sensible largeness of one sensation vary so much from that of another?_
Only imperfectly. One factor in the result undoubtedly is the number
of nerve-terminations simultaneously excited by the outward agent
that awakens the sensation. When many skin-nerves are warmed, or
much retinal surface illuminated, our feeling is larger than when a
lesser nervous surface is excited. The single sensation yielded by two
compass-points, although it seems simple, is yet felt to be much bigger
and blunter than that yielded by one. The touch of a single point may
always be recognized by its quality of sharpness. This page looks
much smaller to the reader if he closes one eye than if both eyes are
open. So does the moon, which latter fact shows that the phenomenon
has nothing to do with parallax. The celebrated boy couched for the
cataract by Chesselden thought, after his first eye was operated,
"all things he saw extremely large," but being couched of his second
eye, said "that objects at first appeared large to this eye, but not
so large as they did at first to the other; and looking upon the same
object with both eyes, he thought it looked about twice as large as
with the first couched eye only, but not double, that we can anyways
discover."
The greater extensiveness that the feeling of certain parts of the same
surface has over other parts, and that one order of surface has over
another (retina over skin, for example), may also to a certain extent
be explained by the operation of the same factor. It is an anatomical
fact that the most spatially sensitive surfaces (retina, tongue,
finger-tips, etc.) are supplied by nerve-trunks of unusual thickness,
which must supply to every unit of surface-area an unusually large
number of terminal fibres. But the variations of felt extension obey
probably only a very rough law of numerical proportion to the number of
fibres. A sound is not twice as voluminous to two ears as to one; and
the above-cited variations of feeling, when the same surface is excited
under different conditions, show that the feeling is a resultant of
several factors of which the anatomical one is only the principal.
Many ingenious hypotheses have been brought forward to assign the
co-operating factors where different conditions give conflicting
amounts of felt space. Later we shall analyze some of these cases in
detail, but it must be confessed here in advance that many of them
resist analysis altogether.[153]
THE PERCEPTION OF SPATIAL ORDER.
So far, all we have established or sought to establish is the existence
of the vague form or _quale_ of spatiality as an inseparable element
bound up with the other peculiarities of each and every one of our
sensations. The numerous examples we have adduced of the variations of
this extensive element have only been meant to make clear its strictly
sensational character. In very few of them will the reader have been
able to explain the variation by an added intellectual element, such as
the suggestion of a recollected experience. In almost all it has seemed
to be the immediate psychic effect of a peculiar sort of nerve-process
excited; and all the nerve-processes in question agree in yielding
what space they do yield, to the mind, in the shape of a simple total
vastness, in which, _primitively_ at least, no _order of parts_ or of
_subdivisions_ reigns.
Let no one be surprised at this notion of a space without order. There
may be a space without order just as there may be an order without
space.[154] And the primitive perceptions of space are certainly of an
unordered kind. The order which the spaces first perceived potentially
include must, before being distinctly apprehended by the mind, be woven
into those spaces by a rather complicated set of intellectual acts. The
primordial largenesses which the sensations yield must be _measured and
subdivided_ by consciousness, and _added_ together, before they can
form by their synthesis what we know as the real Space of the objective
world. In these operations, imagination, association, attention, and
selection play a decisive part; and although they nowhere add any new
material to the space-data of sense, they so shuffle and manipulate
these data and hide present ones behind imagined ones that it is no
wonder if some authors have gone so far as to think that the sense-data
have no spatial worth at all, and that the intellect, since it makes
the subdivisions, also gives the spatial quality to them out of
resources of its own.
