A few convinced materialists and parallelists, to whom I have read the above criticisms on their systems, have found no answer to them; my criticisms have appeared to them just, but nevertheless they have continued to abide by their own systems, probably because they were bound to have one. We do not destroy an erroneous idea when we do not replace it by another.

This has decided me to set forth some personal views which, provisionally, and for want of better, might be substituted for the old doctrines. Before doing this, I hasten to explain their character, and to state openly that they are only hypotheses.

I know that metaphysicians rarely make avowals of this kind. They present their systems as a well-connected whole, and they set forth its different parts, even the boldest of them, in the same dogmatic tone, and without warning that we ought to attach very unequal degrees of confidence to these various parts. This is a deplorable method, and to it is perhaps due the kind of disdain that observers and experimentalists feel for metaphysics—a disdain often without justification, for all is not false, and everything is not hypothetical, in metaphysics. There are in it demonstrations, analyses, and criticisms, especially the last, which appear to me as exact and as certain as an observation or experiment. The mistake lies in mixing up together in a statement, without distinction, the certain with the probable, and the probable with the possible.

Metaphysicians are not wholly responsible for this fault of method; and I am much inclined to think that it is the natural consequence of the abuse of speculation. It is especially by the cultivation of the sciences of observation that we foster in ourselves the precious sense of proof, because we can check it any minute by experimental verification. When we are working at a distance from the facts, this sense of proof gets thinner, and there is lost that feeling of responsibility and fear of seeing one's assertions contradicted by a decisive countervailing observation, which is felt by every observer. One acquires the unbearable pride which I note in Kant, and one abandons one's self to the spirit of construction. I am speaking from personal experience. I have several times detected within me this bad spirit of construction, I have been seeking to group several facts of observation under the same idea, and then I have discovered that I was belittling and depreciating those facts which did not fit in with the idea.

The hypothesis I now present on the relations of the mind and the brain has, for me, the advantage of bringing to light the precise conditions which a solution of this great problem must satisfy for this solution to be worthy of discussion.

These conditions are very numerous. I shall not indicate them all successively; but here are two which are particularly important.

1. The manifestations of the consciousness are conditioned by the brain. Let us suspend, by any means, the activity of the encephalic mass, by arresting the circulation of the blood for example, and the psychic function is at once inhibited. Compress the carotid, and you obtain the clouding-over of the intellect. Or, instead of a total abolition, you can have one in detail; sever a sensory nerve with the bistoury, and all the sensations which that nerve transmits to the brain are suppressed. Consciousness appears only when the molecular disturbance reaches the nerve centres; everything takes place in the same way as if this disturbance released the consciousness. Consciousness also accompanies or follows certain material states of the nerve centres, such as the waves which traverse the sensory nerves, which exercise reflex action in the cells, and which propagate themselves in the motor nerves. It is to the production, the distribution, and the integrity of this nervous influx that the consciousness is closely linked. It there finds one of the conditions of its apparition.

2. On the other hand, the consciousness remains in complete ignorance of these intra-cerebral phenomena. It does not perceive the nerve-wave which sets it in motion, it knows nothing of its peculiarities, of its trajectory, or the length of its course. In this sense it may be said that it is in no degree an anatomist; it has no idea of all the peculiarities of the nerve-wave which form part of its cerebral history from the moment when these peculiarities are out of relation with the properties of external objects.

One sometimes wonders that our consciousness is not aware that the objects we perceive with our two eyes correspond to a double undulation, namely, that of the right and that of the left, and that the image is reversed on the retina, so that it is the rods of the right which are impressed by objects on our left, and the rods of the upper part by objects below our eyes. These are, it has been very justly said, factitious problems, imaginary difficulties which do not exist. There is no need to explain, for instance, direct vision by a reversed image, because our consciousness is not aware that the image on the retina is reversed. In order to take account of this, we should require another eye to see this image. This answer appears particularly to the point. It will be found that it is absolutely correct if we reflect that this case of the unfelt inversion of the image on the retina is but one example of the anatomical ignorance of the consciousness.

It might also be declared, in the same order of ideas, that our consciousness is ignorant, that excitements of the eye cross each other at the level of the chiasma, and pass through the internal capsule, and that the majority of the visual excitements of an eye are received by the opposite hemisphere.

A rather confused notion of these facts has formed itself in the minds of several critics, and I can discern the proof of this in the language they use. It will be said, for example, that the idea exists in the consciousness or in the mind, and phrases like the following will be avoided: "I think with my brain"—the suggestion consists in introducing an idea in the brain—"The nerve cell perceives and reasons, &c." Ordinarily these forms of speech are criticised because they appear to have the defect of establishing a confusion between two irreducible elements, the physical and the mental. I think the error of language proceeds from another cause, since I do not admit this distinction between the physical and the mental. I think that the error consists in supposing vaguely that the consciousness comprehends intra-cerebral phenomena, whereas it ignores them.

