Lectures on the Philosophy of the Human Mind (Vol. 1 of 3)

by Thomas Brown

Footnotes

[63] Œuvres, tom. ii. p. 12.

[64] P. 131.

[65] Pleasures of Imagination, Book III. v. 493—514. The fixed soul, v. 505. Exploits, v. 508; and Spells, v. 509. Orig.

[66] On the Intellectual powers, Essay II. c. 16.

[67] Lib. I. v. 18–25. and 28–31.

[LECTURE XIX.]

BRIEF NOTICE OF THE CORPOREAL PART OF THE PROCESS, IN SENSATION.

The mental phenomena, of the class which is at present under our consideration, being those, which arise, in consequence of certain previous affections of our organs of sense, it is necessary, that we should take some notice of the corporeal part of the process; though it must always be remembered, that it is the last part of the process, the mental affection only, which truly belongs to our science,—and that, if this, in all its varieties, had been the result of any other species of affections of organs constituted in any other manner,—as long as there was the regular correspondence of certain mental affections with certain organic affections,—the philosophy of mind would have continued precisely the same as now. Our systems of anatomy, and of the physiology of our mere bodily frame, would indeed have been different,—but not that more intimate physiology, which relates to the functions of the animating spirit, whose presence is life, and without which our bodily frame, in all its beautiful adaptation of parts to parts, is a machine, as inert and powerless, as the separate atoms that compose it.

The great essential organ of all sensation is the brain, with its appendages, particularly the nerves that issue from it to certain organs, which are more strictly termed the organs of sense; as it is there the immediate objects, or external causes of sensation, the particles of light, for example, in vision, or of odour in smell, arrive, and come, as it were, into contact with the sensorial substance. Each organ, as you well know, has objects peculiar to itself, which it would be superfluous to enumerate; and since the blind are still sensible of sound, the deaf of colour, and both of smell, and taste, and touch, there must evidently be some difference, either in the sensorial substance itself which is diffused over the different organs, or in the mode of its diffusion and exposure in the different organs, from which this striking diversity of their relative sensibilities proceeds. The nervous matter however, considered separately from the coats in which it is enveloped, is of the same half-fibrous, but soft and pulpy texture, as the substance of the brain itself, and is in perfect continuity with that substance, forming, therefore, with it, what may be considered as one mass, as much as the whole brain itself may be considered as one mass; which has, indeed, for its chief seat the great cavity of the head; the

“Superas hominis sedes, arcemque cerebri;

Namque illic posuit solium, et sua templa sacravit,

Mens animi;—”[68]

but which extends, by innumerable ramifications, over the whole surface, and through the internal parts of the body. The mind, in that central brain in which it is supposed to reside, communicating with all these extreme branches, has been compared, by a very obvious, but a very beautiful similitude, to the parent Ocean, receiving from innumerable distances the waters of its filial streams:

“Ac uti longinquis descendunt montibus amnes,

Velivolus Tamisis, flaventisque Indus arenæ,

Euphratesque, Tagusque, et opimo flumine Ganges,

Undas quisque suas volvens,—cursuque sonoro

In mare prorumpunt; hos magno acclinis in antro

Excipit Oceanus, natorumque ordine longo

Dona recognoscit venientum, ultroque serenat

Coeruleam faciem, et diffuso marmore ridet.

Haud aliter species properant se inferre novellæ

Certatim menti.”[69]

