general, are fundamentally defective, by reason of an oversight
which was inevitable, in the state of knowledge at the time in which
they were promulgated. What the older philosophers did not know, and
could not know, before the anatomist and the physiologist had done
their work, is that, between the external object and that mind in
which they supposed the sensation to inhere, there lies a physical
obstacle. The sense organ is not a mere passage by which the “tenuia
simulacra rerum,” or the “intentional species” cast off by objects,
or the “forms” of sensible things, pass straight to the mind; on the
contrary, it stands as a firm and impervious barrier, through which no
material particle of the world without can make its way to the world
within.
Let us consider the olfactory sense organ more nearly. Each of the
nostrils leads into a passage completely separated from the other
by a partition, and these two passages place the nostrils in free
communication with the back of the throat, so that they freely transmit
the air passing to the lungs when the mouth is shut, as in ordinary
breathing. The floor of each passage is flat, but its roof is a high
arch, the crown of which is seated between the orbital cavities of the
skull, which serve for the lodgment and protection of the eyes; and
it therefore lies behind the apparent limits of that feature which,
in ordinary language, is called the nose. From the side walls of the
upper and back part of these arched chambers, certain delicate plates
of bone project, and these, as well as a considerable part of the
partition between the two chambers, are covered by a fine, soft, moist
membrane. It is to this “Schneiderian,” or olfactory, membrane that
odorous bodies must obtain direct access, if they are to give rise
to their appropriate sensations; and it is upon the relatively large
surface, which the olfactory membrane offers, that we must seek for
the seat of the organ of the olfactory sense. The only essential part
of that organ consists of a multitude of minute rod-like bodies, set
perpendicularly to the surface of the membrane, and forming a part of
the cellular coat, or epithelium, which covers the olfactory membrane,
as the epidermis covers the skin. In the case of the olfactory sense,
there can be no doubt that the Democritic hypothesis, at any rate for
such odorous substances as musk, has a good foundation. Infinitesimal
particles of musk fly off from the surface of the odorous body, and,
becoming diffused through the air, are carried into the nasal passages,
and thence into the olfactory chambers, where they come into contact
with the filamentous extremities of the delicate olfactory epithelium.
But this is not all. The “mind” is not, so to speak, upon the other
side of the epithelium. On the contrary, the inner ends of the
olfactory cells are connected with nerve fibres, and these nerve
fibres, passing into the cavity of the skull, at length end in a
part of the brain, the olfactory sensorium. It is certain that the
integrity of each, and the physical inter-connection of all these three
structures, the epithelium of the sensory organ, the nerve fibres,
and the sensorium, are essential conditions of ordinary sensation.
That is to say, the air in the olfactory chambers may be charged with
particles of musk; but, if either the epithelium, or the nerve fibres,
or the sensorium is injured, or if they are physically disconnected
from one another, sensation will not arise. Moreover, the epithelium
may be said to be receptive, the nerve fibres transmissive, and the
sensorium sensifacient. For, in the act of smelling, the particles of
the odorous substance produce a molecular change (which Hartley was in
all probability right in terming a vibration) in the epithelium, and
this change being transmitted to the nerve fibres, passes along them
with a measurable velocity, and, finally reaching the sensorium, is
immediately followed by the sensation.
Thus, modern investigation supplies a representative of the Epicurean
simulacra in the volatile particles of the musk; but it also gives us
the stamp of the particles on the olfactory epithelium, without any
transmission of matter, as the equivalent of the Aristotelian “form;”
while, finally, the modes of motion of the molecules of the olfactory
cells, of the nerve, and of the cerebral sensorium, which are Hartley’s
vibrations, may stand very well for a double of the “intentional
species” of the Schoolmen. And this last remark is not intended merely
to suggest a fanciful parallel; for, if the cause of the sensation is,
as analogy suggests, to be sought in the mode of motion of the object
of sense, then it is quite possible that the particular mode of motion
of the object is reproduced in the sensorium; exactly as the diaphragm
of a telephone reproduces the mode of motion taken up at its receiving
end. In other words, the secondary “intentional species” may be, as the
Schoolmen thought the primary one was, the last link between matter and
mind.
None the less, however, does it remain true that no similarity exists,
nor indeed is conceivable, between the cause of the sensation and the
sensation. Attend as closely to the sensations of muskiness, or any
other odour, as we will, no trace of extension, resistance, or motion
is discernible in them. They have no attribute in common with those
which we ascribe to matter; they are, in the strictest sense of the
words, immaterial entities.