Let me repeat that there is no such thing as intra-cerebral sensibility. The consciousness is absolutely insensitive with regard to the dispositions of the cerebral substance and its mode of work. It is not the nervous undulation which our consciousness perceives, but the exciting cause of this wave—that is, the external object. The consciousness does not feel that which is quite close to it, but is informed of that which passes much further off. Nothing that is produced inside the cranium interests it; it is solely occupied with objects of which the situation is extra-cranial. It does not penetrate into the brain, we might say, but spreads itself like a sheet over the periphery of the body, and thence springs into the midst of the external objects.

There is, therefore, I do not say a contradiction, but a very striking contrast between these two facts. The consciousness is conditioned, kept up, and nourished by the working of the cerebral substance, but knows nothing of what passes in the interior of that substance. This consciousness might itself be compared to a parasitical organism which plunges its tap roots into the nerve centres, and of which the organs of perception, borne on long stalks, emerge from the cranium and perceive everything outside that cranium. But this is, of course, only a rough image.

Strictly, it is possible to explain this distribution of the conscience, singular as it is at first sight, by those reasons of practical utility which are so powerful in the history of evolution.

A living being has to know the world external to himself in order to adapt and preadapt himself to it, for it is in this outer world that he finds food, shelter, beings of his own species, and the means of work, and it is on this world of objects that he acts in every possible way by the contractions of his muscles. But with regard to intracephalic actions, they are outside the ordinary sphere of our actions. There is no daily need to know them, and we can understand that the consciousness has not found very pressing utilitarian motives for development in that direction. One must be an histologist or a surgeon to find an appreciable interest in studying the structure of the nerve cell or the topography of the cerebral centres.

We can therefore explain well enough, by the general laws of adaptation, the reason of the absence of what might be called "cerebral sensibility," but, here as elsewhere, the question of the "Why" is much easier to solve than that of the "How."

The question of the "How" consists in explaining that the consciousness, directly aroused by a nerve-wave, does not perceive this undulation, but in its stead the external object. Let us first note that between the external object and the nervous influx there is the relation of cause to effect. It is only the effect which reaches us, our nerve cells, and our consciousness. What must be explained is how a cognition (if such a word may be employed here) of the effect can excite the consciousness of the cause. It is clear that the effect does not resemble the cause, as quality: the orange I am looking at has no resemblance with the brain wave which at this moment is traversing my optic nerve; but this effect contains everything which was in the cause, or, more exactly, all that part of the cause of which we have perception. Since it is only by the intermediary of our nervous system that we perceive the object, all the properties capable of being perceived are communicated to our nervous system and inscribed in the nerve wave. The effect produced therefore is the measure of our perception of the cause. This is absolutely certain. All bodies possess an infinity of properties which escape our cognitions; because, as excitants of our organism, these properties are wanting in the intensity or the quality necessary to make it vibrate; they have not been tuned in unison with our nervous chords. And, inversely, all we perceive of the mechanical, physical, and chemical properties of a body is contained in the vibration this body succeeds in propagating through our cerebral atmosphere. There is in this a phenomenon of transmission analogous to that which is produced when an air of music is sent along a wire; the whole concert heard at the other extremity of the wire has travelled in the form of delicate vibrations.

There must therefore exist, though unperceived by our senses, a sort of kinship between the qualities of the external objects and the vibrations of our nerves. This is sometimes forgotten. The theory of the specific energy of the nerves causes it to be overlooked. As we see that the quality of the sensation depends on the nerve that is excited, one is inclined to minimise the importance of the excitant. It is relegated to the position of a proximate cause with regard to the vibration of the nerve, as the striking of a key on the piano is the proximate cause of the vibration of a string, which always gives the same degree of sound whether struck by the forefinger or third finger, or by a pencil or any other body. It will be seen at once that this comparison is inexact. The specific property of our nerves does not prevent our knowing the form of the excitant, and our nerves are only comparable to piano strings if we grant to these the property of vibrating differently according to the nature of the bodies which strike them.

How is it that the nerve wave, if it be the depository of the whole of the physical properties perceived in the object, resembles it so little? It is because—this is my hypothesis—these properties, if they are in the undulation, are not there alone. The undulation is the work of two collaborators: it expresses both the nature of the object which provokes it and that of the nervous apparatus which is its vehicle. It is like the furrow traced in the wax of the phonograph which expresses the collaboration of an aËrial vibration with a stylus, a cylinder, and a clock-work movement. This engraved line resembles, in short, neither the phonographic apparatus nor the aËrial vibration, although it results from the combination of the two.

