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.