Thus, the most elementary study of sensation justifies Descartes’
position, that we know more of mind than we do of body; that the
immaterial world is a firmer reality than the material. For the
sensation “muskiness” is known immediately. So long as it persists,
it is a part of what we call our thinking selves, and its existence
lies beyond the possibility of doubt. The knowledge of an objective or
material cause of the sensation, on the other hand, is mediate; it is
a belief as contradistinguished from an intuition; and it is a belief
which, in any given instance of sensation, may, by possibility, be
devoid of foundation. For odours, like other sensations, may arise from
the occurrence of the appropriate molecular changes in the nerve or in
the sensorium, by the operation of a cause distinct from the affection
of the sense organ by an odorous body. Such “subjective” sensations
are as real existences as any others, and as distinctly suggest an
external odorous object as their cause; but the belief thus generated
is a delusion. And, if beliefs are properly termed “testimonies of
consciousness,” then undoubtedly the testimony of consciousness may be,
and often is, untrustworthy.
Another very important consideration arises out of the facts as they
are now known. That which, in the absence of a knowledge of the
physiology of sensation, we call the cause of the smell, and term the
odorous object, is only such, mediately, by reason of its emitting
particles which give rise to a mode of motion in the sense organ. The
sense organ, again, is only a mediate cause by reason of its producing
a molecular change in the nerve fibre; while this last change is also
only a mediate cause of sensation, depending, as it does, upon the
change which it excites in the sensorium.
The sense organ, the nerve, and the sensorium, taken together,
constitute the sensiferous apparatus. They make up the thickness of the
wall between the mind, as represented by the sensation “muskiness,” and
the object, as represented by the particle of musk in contact with the
olfactory epithelium.
It will be observed that the sensiferous wall and the external world
are of the same nature; whatever it is that constitutes them both is
expressible in terms of matter and motion. Whatever changes take place
in the sensiferous apparatus are continuous with, and similar to, those
which take place in the external world.[61] But, with the sensorium,
matter and motion come to an end; while phenomena of another order,
or immaterial states of consciousness, make their appearance. How is
the relation between the material and the immaterial phenomena to
be conceived? This is the metaphysical problem of problems, and the
solutions which have been suggested have been made the corner-stones of
systems of philosophy. Three mutually irreconcilable readings of the
riddle have been offered.
The first is, that an immaterial substance of mind exists; and that it
is affected by the mode of motion of the sensorium in such a way as to
give rise to the sensation.
The second is, that the sensation is a direct effect of the mode of
motion of the sensorium, brought about without the intervention of any
substance of mind.
The third is, that the sensation is neither directly nor indirectly
an effect of the mode of motion of the sensorium, but that it has an
independent cause. Properly speaking, therefore, it is not an effect of
the motion of the sensorium, but a concomitant of it.
As none of these hypotheses is capable of even an approximation to
demonstration, it is almost needless to remark that they have been
severally held with tenacity and advocated with passion. I do not
think it can be said of any of the three that it is inconceivable, or
that it can be assumed on _à priori_ grounds to be impossible.
Consider the first, for example; an immaterial substance is perfectly
conceivable. In fact, it is obvious that, if we possessed no sensations
but those of smell and hearing, we should be unable to conceive a
material substance. We might have a conception of time, but could
have none of extension, or of resistance, or of motion. And without
the three latter conceptions no idea of matter could be formed. Our
whole knowledge would be limited to that of a shifting succession
of immaterial phenomena. But, if an immaterial substance may exist,
it may have any conceivable properties; and sensation may be one of
them. All these propositions may be affirmed with complete dialectic
safety, inasmuch as they cannot possibly be disproved; but neither
can a particle of demonstrative evidence be offered in favour of the
existence of an immaterial substance.
As regards the second hypothesis, it certainly is not inconceivable,
and therefore it may be true, that sensation is the direct effect of
certain kinds of bodily motion. It is just as easy to suppose this as
to suppose, on the former hypothesis, that bodily motion affects an
immaterial substance. But neither is it susceptible of proof.
And, as to the third hypothesis, since the logic of induction is in no
case competent to prove that events apparently standing in the relation
of cause and effect may not both be effects of a common cause—that
also is as safe from refutation, if as incapable of demonstration, as
the other two.
In my own opinion, neither of these speculations can be regarded
seriously as anything but a more or less convenient working hypothesis.
But, if I must choose among them, I take the “law of parsimony” for my
guide, and select the simplest—namely, that the sensation is the direct
effect of the mode of motion of the sensorium. It may justly be said
that this is not the slightest explanation of sensation; but then am
I really any the wiser, if I say that a sensation is an activity (of
which I know nothing) of a substance of mind (of which also I know
nothing)? Or, if I say that the Deity causes the sensation to arise in
my mind immediately after He has caused the particles of the sensorium
to move in a certain way, is anything gained? In truth, a sensation, as
we have already seen, is an intuition—a part of immediate knowledge.