* * * * *
As for ourselves, having found that all our sensations (however as
yet unconnected and undiscriminated) are of extensive objects, _our
next problem, is: How do we_ ARRANGE _these at first chaotically given
spaces into the one regular and orderly 'world of space' which we now
know?_
To begin with, there is no reason to suppose that the several
sense-spaces of which a sentient creature may become conscious, each
filled with its own peculiar content, should tend, simply _because
they are many_, to enter into any definite spatial intercourse with
each other, or lie in any particular order of positions. Even in
ourselves we can recognize this. Different feelings may coexist in us
without assuming any particular spatial order. The sound of the brook
near which I write, the odor of the cedars, the comfort with which my
breakfast has filled me, and my interest in this paragraph, all lie
distinct in my consciousness, but in no sense outside or alongside
of each other. Their spaces are interfused and at most fill the
same vaguely objective world. Even where the qualities are far less
disparate, we may have something similar. If we take our subjective and
corporeal sensations alone, there are moments when, as we lie or sit
motionless, we find it very difficult to feel distinctly the length of
our back or the direction of our feet from our shoulders. By a strong
effort we can succeed in dispersing our attention impartially over our
whole person, and then we feel the real shape of our body in a sort
of unitary way. But in general a few parts are strongly emphasized
to consciousness and the rest sink out of notice; and it is then
remarkable how vague and ambiguous our perception of their relative
order of location is. Obviously, for the orderly arrangement of a
multitude of sense-spaces in consciousness, something more than their
mere separate existence is required. What is this further condition?
_If a number of sensible extents are to be perceived alongside of
each other and in definite order they must appear as parts in a vaster
sensible extent which can enter the mind simply and all at once_. I
think it will be seen that the difficulty of estimating correctly
the form of one's body by pure feeling arises from the fact that it
is very hard to feel its totality as a unit at all. The trouble is
similar to that of thinking forwards and backwards simultaneously. When
conscious of our head we tend to grow unconscious of our feet, and
there enters thus an element of time-succession into our perception
of ourselves which transforms the latter from an act of intuition to
one of construction. This element of constructiveness is present in a
still higher degree, and carries with it the same consequences, when
we deal with objective spaces too great to be grasped by a single
look. The relative positions of the shops in a town, separated by many
tortuous streets, have to be thus constructed from data apprehended in
succession, and the result is a greater or less degree of vagueness.
That a sensation _be discriminated as a part_ from out of a larger
enveloping space is then the _conditio sine quâ non_ of its being
apprehended in a definite spatial order. The problem of ordering
our feelings in space is then, in the first instance, a problem of
discrimination, but not of discrimination pure and simple; for then not
only coexistent sights but coexistent sounds would necessarily assume
such order, which they notoriously do not. Whatever is discriminated
will appear as a small space within a larger space, it is true, but
this is but the very rudiment of order. For the location of it within
that space to become precise, other conditions still must supervene;
and the best way to study what they are will be to pause for a little
and _analyze what the expression 'spatial order' means_.
* * * * *
Spatial order is an abstract term. The concrete perceptions which it
covers are figures, directions, positions, magnitudes, and distances.
To single out any one of these things from a total vastness is
partially to introduce order into the vastness. To subdivide the
vastness into a multitude of these things is to apprehend it in a
completely orderly way. Now what are these things severally? To begin
with, no one can for an instant hesitate to say that some of them
are qualities of sensation, just as the total vastness is in which
they lie. Take figure: a square, a circle, and a triangle appear in
the first instance to the eye simply as three different kinds of
impressions, each so peculiar that we should recognize it if it were to
return. When Nunnely's patient had his cataracts removed, and a cube
and a sphere were presented to his notice, he could at once perceive a
difference in their shapes; and though he could not say which was the
cube and which the sphere, he saw they were not of the same figure.
So of lines: if we can notice lines at all in our field of vision, it
is inconceivable that a vertical one should not affect us differently
from an horizontal one, and should not be recognized as affecting us
similarly when presented again, although we might not yet know the name
'vertical,' or any of its connotations, beyond this peculiar affection
of our sensibility. So of angles: an obtuse one affects our feeling
immediately in a different way from an acute one. Distance-apart, too,
is a simple sensation--the sensation of a line joining the two distant
points: lengthen the line, you alter the feeling and with it the
distance felt.
_Space-relations._
But with distance and direction we pass to the category of
space-_relations_, and are immediately confronted by an opinion which
makes of all relations something _toto cœlo_ different from all facts
of feeling or imagination whatsoever. A relation, for the Platonizing
school in psychology, is an energy of pure thought, and, as such, is
quite incommensurable with the data of sensibility between which it may
be perceived to obtain.
