I ask permission to reproduce here a communication made by me in December 1904 to the SociÉtÉ FranÇaise de Philosophie. I there set forth briefly the ideas which I have just developed in this book. This succinct exposÉ may be useful as a recapitulation of the argument.

Description of Matter.—The physicists who are seeking for a conception of the Inmost structure of matter in order to explain the very numerous phenomena they perceive, fancy they can connect them with other phenomena, less numerous, but of the same order. They thus consider matter in itself.

We psychologists add to matter something more, viz. the observer. We consider matter and define it by its relations to our modes of knowledge—that is to say, by bearing in mind that it is conditioned by our external perception. These are two different points of view.

In developing our own standpoint, we note that of the outer world we are acquainted with nothing but our sensations: if we propound this limit, it is because many observations and experiments show that, between the external object and ourselves, there is but one intermediary, the nervous system, and that we only perceive the modifications which the external object, acting as an excitant, provokes in this system.

Let us provisionally apply to these modifications the term sensations, without settling the question of their physical or mental nature.

Other experiments, again, prove to us that our sensations are not necessarily similar to the objects which excite them; for the quality of each sensation depends on what is called the specific energy of the nerve excited. Thus, whether the optic nerve be appealed to by a ray of light, an electric current, or a mechanical shock, it always gives the same answer, and this answer is the sensation of light.

It follows that our nervous system itself is only known to us as regards its structure by the intermediary of sensations, and we are not otherwise more informed upon its nature than upon that of any other object whatever.

In the second place, a much more serious consequence is that all our sensations being equally false, so far as they are copies of the excitants which provoke them, one has no right to use any of these sensations to represent to ourselves the inmost structure of matter. The theories to which many physicists still cling, which consist in explaining all the modalities of matter by different combinations of movement, start from false premises. Their error consists in explaining the whole body of our sensations by certain particular sensations of the eye, of the touch, and of the muscular sense, in which analysis discovers the elements and the source of the representation of motion. Now these particular sensations have no more objective value than those of the tongue, of the nose, and of the ear; in so far as they are related to the external excitant of which it is sought to penetrate the inmost nature, one of them is as radically false as the other.

It is true that a certain number of persons will think to escape from our conclusion, because they do not accept our starting point. There exist, in fact, several systems which propound that the outer world is known to us directly without the intermediary of a tertium quid, that is, of sensation. In the first place, the spiritists are convinced that disembodied souls can remain spectators of terrestrial life, and, consequently, can perceive it without the interposition of organs. On the other hand, some German authors have recently maintained, by rather curious reasoning, that the specific energy of our nervous system does not transform the excitants, and that our sensations are the faithful copies of that which causes them. Finally, various philosophers, Reid, Hamilton, and, in our own days, the deep and subtle mind of M. Bergson, have proposed to admit that by direct comprehension we have cognisance of the objects without mystery and as they are. Let this be admitted. It will change nothing in our conclusions, and for the following reasons.

We have said that no kind of our sensations—neither the visual, the tactile, nor the muscular—permits us to represent to ourselves the inmost structure of matter, because all sensations, without exception, are false, as copies of material objects. We are now assured that we are mistaken, and that our sensations are all true—that is to say, are faithful copies of the objects. If all are true, it comes to the same thing as if all are false. If all are true, it is impossible to make any choice among them, to retain only the sensations of sight and touch, and to use them in the construction of a mechanical theory, to the exclusion of the others. For it is impossible for us to explain some by the others. If all are equally true, they all have the same right to represent the structure of matter, and, as they are irreconcilable, no theory can be formed from their synthesis.

Let us, consequently, conclude this: whatever hypothesis may be built up on the relations possibly existing between matter and our sensations, we are forbidden to make a theory of matter in the terms of our sensations.

That is what I think of matter, understood as the inmost structure of bodies—of unknowable and metaphysical matter. I shall not speak of it again; and henceforth when I use the word matter, it will be in quite a different acceptation—it will be empirical and physical matter, such as it appears to us in our sensations. It must therefore be understood that from this moment we change our ground. We leave the world of noumena and enter that of phenomena.

Definition of Mind.—Generally, to define the mind, we oppose the concept of mind to the concept of matter, with the result that we get extremely vague images in our thoughts. It is preferable to replace the concepts by facts, and to proceed to an inventory of all mental phenomena.

Now, in the course of this inventory, we perceive that we have continually to do with two orders of elements, which are united in reality, but which our thought may consider as isolated. One of these elements is represented by those states which we designate by the name of sensations, images, emotions, &c.; the other element is the consciousness of these sensations, the cognition of these images, the fact of experiencing these emotions. It is, in other words, a special activity of which these states are the object and, as it were, the point of application—an activity which consists in perceiving, judging, comparing, understanding, and willing. To make our inventory orderly, let us deal with these two elements separately and begin with the first.