In the brain itself, the anatomist is able to shew us, with perfect clearness, many complicated parts, which we must believe to be adapted for answering particular purposes in the economy of life; but when we have gazed with admiration on all the wonders which his dissecting hand has revealed to us, and have listened to the names with which he most accurately distinguishes the little cavities or protuberances which his knife has thus laid open to our view, we are still as ignorant as before of the particular purposes to which such varieties of form are subservient; and our only consolation is,—for there is surely some comfort in being only as ignorant as the most learned,—that we know as much of the distinct uses of the parts as the anatomist himself, who exhibits them to us, and teaches us how to name them. A structure, in every respect different, though assuredly less fit than the present which has been chosen by infinite wisdom, might, as far as we know, have answered exactly the same end; which is as much as to say, that our ignorance on the subject is complete. The only physiological facts of importance, in reference to sensation, are, that if the nerves, which terminate in particular organs, be greatly diseased, the sensations which we ascribe to those particular organs cease; and cease, in like manner, if the continuity of the nerves be destroyed, by cutting them in any part of their course, or if, without loss of absolute continuity, their structure, in any part of their course be impaired by pressure, whether from tight ligatures drawn around them for the purpose of experiment, or from natural morbid causes. In short, if the brain and nerves be in a sound state, and certain substances be applied to certain parts of the nervous system,—as, for instance, sapid bodies to the extremities of the nerves of taste, or light to that expansion of the optic nerve, which forms what is termed the retina,—there is then instant sensation; and when the brain itself is not in a sound state to a certain extent, or when the nerve which is diffused on a particular organ is, either at this extremity of it, or in any part of its course, to a certain degree impaired, then there is no sensation, though the same external causes be applied. This very slight general knowledge of the circumstances in which sensation takes place, and of the circumstances in which it does NOT take place, is all the knowledge which physiology affords us of the corporeal part of the process;—and it is likely to continue so forever,—at least in all the more important respects of our ignorance,—since any changes which occur in the corpuscular motion, and consequent new arrangement of the particles of the substance of the brain and nerves, corresponding with the diversities of feeling during those particular states,—if such corpuscular motions or changes do really take place,—are probably far too minute to be observable by our organs; even though we could lay open all the internal parts of the brain to complete observation, without destroying, or at all affecting, the usual phenomena of life:—

In “following life through creatures we dissect,

We loose it, in the moment we detect.”

Indeed, we are not able to do even so much as this; for life has already vanished, long before we have come upon the verge of its secret precincts. It is like a Magician, that operates at a distance on every side, but still keeps himself apart, within a narrow circle. If we remain without the circle, we may gaze with never-ceasing admiration, on the wonders that play in rapid succession before our eyes. But, if we rush within, to force an avowal, of the secret energy that produces them, the enchanter and the enchantments alike are fled.

The brain, then, and the various nerves of sense in continuity with it, may, when taken together, be considered as forming one great organ, which I would term briefly the sensorial organ, essential to life, and to the immediate production of those mental phenomena which constitute our sensations, and, perhaps, too, modifying in some measure, directly or indirectly, all the other phenomena of the mind.

“Dum mens alma caput cerebrique palatia celsa

Occupat, et famulos sublimis dirigit artus,

Et facili imperio nervorum flectit habenas,

Illius ad nutum sensus extranea rerum

Explorant signa, et studio exemplaria fido

Ad dominam adducunt; vel qui statione locantur

Vicina, capitisque tuentur limina, ocelli,

Naresque, auriculæque, et vis arguta palati;

Vel qui per totam currit sparso agmine molem

Tactus, ad extremas speculator corporis aras.

His sensim auxiliis instructa fidelibus, olim

Mens humilis nulloque jacens ingloria cultu

Carceris in tenebris mox sese attolit in auras

Dives opum variarum, et sidera scandit Olympi.”

Of the nature of the connexion of this great sensorial organ with the sentient mind, we never shall be able to understand more than is involved in the simple fact, that a certain affection of the nervous system precedes immediately a certain affection of the mind. But, though we are accustomed to regard this species of mutual succession of bodily and mental changes, as peculiarly inexplicable, from the very different nature of the substances which are reciprocally affected, it is truly not more so than any other case of succession of events, where the phenomena occur in substances that are not different in their properties, but analogous, or even absolutely similar; since, in no one instance of this kind, can we perceive more than the uniform order of the succession itself; and of changes, the successions of which are all absolutely inexplicable, or, in other words, absolutely simple, and unsusceptible, therefore, of further analysis, none can be justly said to be more or less so than another. That a peculiar state of the mere particles of the brain, should be followed by a change of state of the sentient mind, is truly wonderful; but if we consider it strictly, we shall find it to be by no means more wonderful, than that the arrival of the moon, at a certain point of the heavens, should render the state of a body on the surface of our earth, different from what it otherwise would naturally be or that the state of every particle of our globe, in its relative tendencies of gravitation, should be instantly changed, as it unquestionably would be, by the destruction of the most distant satellite of the most distant planet of our system, or probably too, by the destruction even of one of those remotest of stars, which are illuminating their own system of planets, so far in the depth of infinity, that their light,—to borrow a well-known illustration of sidereal distance,—may never yet have reached our earth, since the moment at which they darted forth their first beams, in the creation of the universe. We believe, indeed, with as much confidence, that one event will uniformly have for its consequent another event, which we have observed to follow it, as we believe the simple fact that it has preceded it, in the particular case observed. But the knowledge of the present sequence, as a mere fact, to be remembered, and the expectation of future similar sequences, as the result of an original law of our belief, are precisely of the same kind, whether the sequence of changes be in mind, or in matter singly, or reciprocally in both.