We may consequently imagine a disciple of this school to say to us at
this point: "Suppose you _have_ made a separate specific sensation
of each line and each angle, what boots it? You have still the order
of directions and of distances to account for; you have still the
relative magnitudes of all these felt figures to state; you have their
respective positions to define before you can be said to have brought
order into your space. And not one of these determinations can be
effected except through an act of relating thought, so that your
attempt to give an account of space in terms of pure sensibility breaks
down almost at the very outset. _Position_, for example, can never be
a sensation, for it has nothing intrinsic about it; it can only obtain
_between_ a spot, line, or other figure and _extraneous co-ordinates_,
and can never be an element of the sensible datum, the line or the
spot, in itself. Let us then confess that Thought alone can unlock
the riddle of space, and that Thought is an adorable but unfathomable
mystery."
Such a method of dealing with the problem has the merit of shortness.
Let us, however, be in no such hurry, but see whether we cannot get a
little deeper by patiently considering what these space-relations are.
'Relation' is a very slippery word. It has so many different concrete
meanings that the use of it as an abstract universal may easily
introduce bewilderment into our thought. We must therefore be careful
to avoid ambiguity by making sure, wherever we have to employ it,
what its precise meaning is in that particular sphere of application.
At present we have to do with space-relations, and no others. Most
'relations' are feelings of an entirely different order from the terms
they relate. The relation of similarity, e.g., may equally obtain
between jasmine and tuberose, or between Mr. Browning's verses and Mr.
Story's; it is itself neither odorous nor poetical, and those may well
be pardoned who have denied to it all sensational content whatever. But
just as, in the field of quantity, the relation between two numbers is
another number, so _in the field of space the relations are facts of
the same order with the facts they relate. If these latter be patches
in the circle of vision, the former are certain other patches between
them._ When we speak of the relation of direction of two points toward
each other, we mean simply the sensation of the line that joins the
two points together. _The line is the relation;_ feel it and you
feel the relation, see it and you see the relation; nor can you in
any conceivable way think the latter except by imagining the former
(however vaguely), or describe or indicate the one except by pointing
to the other. And the moment you have imagined the line, the relation
stands before you in all its completeness, with nothing further to
be done. Just so the relation of _direction_ between two lines is
identical with the peculiar sensation of shape of the space enclosed
between them. This is commonly called an angular relation.
If these relations are sensations, no less so are the relations of
position. _The relation of position between the top and bottom points
of a vertical line is that line,_ and nothing else. The relations of
position between a point and a horizontal line below it are potentially
numerous. There is one more important than the rest, called its
distance. This is the sensation, ideal or actual, of a perpendicular
drawn from the point to the line.[155] Two lines, one from each
extremity of the horizontal to the point, give us a peculiar sensation
of triangularity. This feeling may be said to constitute the _locus_ of
all the relations of position of the elements in question. _Rightness
and leftness, upness and downness, are again pure sensations_ differing
specifically from each other, and generically from everything else.
Like all sensations, they can only be indicated, not described. If
we take a cube and label one side _top_, another _bottom_, a third
_front_, and a fourth _back_, there remains no form of words by which
we can describe to another person which of the remaining sides is
_right_ and which _left_. We can only point and say _here_ is right and
_there_ is left, just as we should say _this_ is red and _that_ blue.
Of two points seen beside each other at all, one is always affected by
one of these feelings, and the other by the opposite; the same is true
of the extremities of any line.[156]
Thus it appears indubitable that all space-relations except those of
magnitude are nothing more or less than pure sensational objects. But
_magnitude_ appears to outstep this narrow sphere. We have relations
of muchness and littleness between times, numbers, intensities, and
qualities, as well as spaces. It is impossible, then, that such
relations should form a particular kind of simply spatial feeling.
This we must admit: the relation of quantity is generic and occurs in
many categories of consciousness, whilst the other relations we have
considered are specific and occur in space alone. When our attention
passes from a shorter line to a longer, from a smaller spot to a
larger, from a feebler light to a stronger, from a paler blue to a
richer, from a march tune to a galop, the transition is accompanied
in the synthetic field of consciousness by a peculiar feeling of
difference which is what we call the sensation of _more_,--more length,
more expanse, more light, more blue, more motion. This transitional
sensation of _more_ must be identical with itself under all these
different accompaniments, or we should not give it the same name in
every case. We get it when we pass from a short vertical line to a long
horizontal one, from a small square to a large circle, as well as when
we pass between those figures whose shapes are congruous. But when the
shapes are congruous our consciousness of the relation is a good deal
more distinct, and it is most distinct of all when, in the exercise
of our analytic attention, we notice, first, a _part_, and then the
_whole_, of a _single_ line or shape. Then the _more_ of the whole
actually sticks out, as a separate piece of space, and is so envisaged.