Similarly, I suppose that if the nervous vibration resembles so little the excitant which gives it birth, it is because the factor nervous system adds its effect to the factor external object. Each of these factors represents a different property: the external object represents a cognition and the nervous system an excitement.

Let us imagine that we succeed in separating these two effects. It will be conceived, theoretically, that a separation of this kind will lay bare the hidden resemblances, giving to each collaborator the part which belongs to it. The excitement, for instance, will be suppressed, and the cognition will be retained. Is it possible to make, or at least to imagine, such an analysis? Perhaps: for, of these two competing activities, one is variable, since it depends on the constantly changing nature of the objects which come into relation with us; the other, on the contrary, is a constant, since it expresses the contribution of our nerve substance, and, though this last is of very unstable composition, it necessarily varies much less than the series of excitants. We consequently see faintly that these two elements differ sufficiently in character for us to be able to suppose that they are separable by analysis.

But how could this analysis be made? Evidently not by chemical or physical means: we have no need here of reagents, prisms, centrifugal apparatus, permeable membranes, or anything of that kind. It will suffice to suppose that it is the consciousness itself that is the dialyser. It acts by virtue of its own laws—that is to say, by changes in intensity. Supposing that sensibility increases for the variable elements of the undulation, and becomes insensible for the constant elements. The effect will be the same as a material dissociation by chemical analysis: there will be an elimination of certain elements and the retention of others.

Now, all we know of the consciousness authorises us to entrust this rÔle to it, for it is within the range of its habits. We know that change is the law of consciousness, that it is effaced when the excitements are uniform, and is renewed by their differences or their novelty. A continued or too often repeated excitement ceases in time to be perceived. It is to condense these facts into a formula that Bain speaks of the law of relativity of cognition, and, in spite of a few ambiguities on the part of Spencer and of Bain himself in the definition of this law,[50] the formula with the sense I have just indicated is worth preserving.

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.

Let us now see the advantages of this new system. First, it contains no paralogism, no logical or psychological error, since it does not advance the supposition that the mental differs by its nature from the physical phenomenon. We have discussed above the consequences of this error, They are here avoided. In the second place, it is explanatory, at least in a certain measure, since the formula we employ allows us to understand, better than by the principle of a simple juxtaposition, why certain nerve currents flow in the light of consciousness, while others are plunged into the darkness of unconsciousness. This law of consciousness, which Bain called the law of relativity, becomes, when embodied with my theory of the relations of the physical to the moral, an explanation of the distribution of consciousness through the actions of the brain.

I ask myself whether the explanation I have devised ought to be literally preserved. Perhaps not. I have endeavoured less to present a ready-made solution than to indicate the direction in which we ought to look for one. The law of consciousness which I have used to explain the transformation of a nerve current into perception and images is only an empirical law produced by the generalisation of particular observations. Until now there has been, so far as I know, no attempt to ascertain whether this law of consciousness, notwithstanding the general nature which some authors incline to ascribe to it, might not explain itself by some more general facts, and might not fit, as a particular case, into a more comprehensive frame. To be brief, this is very possible. I have not troubled myself about it, and I have made a transcendental use of this empirical law; for I have impliedly supposed it to be a first principle, capable of accounting for the development of the consciousness, but itself incapable of explanation.

If other observers discover that that which to me has appeared inexplicable, may be explained by quite peculiar causes, it is clear that my theory must be abandoned or modified. New theories must then be sought for, which will probably consist in recognising different properties in the consciousness. A little thought will discover several, I have no doubt. By way of suggestion, I will indicate one of these hypothetical possibilities: "The consciousness has the faculty of reading in the effect that which existed in the cause." It is not rash to believe that by working out this idea, a certain solution would be discovered. Moreover, the essential is, I repeat, less to find a solution than to take account of the point which requires one; and metaphysics seem to me especially useful when it shows us where the gap in our knowledge exists and what are the conditions required to fill this gap.

Above all, I adhere to this idea, which has been one of the guiding forces of this book: there exists at the bottom of all the phenomena of the intelligence, a duality. To form a true phenomenon, there must be at once a consciousness and an object. According to passing tendencies, either of temperament or of fashion, preponderance has been given sometimes to one of the terms of this couple, sometimes to the other. The idealist declares: "Thought creates the world." The materialist answers: "The matter of the brain creates thought." Between these two extreme opinions, the one as unjustifiable as the other in the excesses they commit, we take up an intermediate position. Looking at the balance, we see no argument capable of being placed in the scale of the consciousness which may not be neutralised by an argument placed in the scale of the object; and if we had to give our final verdict we should say: "The consciousness and matter have equal rights," thus leaving to every one the power to place, in this conception of an equality of rights, the hopes of survival of which his heart has need.