What the nature of the change is, that is produced at the extremity of the nerve, it is beyond our power to state, or even to guess; and we are equally ignorant of the manner in which this affection of the nerve is communicated, or is supposed to be communicated, to the brain. But that some affection is gradually propagated, from the one to the other, so as to render the change in the state of the brain subsequent, by a certain interval, to the change in the state of the nerve, is universally believed. In applying to this change the term impression, a term indeed which had been in common use before, Dr Reid is careful to point out the reason for which this term appears to him preferable to others; and though I confess that the word seems to me to convey too much the notion of a peculiar well known species of action; that which consists in producing a certain configuration of the object impressed, corresponding with the figure of the impressing object, the very notion that has had so pernicious an effect in the theory of perception; and though I conceive the simple term change or affection to be all which is safely admissible, as long as the nature of the particular change is absolutely unknown; still it must be confessed that impression is a term a little more general than the other names of action, to which Dr Reid alludes, and therefore preferable to them, in the present case.

“There is sufficient reason,” he says, “to conclude, that, in perception, the object produces some change in the organ; that the organ produces some change upon the nerve; and that the nerve produces some change in the brain. And we give the name of an impression to those changes, because we have not a name more proper to express, in a general manner, any change produced in a body, by an external cause, without specifying the nature of that change. Whether it be pressure, or attraction, or repulsion, or vibration, or something unknown, for which we have no name, still it may be called an impression. But with regard to the particular kind of this change or impression, philosophers have never been able to discover any thing at all.”[70]

That the word impression is not so free, as Dr Reid supposes, from that hypothetical meaning which he wished to avoid, I have already remarked. But the reason assigned by him for his preference of it, is unquestionably a just one; since a phrase which expresses the least possible knowledge, must be allowed to be the best suited to human ignorance,—that ignorance, which, not in the philosophy of intellect only, but in whatever track of science we may proceed, and whatever truths we may proudly discover in our way, still meets us at the end of every path, as if to mock at once our weakness and our pride,—and which seems to us to be every where, because it is, wherever we are ourselves. The splendour of nature, as it exists in itself, is, if I may speak figuratively, like sunshine on a boundless plain, on the flowers and herbage of which, though there be innumerable varieties of colour, there is brilliancy in all. But the misfortune is, that, as soon as we have approached near enough to distinguish the diversity of tints, their brilliancy is so obscured by our very approach to them, that their nice diversities are no longer distinguishable; as if man could not move along, without throwing his own shadow on every thing before him.