The same exact sensation of it is given when we are able to superpose
one line or figure on another. This indispensable condition of exact
measurement of the _more_ has led some to think that the feeling
itself arose in every case from original experiences of superposition.
This is probably not an absolutely true opinion, but for our present
purpose that is immaterial. So far as the subdivisions of a sense-space
are to be _measured_ exactly against each other, objective forms
occupying one subdivision must directly or indirectly be superposed
upon the other, and the mind must get the immediate feeling of an
outstanding _plus_. And even where we only feel one subdivision to be
vaguely larger or less, the mind must pass rapidly between it and the
other subdivision, and receive the immediate sensible shock of the
_more_.
* * * * *
_We seem thus to have accounted for all space-relations, and made
them clear to our understanding. They are nothing but sensations of
particular lines, particular angles, particular forms of transition,
or_ (in the case of a _distinct more) of particular outstanding
portions of space after two figures have been superposed._ These
relation-sensations may actually be produced as such, as when a
geometer draws new lines across a figure with his pencil to demonstrate
the relations of its parts, or they may be ideal representations of
lines, not really drawn. But in either case their entrance into the
mind is equivalent to a more detailed subdivision, cognizance, and
measurement of the space considered. _The bringing of subdivisions to
consciousness constitutes, then, the entire process by which we pass
from our first vague feeling of a total vastness to a cognition of
the vastness in detail._ The more numerous the subdivisions are, the
more elaborate and perfect the cognition becomes. But inasmuch as all
the subdivisions are themselves sensations, and even the feeling of
'more' or 'less' is, where not itself a figure, at least a sensation
of transition between two sensations of figure, it follows, for aught
we can as yet see to the contrary, that _all spatial knowledge is
sensational at bottom_, and that, as the sensations lie together in the
unity of consciousness, no new material element whatever comes to them
from a supra-sensible source.[157]
_The bringing of subdivisions to consciousness! This, then, is our next
topic._ They may be brought to consciousness under three aspects in
respect of their _locality_, in respect of their _size_, in respect of
their _shape_.
_The Meaning of Localization._
_Confining ourselves to the problem of locality_ for the present,
let us begin with the simple case of a sensitive surface, only two
points of which receive stimulation from without. How, first, are
these two points felt as alongside of each other with an interval of
space between them? We must be conscious of two things for this: of
the duality of the excited points, and of the extensiveness of the
unexcited interval. The duality alone, although a necessary, is not a
sufficient condition of the spatial separation. We may, for instance,
discern two sounds in the same place, sweet and sour in the same
lemonade, warm and cold, round and pointed contact in the same place
on the skin, etc.[158] In all discrimination the recognition of the
duality of two feelings by the mind is the easier the more strongly the
feelings are contrasted in quality. If our two excited points awaken
identical qualities of sensation, they must, perforce, appear to the
mind as one; and, not distinguished at all, they are, _a fortiori_, not
localized apart. Spots four centimetres distant on the back have no
qualitative contrast at all, and fuse into a single sensation. Points
less than three thousandths of a millimetre apart awaken on the retina
sensations so contrasted that we apprehend them immediately as two. Now
these unlikenesses which arise so slowly when we pass from one point to
another in the back, so much faster on the tongue and finger-tips, but
with such inconceivable rapidity on the retina, what are they? Can we
discover anything about their intrinsic nature?
The most natural and immediate answer to make is that they are
unlikeness of _place_ pure and simple. In the words of a German
physiologist,[159] to whom psychophysics owes much:
"The sensations are from the outset (_von vornherein_) localized....
Every sensation as such is from the very beginning affected with the
spatial quality, so that this quality is nothing like an external
attribute coming to the sensation from a higher faculty, but must be
regarded as something immanently residing in the sensation itself."