We will first examine sensation: let us put aside that which is the fact of feeling, and retain that which is felt. Thus defined and slightly condensed, what is sensation? Until now we have employed the word in the very vague sense of a tertium quid interposed between the object and ourselves. Now we have to be more precise, and to inquire whether sensation is a physical or a mental thing. I need not tell you that on this point every possible opinion has been held. My own opinion is that sensation should be considered as a physical phenomenon; sensation, be it understood, in the sense of impression felt, and not in that of capacity to feel.

Here are the arguments I invoke for the support of my thesis: in the first place, popular opinion, which identifies matter with what we see, and with what we touch—that is to say, with sensation. This popular opinion represents a primitive attitude, a family possession which we have the right to retain, so long as it is not proved to us to be false: next, this remark, that by its mode of apparition at once unexpected, the revealer of new cognitions, and independent of our will, as well as by its content, sensation sums up for us all we understand by matter, physical state, outer world. Colour, form, extent, position in space, are known to us as sensations only. Sensation is not a means of knowing these properties of matter, it is these properties themselves.

What objections can be raised against my conclusion? One has evidently the right to apply the term psychological to the whole sensation, taken en bloc, and comprising in itself both impression and consciousness. The result of this terminology will be that, as we know nothing except sensations, the physical will remain unknowable, and the distinction between the physical and the mental will vanish. But it will eventually be re-established under other names by utilising the distinction I have made between objects of cognition and acts of cognition;—a distinction which is not verbal, and results from observation.

What is not permissible is to declare that sensation is a psychological phenomenon, and to oppose this phenomenon to physical reality, as if this latter could be known to us by any other method than sensation.

If the opinion I uphold be accepted, if we agree to see in sensation, understood in a certain way, a physical state, it will be easy to extend this interpretation to a whole series of different phenomena. To the images, first, which proceed from sensations, since they are recurring sensations; to the emotions also, which, according to recent theories, result from the perception of the movements which are produced in the heart, the vessels, and the muscles; and finally, to effort, whether of will or of attention, which is constituted by the muscular sensations perceived, and consequently also results from corporeal states. The consequences must be clearly remarked. To admit that sensation is a physical state, is to admit, by that very fact, that the image, idea, emotion, and effort—all those manifestations generally ascribed to the mind alone—are also physical states.

What, then, is the mind? And what share remains to it in all these phenomena, from which it seems we are endeavouring to oust it? The mind is in that special activity which is engaged in sensation, image, idea, emotion, and effort. For a sensation to be produced; there must be, as I said a little time ago, two elements: the something felt—a tree, a house, an animal, a titillation, an odour,—and also the fact of feeling this something, the consciousness of it, the judgment passed on it, the reasoning applied to it—in other terms, the categories which comprehend it. From this point of view, the dualism contained in sensation is clearly expressed. Sensation as a thing felt, that is, the physical part, or matter; sensation as the fact of feeling or of judging, that is, the mind.

Mark the language I use. We say that matter is the something felt; but we do not say for the sake of symmetry, that the mind is the something which feels. I have used a more cautious, and, I think, a more just formula, which places the mind in the fact of feeling. Let me repeat again, at the risk of appearing too subtle: the mind is the act of consciousness; it is not a subject which has consciousness. For a subject, let it be noted, a subject which feels, is an object of cognition—it forms part of the other group of elements, the group of sensations. In practice we represent by mind a fragment of our own biography, and by dint of pains we attribute to this fragment the faculty of having a consciousness; we make it the subject of the relation subject-object. But this fragment, being constituted of memories and sensations, does not exactly represent the mind, and does not correspond to our definition; it would rather represent the mind sensationalised or materialised.

From this follows the curious consequence that the mind is endowed with an incomplete existence; it is like form, which can only be realised by its application to matter of some kind. One may fancy a sensation continuing to exist, to live and to provoke movements, even after ceasing to be perceived. Those who are not uncompromising idealists readily admit this independence of the objects with regard to our consciousness, but the converse is not true. It is impossible to understand a consciousness existing without an object, a perception without a sensation to be perceived, an attention without a point of application, an empty wish which should have nothing to wish for; in a word, a spiritual activity acting without matter on which to act, or more briefly still—mind without matter. Mind and matter are correlative terms; and, on this point, I firmly believe that Aristotle was much closer to the truth than many modern thinkers.