When I say, that we are ignorant of the nature of that change, which is propagated along the nerve to the brain, I speak in reference to an opinion that is universal. But, though it may be improbable, it is certainly far from impossible, that there is really no such progressive communication, as this which is supposed. The brain and nerves, though, from the difference of names, you might be led, perhaps, to consider them as distinct, I have already said, are not separate organs, but are in continuity with each other, at least as much as various parts of the brain itself, which are comprehended under that single term, can be said to be continuous. When taken together, they form what is truly one complicated sensorial organ,—the organ of all our sensations, according to the different states in which the organ exists, or the different parts of it which are chiefly affected. In hearing, for example, a certain state of that part of the sensorial organ, which constitutes the auditory nerves,—in vision, a certain state of that part of it, which constitutes the optic nerves, is necessary to sensation,—and, in both cases, according to the universal supposition on the subject, all or part of the brain likewise must exist in a certain state, of which we know nothing more, than that it is followed, in the one case, by the sensation of sound, in the other case by that of sight. The connexion of the mind with the bodily frame,—which must be equally inexplicable on every supposition that can be formed,—is not supposed, by any philosopher, to depend on the state of a single physical point of the brain alone; and, if it extend to more than one such point, there is nothing,—in the nature of the connexion itself, independently of experience,—which necessarily limits it to one portion of the complex sensorial organ, more than to another,—to the particles of the central mass of the brain, for example, more than to those of the nerve itself. It is experience, then, to which we are referred; and experience, though it shows that certain nerves are not essential to life, since life continues equally, after they may have been impaired, or even destroyed, is far from showing that an affection of them is not essential to sensation, at the very moment of the particular sensation; nor does it afford even the slightest evidence, to justify the belief, that the only use of the nerve is to communicate a certain affection to the brain, which affection of the mere central part of the sensorial organ, would, of itself, immediately induce sensation, though the nerves were annihilated in the preceding instant. The sensation may be the immediate effect, not of the state of the brain only, but of the state of the brain, and of any particular nerve, considered as existing together at the moment; in the same manner, as, by those who ascribe the immediate origin of sensation to the mere brain, exclusive of its nervous appendages, it is supposed to depend on the state, not of one physical point of the central brain, but on the state of many such co-existing points. We know not, to what extent, in the great sensorial organ, this change is necessary; but we believe, that, to some extent, it is necessary; and the question is, whether, in the whole portion so affected, the affection be produced by a succession of changes, propagated from part to part? This may, perhaps, be the more probable supposition:—but, whatever may be the comparative probability or improbability, it certainly has not been demonstrated by observation or experiment; nor can there be said to be, a priori, any absurdity in the opposite supposition, that the sensorial affection, to whatever extent it may be necessary, is not progressive, but immediate,—that, as long as the sensorial organ, (under which term I comprehend, as I have already frequently repeated, not the brain merely, but also its nervous appendages, that exist in apparent continuity with the brain,) is unimpaired, by accident or disease, the presence of the immediate object of sense, at the external organ, which on every supposition, must be followed by some sensorial change of state, is instantly followed by that general change of state of the internal organ, whatever it may be, which is necessary to sensation, in the particular case; in the same manner, as the presence of a celestial body, at a certain point in the heavens, is immediately followed by a change of state, in the whole gravitating particles of our globe; the change in any long line of these gravitating particles being not communicated from each to each, but depending only on the presence of the distant sun or planet; and beginning in the most remote particles of the line, at the very same instant, as in that which is nearest, on the surface of the earth. An instant change, in the long line of sensorial particles,—if the affection of a long line of these particles be necessary,—on the presence of a particular object, is not more improbable in itself, than this instant and universal influence of gravitation, that varies with all the varying positions of a distant object.

But is it, indeed, certain, that, in sensation, there is an affection of the central brain, whether immediate or progressive? Is it not possible, at least, or more than possible, that the state of the mind, when we perceive colours and sounds, may be the immediate consequent of the altered state of that part of the sensorial organ, which forms the expansion of the nerve in the eye or ear? The sensations must be supposed, in every theory, to be the consequents of states induced in some sensorial particles, and there is nothing but the mere names of brain and nerve, invented by ourselves, and the notions which we have chosen, without evidence, to attach to these mere names, which would mark the sensorial particles in the nervous expanse itself, as less fitted to be the immediate antecedents of sight and hearing, than the similar sensorial particles in any portion of the central mass of the brain. There is no reason, in short a priori, for supposing that a state of the sensorial particles of the nerves cannot be the cause of sensation, and that the sensation must be the effect of a state equally unknown, of apparently similar particles, in that other part of the general sensorial organ, which we have denominated the brain. Sensation, indeed, is prevented by decay, or general disease of the brain, or by separation of the nerve, or pressure on it, in any part of its course. But it is far from improbable, that these causes, which must evidently be injurious to the organ, may act, merely by preventing that sound state of the nerve, which is necessary for sensation, and which, in an organ so very delicate, may be affected by the slightest influences,—by influences far slighter, than may naturally be expected to result from such an injury of such a part. The nerves and brain, together, form one great organ; and a sound state of the whole organ, even from the analogy of other grosser organs, may well be supposed to be necessary for the healthy state and perfect function of each separate part.