And yet the moment we reflect on this answer an insuperable logical
difficulty seems to present itself. No single _quale_ of sensation can,
by itself, amount to a consciousness of _position_. Suppose no feeling
but that of a single point ever to be awakened. Could that possibly be
the feeling of any special _whereness_ or _thereness_? Certainly not.
_Only when a second point is felt to arise can the first one acquire
a determination of up, down, right or left, and these determinations
are all relative to that second point._ Each point, so far as it is
_placed, is_ then only by virtue of what it _is not,_ namely, by virtue
of another point. This is as much as to say that position has nothing
_intrinsic_ about it; and that, although a feeling of absolute bigness
may, _a feeling of place cannot, possibly form an immanent element in
any single isolated sensation._ The very writer we have quoted has
given heed to this objection, for he continues (p. 335) by saying that
the sensations thus originally localized "are only so _in themselves_,
but not in the representation of consciousness, which is not yet
present.... They are, in the first instance, devoid of all mutual
relations with each other." But such a localization of the sensation
'in itself' would seem to mean nothing more than the susceptibility or
_potentiality_ of being distinctly localized when the time came and
other conditions became fulfilled. Can we now discover anything about
such susceptibility in itself before it has borne its ulterior fruits
in the developed consciousness?
_'Local Signs.'_
To begin with, every sensation of the skin and every visceral sensation
seems to derive from its topographic seat a peculiar shade of feeling,
which it would not have in another place. And this feeling _per se_
seems quite another thing from the perception of the place. Says
Wundt[160]:
"If with the finger we touch first the cheek and then the palm,
exerting each time precisely the same pressure, the sensation shows
notwithstanding a distinctly marked difference in the two cases.
Similarly, when we compare the palm with the back of the hand, the
nape of the neck with its anterior surface, the breast with the
back; in short, any two distant parts of the skin with each other.
And moreover, we easily remark, by attentively observing, that spots
even tolerably close together differ in respect of the quality of
their feeling. If we pass from one point of our cutaneous surface to
another, we find a perfectly gradual and continuous alteration in
our feeling, notwithstanding the objective nature of the contact has
remained the same. Even the sensations of corresponding points on
opposite sides of the body, though similar, are not identical. If,
for instance, we touch first the back of one hand and then of the
other, we remark a qualitative unlikeness of sensation. It must not
be thought that such differences are mere matters of imagination, and
that we take the sensations to be different because we represent each
of them to ourselves as occupying a different place. With sufficient
sharpening of the attention, we may, confining ourselves to the
quality of the feelings alone, entirely abstract from their locality,
and yet notice the differences quite as markedly."
Whether these local contrasts shade into each other with absolutely
continuous gradations, we cannot say. But we know (continues Wundt) that
"they change, when we pass from one point of the skin to its neighbor,
with very different degrees of rapidity. On delicately-feeling parts,
used principally for touching, such as the finger-tips, the difference
of sensation between two closely approximate points is already
strongly pronounced; whilst in parts of lesser delicacy, as the arm,
the back, the legs, the disparities of sensation are observable only
between distant spots."