As such, it is an ultimate fact and inexplicable; and all that we
can hope to find out about it, and that indeed is worth finding out,
is its relation to other natural facts. That relation appears to me
to be sufficiently expressed, for all practical purposes, by saying
that sensation is the invariable consequent of certain changes in the
sensorium—or, in other words, that, so far as we know, the change in
the sensorium is the cause of the sensation.
I permit myself to imagine that the untutored, if noble, savage of
“common sense” who has been misled into reading thus far by the hope
of getting positive solid information about sensation, giving way to
not unnatural irritation, may here interpellate: “The upshot of all
this long disquisition is that we are profoundly ignorant. We knew
that to begin with, and you have merely furnished another example of
the emptiness and uselessness of metaphysics.” But I venture to reply,
Pardon me, you were ignorant, but you did not know it. On the contrary,
you thought you knew a great deal, and were quite satisfied with the
particularly absurd metaphysical notions which you were pleased to
call the teachings of common sense. You thought that your sensations
were properties of external things, and had an existence outside of
yourself. You thought that you knew more about material than you do
about immaterial existences. And if, as a wise man has assured us,
the knowledge of what we don’t know is the next best thing to the
knowledge of what we do know, this brief excursion into the province of
philosophy has been highly profitable.
Of all the dangerous mental habits, that which schoolboys call
“cocksureness” is probably the most perilous; and the inestimable
value of metaphysical discipline is that it furnishes an effectual
counterpoise to this evil proclivity. Whoso has mastered the elements
of philosophy knows that the attribute of unquestionable certainty
appertains only to the existence of a state of consciousness so long
as it exists; all other beliefs are mere probabilities of a higher or
lower order. Sound metaphysic is an amulet which renders its possessor
proof alike against the poison of superstition and the counter-poison
of nihilism; by showing that the affirmations of the former and the
denials of the latter alike deal with matters about which, for lack of
evidence, nothing can be either affirmed or denied.
I have dwelt at length upon the nature and origin of our sensations of
smell, on account of the comparative freedom of the olfactory sense
from the complications which are met with in most of the other senses.
Sensations of taste, however, are generated in almost as simple a
fashion as those of smell. In this case, the sense organ is the
epithelium which covers the tongue and the palate: and which sometimes,
becoming modified, gives rise to peculiar organs termed “gustatory
bulbs,” in which the epithelial cells elongate and assume a somewhat
rod-like form. Nerve fibres connect the sensory organ with the
sensorium, and tastes or flavours are states of consciousness caused
by the change of molecular state of the latter. In the case of the
sense of touch there is often no sense organ distinct from the general
epidermis. But many fishes and amphibia exhibit local modifications
of the epidermic cells which are sometimes extraordinarily like the
gustatory bulbs; more commonly, both in lower and higher animals,
the effect of the contact of external bodies is intensified by the
development of hair-like filaments, or of true hairs, the bases of
which are in immediate relation with the ends of the sensory nerves.
Every one must have noticed the extreme delicacy of the sensations
produced by the contact of bodies with the ends of the hairs of the
head; and the “whiskers” of cats owe their functional importance to
the abundant supply of nerves to the follicles in which their bases
are lodged. What part, if any, the so-called “tactile corpuscles,”
“end bulbs,” and “Pacinian bodies,” play in the mechanism of touch is
unknown. If they are sense organs, they are exceptional in character,
in so far as they do not appear to be modifications of the epidermis.
Nothing is known respecting the organs of those sensations of
resistance which are grouped under the head of the muscular sense;
nor of the sensations of warmth and cold; nor of that very singular
sensation which we call tickling.
In the case of heat and cold, the organism not only becomes affected
by external bodies, far more remote than those which affect the
sense of smell; but the Democritic hypothesis is obviously no longer
permissible. When the direct rays of the sun fall upon the skin,
the sensation of heat is certainly not caused by “attenuated films”
thrown off from that luminary, but is due to a mode of motion which is
transmitted to us. In Aristotelian phrase, it is the form without the
matter of the sun which stamps the sense organ; and this, translated
into modern language, means nearly the same thing as Hartley’s
vibrations. Thus we are prepared for what happens in the case of
the auditory and the visual senses. For neither the ear, nor the
eye, receives anything but the impulses or vibrations originated by
sonorous or luminous bodies. Nevertheless, the receptive apparatus
still consists of nothing but specially modified epithelial cells.
In the labyrinth of the ear of the higher animals, the free ends of
these cells terminate in excessively delicate hair-like filaments;
while, in the lower forms of auditory organ, its free surface is
beset with delicate hairs like those of the surface of the body, and
the transmissive nerves are connected with the bases of these hairs.