If, indeed, the appearance of the brain and nerves were such, as marked them to be peculiarly fitted for the communication of motion of any sort, there might be some presumption, from this very circumstance, in favour of the opinion, that sensation takes place, only after a progressive series of affections of some sort, propagated along the nerve to the interior brain. But it must be remembered, that the nature, both of the substance of the nerves themselves, and of the soft and lax substance, in which they are loosely embedded, renders them very ill adapted for the communication of nice varieties of motion, and gives some additional likelihood, therefore, to the supposition, that affections of the sensorial organ, so distinct as our sensations are from each other, and so exactly corresponding with the slightest changes of external objects, do not depend on the progressive communication of faint and imperceptible motion, in circumstances so unfavourable to the uninterrupted progress even of that more powerful motion, which can be measured by the eye. In a case so doubtful as this, however, in which the intervening changes supposed by philosophers,—if such a progressive series of motions do really take place,—are confessed to be beyond our observation, it is impossible for any one, who has a just sense of the limits, which nature has opposed to our search, to pronounce with certainty, or even perhaps with that faint species of belief, which we give to mere probability. My conjectures on the subject, therefore, I state simply as conjectures, and nothing more.

If, indeed, what is but a mere conjecture could be shown to be well founded, it would add another case to the innumerable instances, in which philosophers have laboured, for ages, to explain what did not exist,—contenting themselves, after their long toil, with the skill and industry which they have exhibited, in removing difficulties, which they had before, with great skill and industry, placed in their own way. “I am not so much convinced of our radical ignorance,” says an ingenious writer, “by the things that are, of which the nature is hid from us, as by the things that are not, of which notwithstanding we contrive to give a very tolerable account; for this shews that we are not merely without the principles which lead to truth, but that there are other principles in our nature, which can accommodate themselves very well, and form a close connexion, with what is positively false.”

But whatever reason there may be for removing this supposed link of the corporeal part of the process of sensation, there is another prior link, which it appears to me of great importance to separate from the chain. I allude to the distinction, which is commonly made, of the objects of sense, as acting themselves on our organs, or as acting through what is termed a medium.

“A second law of our nature,” says Dr Reid, “regarding perception is, that we perceive no object, unless some impression is made upon the organ of sense, either by the immediate application of the object, or by some medium which passes between the object and the organ. In two of our senses, to wit, touch and taste, there must be an immediate application of the object to the organ. In the other three, the object is perceived at a distance, but still by means of a medium, by which some impression is made upon the organ. The effluvia of bodies drawn into the nostrils, with the breath, are the medium of smell; the undulations of the air, are the medium of hearing; and the rays of light passing from visible objects to the eye, are the medium of sight. We see no object, unless rays of light come from it to the eye. We hear not the sound of any body, unless the vibrations of some elastic medium, occasioned by the tremulous motion of the sounding body, reach our ear. We perceive no smell, unless the effluvia of the smelling body enter into the nostrils. We perceive no taste, unless the sapid body be applied to the tongue, or some part of the organ of taste. Nor do we perceive any tangible quality of a body, unless it touch the hands, or some part of our body.”[71]

It is evident, that, in these cases of a supposed medium, which Dr Reid considers as forming so important a distinction of our sensations, the real object of sense is not the distant object, but that which acts immediately upon the organs,—the light itself, not the sun which beams it on us,—the odorous particles, which the wind has wafted to us from the rose, not the rose itself upon its stem,—the vibrations of the air, within our ear, not the cannon that is fired at the distance of miles. The light, the odour, the vibrating air, by which alone our senses are affected, act on our nerves of sight, of smell, and hearing, with an influence as direct, and as little limited in the kind of action, as that with which the fruit, which we eat or handle, acts on our nerves of taste or touch. This influence of the objects immediately external is all, in which our organs of sense, and consequently the mind as the principle of mere sensation, is concerned. The reference to the distant sun, or rose, or cannon, which alone leads us to speak of a medium in any of these cases, is the effect of another principle of our intellectual nature,—the principle of association, or suggestion,—that is afterwards to be considered by us, without which, indeed, our mere transient sensations would be comparatively of little value; but which, as a quality or susceptibility of the mind, is not to be confounded with that, by which the mind becomes, instantly sentient, in consequence of a certain change produced in the state of its sensorial organ.