The internal organs, too, have their specific _qualia_ of sensation. An
inflammation of the kidney is different from one of the liver; pains in
joints and muscular insertions are distinguished. Pain in the dental
nerves is wholly unlike the pain of a burn. But very important and
curious similarities prevail throughout these differences. Internal
pains, whose seat we cannot see, and have no means of knowing unless
the character of the pain itself reveal it, are felt _where_ they
belong. Diseases of the stomach, kidney, liver, rectum, prostate,
etc., of the bones, of the brain and its membranes, are referred
to their proper position. Nerve-pains describe the length of the
nerve. Such localizations as those of vertical, frontal, or occipital
headache of intracranial origin force us to conclude that parts which
are neighbors, whether inner or outer, may possess by mere virtue of
that fact a common peculiarity of feeling, a respect in which their
sensations agree, and which serves as a token of their proximity. These
_local_ colorings are, moreover, so strong that we cognize them as the
same, throughout all contrasts of sensible quality in the accompanying
perception. Cold and heat are wide as the poles asunder; yet if both
fall on the cheek, there mixes with them something that makes them in
_that respect_ identical; just as, contrariwise, despite the identity
of cold with itself wherever found, when we get it first on the palm
and then on the cheek, some difference comes, which keeps the two
experiences for ever asunder.[161]
And now let us revert to the query propounded a moment since: _Can
these differences of mere quality in feeling, varying according to
locality yet having each sensibly and intrinsically and by itself
nothing to do with position, constitute the 'susceptibilities' we
mentioned, the conditions of being perceived in position, of the
localities to which they belong?_ The numbers on a row of houses, the
initial letters of a set of words, have no intrinsic kinship with
points of space, and yet they are the conditions of our knowledge of
where any house is in the row, or any word in the dictionary. Can the
modifications of feeling in question be tags or labels of this kind
which in no wise originally reveal the position of the spot to which
they are attached, but guide us to it by what Berkeley would call
a 'customary tie'? Many authors have unhesitatingly replied in the
affirmative; Lotze, who in his Medizinische Psychologie[162] first
described the sensations in this way, designating them, thus conceived,
as _local-signs_. This term has obtained wide currency in Germany, and
_in speaking of the_ 'LOCAL-SIGN THEORY' _hereafter, I shall always
mean the theory which denies that there can be in a sensation any
element of actual locality, of inherent spatial order,_ any tone as
it were which cries to us immediately and without further ado, 'I am
_here_,' or 'I am _there_.'
If, as may well be the case, we by this time find ourselves tempted
to accept the Local-sign theory in a general way, we have to clear
up several farther matters. If a sign is to lead us to _the thing_
it means, we must have some other source of knowledge of that thing.
Either the thing has been given in a previous experience of which the
sign also formed part--they are _associated_; or it is what Reid calls
a 'natural' sign, that is, a feeling which, the first time it enters
the mind, evokes from the native powers thereof a cognition of the
thing that hitherto had lain dormant. In both cases, however, the sign
is one thing, and the thing another. In the instance that now concerns
us, _the sign is a quality of feeling and the thing is a position_.
Now we have seen that the position of a point is not only revealed,
but created, by the existence of other points to which it stands in
determinate _relations. If the sign can by any machinery which it sets
in motion evoke a consciousness either of the other points, or of the
relations, or of both, it would seem to fulfil its function, and reveal
to us the position we seek._
But such a machinery is already familiar to us. It is neither more nor
less than the law of habit in the nervous system. When any point of the
sensitive surface has been frequently excited simultaneously with, or
immediately before or after, other points, and afterwards comes to be
excited alone, there will be a tendency for its perceptive nerve-centre
to irradiate into the nerve-centres of the other points. Subjectively
considered, this is the same as if we said that _the peculiar feeling
of the first point_ SUGGESTS _the feeling of the entire region with
whose stimulation its own excitement has been habitually_ ASSOCIATED.
Take the case of the stomach. When the epigastrium is heavily pressed,
when certain muscles contract, etc., the stomach is squeezed, and its
peculiar local sign awakes in consciousness simultaneously with the
local signs of the other squeezed parts. There is also a sensation of
total vastness aroused by the combined irritation, and _somewhere_
in this the stomach-feeling seems to lie. Suppose that later a pain
arises in the stomach from some non-mechanical cause. It will be
tinged by the gastric local sign, and the nerve-centre supporting
this latter feeling will excite the centre supporting the dermal and
muscular feelings habitually associated with it when the excitement was
mechanical. From the combination the same peculiar vastness will again
arise. In a word, 'something' in the stomach-sensation 'reminds' us of
a total space, of which the diaphragmatic and epigastric sensations
also form a part, or, to express it more briefly still, suggests the
neighborhood of these latter organs.[163]
Revert to the case of two excited points on a surface with an unexcited
space between them. The general result of previous experience has
been that when either point was impressed by an outward object, the
same object also touched the immediately neighboring parts. Each
point, together with its local sign, is thus associated with a circle
of surrounding points, the association fading in strength as the
circle grows larger. Each will revive its own circle; but when both
are excited together, the strongest revival will be that due to the
_combined_ irradiation. Now the tract _joining the two excited points_
is the only part common to the two circles. And the feelings of this
whole tract will therefore awaken with considerable vividness in the
imagination when its extremities are touched by an outward irritant.