Thus there is an insensible gradation in the forms of the receptive
apparatus, from the organ of touch, on the one hand, to those of taste
and smell; and, on the other hand, to that of hearing. Even in the
case of the most refined of all the sense organs, that of vision, the
receptive apparatus departs but little from the general type. The
only essential constituent of the visual sense organ is the retina,
which forms so small a part of the eyes of the higher animals; and the
simplest eyes are nothing but portions of the integument, in which
the cells of the epidermis have become converted into glassy rod-like
retinal corpuscles. The outer ends of these are turned towards the
light; their sides are more or less extensively coated with a dark
pigment, and their inner ends are connected with the transmissive nerve
fibres. The light, impinging on these visual rods, produces a change in
them which is communicated to the nerve fibres, and, being transmitted
to the sensorium, gives rise to the sensation—if indeed all animals
which possess eyes are endowed with what we understand as sensation.
In the higher animals, a complicated apparatus of lenses, arranged
on the principle of a camera obscura, serves at once to concentrate
and to individualise the pencils of light proceeding from external
bodies. But the essential part of the organ of vision is still a layer
of cells, which have the form of rods with truncated or conical ends.
By what seems a strange anomaly, however, the glassy ends of these are
turned not towards, but away from, the light; and the latter has to
traverse the layer of nervous tissues with which their outer ends are
connected, before it can affect them. Moreover, the rods and cones of
the vertebrate retina are so deeply seated, and in many respects so
peculiar in character, that it appears impossible, at first sight, that
they can have anything to do with that epidermis of which gustatory and
tactile, and at any rate the lower forms of auditory and visual, organs
are obvious modifications.
Whatever be the apparent diversities among the sensiferous apparatuses,
however, they share certain common characters. Each consists of a
receptive, a transmissive, and a sensificatory portion. The essential
part of the first is an epithelium, of the second, nerve fibres, of the
third, a part of the brain; the sensation is always the consequence
of the mode of motion excited in the receptive, and sent along the
transmissive, to the sensificatory part of the sensiferous apparatus.
And, in all the senses, there is no likeness whatever between the
object of sense, which is matter in motion, and the sensation, which is
an immaterial phenomenon.
On the hypothesis which appears to me to be the most convenient,
sensation is a product of the sensiferous apparatus caused by certain
modes of motion which are set up in it by impulses from without. The
sensiferous apparatuses are, as it were, factories, all of which at
the one end receive raw materials of a similar kind—namely, modes of
motion—while, at the other, each turns out a special product, the
feeling which constitutes the kind of sensation characteristic of it.
Or, to make use of a closer comparison, each sensiferous apparatus is
comparable to a musical-box wound up; with as many tunes as there are
separate sensations. The object of a simple sensation is the agent
which presses down the stop of one of these tunes, and the more feeble
the agent, the more delicate must be the mobility of the stop.[62]
But, if this be true, if the recipient part of the sensiferous
apparatus is, in all cases, merely a mechanism affected by coarser or
finer kinds of material motion, we might expect to find that all sense
organs are fundamentally alike, and result from the modification of the
same morphological elements. And this is exactly what does result from
all recent histological and embryological investigations.
It has been seen that the receptive part of the olfactory apparatus is
a slightly modified epithelium, which lines an olfactory chamber deeply
seated between the orbits in adult human beings. But, if we trace back
the nasal chambers to their origin in the embryo, we find, that, to
begin with, they are mere depressions of the skin of the fore part
of the head, lined by a continuation of the general epidermis. These
depressions become pits, and the pits, by the growth of the adjacent
parts, gradually acquire the position which they finally occupy. The
olfactory organ, therefore, is a specially modified part of the general
integument.
The human ear would seem to present greater difficulties. For the
essential part of the sense organ, in this case, is the membranous
labyrinth, a bag of complicated form, which lies buried in the depths
of the floor of the skull, and is surrounded by dense and solid bone.
Here, however, recourse to the study of development readily unravels
the mystery. Shortly after the time when the olfactory organ appears,
as a depression of the skin on the side of the fore part of the head,
the auditory organ appears as a similar depression on the side of its
back part. The depression, rapidly deepening, becomes a small pouch;
and then, the communication with the exterior becoming shut off, the
pouch is converted into a closed bag, the epithelial lining of which is
a part of the general epidermis segregated from the rest. The adjacent
tissues, changing first into cartilage and then into bone, enclose
the auditory sac in a strong case, in which it undergoes its further
metamorphoses; while the drum, the ear bones, and the external ear,
are superadded by no less extraordinary modifications of the adjacent
parts. Still more marvellous is the history of the development of the
organ of vision. In the place of the eye, as in that of the nose and
that of the ear, the young embryo presents a depression of the general
integument; but, in man and the higher animals, this does not give
rise to the proper sensory organ, but only to part of the accessory
structures concerned in vision. In fact, this depression, deepening
and becoming converted into a shut sac, produces only the cornea, the
aqueous humour, and the crystalline lens of the perfect eye.