Since, however, precisely the same series of changes must take place in nature, whether we class the sun, the flower, the cannon, as the objects of sense, or merely the light, the odorous particles, and the vibrating air, it may perhaps be thought, that the distinction now made is only a verbal one, of no real importance. But it will not appear such to those who are conversant with the different theories of perception which we are afterwards to review; many of which, that have had the greatest sway, and a sway the most fatal to the progress of intellectual philosophy, appear, to me, to have arisen entirely, or at least chiefly, from this very misconception as to the real external object of sense. It is sufficient at present to allude to the effect, which the mere distance of the supposed object must have had, in giving room to all the follies of imagination to fill up the interval.

It may be necessary, however, to remark by the way, that though I do not conceive the bodies which act through a medium, as it is said to be the real objects of the particular sense;—the immense orb of the sun, for example, in all its magnitude, to be the object of that small organ by which we are sensible of light; or the cannon, which exists we know not where, to be the object of that organ by which we are sensible of sound;—I am still far from objecting to the popular and very convenient phraseology, by which we speak of seeing the sun, and hearing the cannon—a phraseology that expresses briefly a reference, which could not otherwise be expressed but by a very awkward circumlocution, and to make any innovation in which would be as absurd, as to reject the popular phrases of the sun's rising and setting merely because they are inconsistent with our astronomical belief. The most rigid philosophy can require no more, than that, when we talk of the sun's actual setting, we should mean by it, only a certain position relative to that great luminary at which the earth arrives in its diurnal revolution,—and that, when we talk of seeing it descend, we should mean nothing more, than that we see light of a certain brilliancy, from which we infer the existence and relative position of the orb that has projected it.

I have been led into these observations, on the various parts of the corporeal process which precedes sensation, by the desire of removing, as much as possible, any obscurity in which your notions on the subject might be involved,—as I know well the influence which even a slight confusion in our notion of any part of a complicated process has, in spreading, as it were, its own darkness and perplexity over parts of the process, which otherwise we should have found no difficulty in comprehending. You might think, that you knew less distinctly the mental sensation itself, because you knew only obscurely the series of bodily changes that precede sensation; but still it must be remembered, that it is only the last link of the corporeal chain,—the ultimate affection of the sensorial organ, in whatever manner and to whatever extent it may be affected,—immediately antecedent to the affection of the mind, which is to be considered as that with which nature has united the corresponding change in our mental frame. This mysterious influence of our bodily on our mental part has been poetically compared to that which the sun was supposed to exercise on a lyre, that formed part of a celebrated Egyptian statue of Memnon, which was said to become musical when struck with its beams; and though the poet has extended the similitude, beyond our mere elementary sensations, to the complex perception of beauty, it is still a very happy illustration—as far as a mere poetic image can be an illustration—of the power which matter exercises over the harmonies of mind:—

“For as old Memnon's image, long renown

By fabling Nilus, to the quivering touch

Of Titan's ray with each repulsive string

Consenting, sounded through the warbling air

Unbidden strains,—even so did Nature's hand,

To certain species of external things

Attune the finer organs of the mind.

So the glad impulse of congenial powers,

Or of sweet sound, or fair proportion'd form,

The grace of motion, or the bloom of light,

Thrills through Imagination's tender frame,

From nerve to nerve. All naked and alive,

They catch the spreading rays; till now the soul

At length discloses every tuneful spring,

To that harmonious movement from without.

Responsive. Then the charm, by Fate prepar'd

Diffuses its enchantment.[72] Fancy dreams

Of sacred fountains, and Elysian groves,

And vales of bliss! the Intellectual Power

Bends from his awful throne a wondering ear,

And smiles; the Passions, gently soothed away,

Sink to divine repose; and Love and Joy

Alone are waking.”[73]

When we consider the variety of our feelings thus wonderfully produced,—the pleasures, and, still more, the inexhaustible knowledge, which arise, by this mysterious harmony, from the imperceptible affection of a few particles of nervous matter, it is impossible for us not to be impressed with more than admiration of that Power, which even our ignorance, that is scarcely capable of seeing any thing, is yet, by the greatest of all the bounties of heaven, able to perceive and admire. In the creation of this internal world of thought, the Divine Author of our being has known how to combine infinity itself with that which may almost be considered as the most finite of things; and has repeated, as it were, in every mind, by the almost creative sensibilities with which He has endowed it, that simple but majestic act of omnipotence, by which, originally, He called from the rude elements of chaos, or rather from nothing, all the splendid glories of the universe.