I have convinced myself that the definition of mind at which we have just arrived is, in its exactness and soberness, the only one which permits psychology to be distinguished from the sciences nearest to it. You know that it has been discovered in our days that there exists a great difficulty in effecting this delimitation. The definitions of psychology hitherto proposed nearly all have the defect of not agreeing with the one thing defined. Time fails us to review them all, but I shall point out one at least, because our discussion on this particular formula will serve as a preparation for taking in hand the last question that remains to be examined—the relation of the mind to the body.

According to the definition I am aiming at, psychology would be the science of internal facts, while the other sciences deal with the external. Psychology, it has also been said, has as its instrument introspection, while the natural sciences work with the eye, the touch, the ear—that is to say, with the senses of extrospection.

To this distinction, I reply that in all sciences there exist but two things: sensations and the consciousness which accompanies them. A sensation may belong to the inner or the outer world through accidental reasons, without any change in its nature; the sensation of the outer world is the social sensation which we share with our fellows. If the excitant which provokes it is included in our nervous system, it is the sensation which becomes individual, hidden to all except ourselves, and constituting a microcosm by the side of a macrocosm. What importance can this have, since all the difference depends on the position occupied by the excitant?

But we are persistently told: there are in reality two ways of arriving at the cognition of objects—from within and from without. These two ways are as opposite as the right and wrong side of a stuff. It is in this sense that psychology is the science of the within and looks at the wrong side, while the natural sciences reckon, weigh, and measure the right side. And this is so true, they add, that the same phenomenon absolutely appears under two forms radically different from each other according as they are looked at from one or the other of the two points of view. Every one of our thoughts, they point out to us, is in correlation with a particular state of our cerebral matter; our thought is the subjective and mental face, the corresponding cerebral process is the objective and material face.

Though this dualism is frequently presented as an observed truth, I think it is possible to show its error. Take an example: I look at the plain before me, and see a flock of sheep pass through it. At the same time an observer, armed with a microscope À la Jules Verne, looks into my brain and observes there a certain molecular dance which accompanies my visual perception. Thus, on the one hand, is my representation; on the other, a dynamic state of the nerve cells. This is what constitutes the right and the wrong sides of the stuff. We are told, "See how little resemblance there is in this; a representation is a psychical, and a movement of molecules a material, thing."

But I, on the contrary, think there is a great resemblance. When I see the flock passing, I have a visual perception. The observer who, by the hypothesis, is at that moment looking into my brain, also experiences a visual perception. Granted, they are not the same perception. How could they be the same? I am looking at the sheep, he is looking at the interior of my brain; it is not astonishing that, looking at objects so different, we should receive images also very different. But, notwithstanding their difference of object—that is, of content—there are here two visual perceptions composed in the same way; and I do not see by what right it can be said that one represents a material, the other a physical, phenomenon. In reality, each of these perceptions has a two-fold and psycho-physical value—physical in regard to the object to which it applies, and psychical inasmuch as it is an act of perception, that is to say, of consciousness. For one is just as much psychical as the other, and as much material, for a flock of sheep is as material a thing as is my brain. If we keep this conclusion in our minds, when we come to make a critical examination of certain philosophical systems, we shall easily see the mistake they make.

Spiritualism[51] rests on the conception that the mind can subsist and work in total independence of any tie to matter. It is true that, in details, spiritualists make some modification in this absolute principle in order to explain the perceptions of the senses and the execution of the orders of the will; but the duality, the independence, and the autonomy of the soul and the body remain, in any case, the peculiar dogma of the system. This dogma appears to me utterly false; the mind cannot exist without matter to which it is applied; and to the principle of heterogeneity, so often invoked to forbid all commerce between the two substances, I reply by appealing to intuition, which shows us the consciousness and its different forms, comparison, judgment, and reasoning, so closely connected with sensation that they cannot be imagined as existing with an isolated life.

Materialism, we know, argues quite differently; it imagines that a particular state of the nerve centres has the virtue of generating a psychical phenomenon, which represents, according to various metaphors, property, function, effect, and even secretion. Critics have often asked how, with matter in motion, a phenomenon of thought could be explained or fabricated. It is very probable that those who admit this material genesis of thought, represent it to themselves under the form of something subtle, like an electric spark, a puff of wind, a will-of-the-wisp, or an alcoholic flame. Materialists are not alone responsible for these inadequate metaphors, which proceed from a metaphysics constructed of concepts. Let us recollect exactly what a psychical phenomenon is. Let us banish the will-o'-the-wisps, replace them by a precise instance, and return to the visual perception we took as an example a little while back: without intending a pun, "revenons À nos moutons." These sheep which I see in the plain are as material, as real, as the cerebral movement which accompanies my perception. How, then, is it possible that this cerebral movement, a primary material fact, should engender this secondary material fact, this collection of complicated beings which form a flock?

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.

[51] See [Note 43] on p. 191.