pitch, and may even disappear in the fixation of habit and instinct.
My hypothesis much resembles the system of parallelism. It perfects it, as it seems to me, as much as the latter has perfected materialism. We indeed admit a kind of parallelism between the consciousness and the object of cognition; but these two series are not independent, not simply placed in juxtaposition as is possible in ordinary parallelism; they are united and fused together so as to complete each other. This new theory appears to me to represent a better form of the series of attempts which have been inspired by the common necessity of making the phenomena of consciousness accord with the determinism of physical facts.
I hold fast to this physical determinism, and accept a strictly mechanical conception of the functions of the nervous system. In my idea, the currents which pass through the cerebral mass follow each other without interruption, from the sensorial periphery to the motor periphery; it is they, and they alone, which excite the movements of the body by acting on the muscles. Parallelism recognises all these things, and I do likewise.