The mind receives with the impression of the two distinct points the
vague idea of a line. The twoness of the points comes from the contrast
of their local signs: the line comes from the associations into which
experience has wrought these latter. If no ideal line arises we have
duality without sense of interval; if the line be excited actually
rather than ideally, we have the interval given with its ends, in
the form of a single extended object felt. E. H. Weber, in the famous
article in which he laid the foundations of all our accurate knowledge
of these subjects, _laid it down as the logical requisite for the
perception of two separated points, that the mind should, along with
its consciousness of them, become aware of an unexcited interval as
such. I have only tried to show how the known laws of experience may
cause this requisite to be fulfilled._ Of course, if the local signs of
the entire region offer but little qualitative contrast _inter se_, the
line suggested will be but dimly defined or discriminated in length or
direction from other possible lines in its neighborhood. This is what
happens in the back, where consciousness can sunder two spots, whilst
only vaguely apprehending their distance and direction apart.
The relation of position of the two points _is_ the suggested interval
or line. Turn now to the simplest case, that of _a single excited spot.
How can it suggest its position?_ Not by recalling any particular
line unless experience have constantly been in the habit of marking
or tracing some one line from it towards some one neighboring point.
Now on the back, belly, viscera, etc., no such tracing habitually
occurs. The consequence is that the only suggestion is that of the
whole neighboring circle; i.e., _the spot simply recalls the general
region in which it happens to lie._ By a process of successive
construction, it is quite true that we can also get the feeling of
distance between the spot and some other particular spot. Attention,
by reinforcing the local sign of one part of the circle, can awaken a
new circle round this part, and so _de proche en proche_ we may slide
our feeling down from our cheek, say, to our foot. But when we first
touched our cheek we had no consciousness of the foot at all.[164] In
the extremities, the lips, the tongue and other mobile parts, the case
is different. We there have an instinctive tendency, when a part of
lesser discriminative sensibility is touched, to move the member so
that the touching object glides along it to the place where sensibility
is greatest. If a body touches our hand we move the hand over it
till the finger-tips are able to explore it. If the sole of our foot
touches anything we bring it towards the toes, and so forth. There
thus arise lines of habitual passage from all points of a member to
its sensitive tip. These are the lines most readily recalled when any
point is touched, and their recall is identical with the consciousness
of the distance of the touched point from the 'tip.' I think anyone
must be aware when he touches a point of his hand or wrist that it is
the relation to the finger-tips of which he is usually most conscious.
Points on the forearm suggest either the finger-tips or the elbow (the
latter being a spot of greater sensibility[165]). In the foot it is the
toes, and so on. A point can only be cognized in its relations to the
entire body at once by awakening a _visual_ image of the whole body.
Such awakening is even more obviously than the previously considered
cases a matter of pure association.
* * * * *
_This leads us to the eye._ On the retina the fovea and the yellow spot
about it form a focus of exquisite sensibility, towards which every
impression falling on an outlying portion of the field is moved by an
instinctive action of the muscles of the eyeball. Few persons, until
their attention is called to the fact, are aware how almost impossible
it is to keep a conspicuous visible object in the margin of the field
of view. The moment volition is relaxed we find that without our
knowing it our eyes have turned so as to bring it to the centre. This
is why most persons are unable to keep the eyes steadily converged upon
a point in space with nothing in it. The objects against the walls of
the room invincibly attract the foveæ to themselves. If we contemplate
a blank wall or sheet of paper, we always observe in a moment that we
are directly looking at some speck upon it which, unnoticed at first,
ended by 'catching our eye.' Thus _whenever an image falling on the
point P of the retina excites attention, it more habitually moves
from that point towards the fovea than in any one other direction._
The line traced thus by the image is not always a straight line. When
the direction of the point from the fovea is neither vertical nor
horizontal but oblique, the line traced is often a curve, with its
concavity directed upwards if the direction is upwards, downwards if
the direction is downwards. This may be verified by anyone who will
take the trouble to make a simple experiment with a luminous body like
a candle-flame in a dark enclosure, or a star. Gazing first at some
point remote from the source of light, let the eye be suddenly turned
full upon the latter. The luminous image will necessarily fall in
succession upon a continuous series of points, reaching from the one
first affected to the fovea. But by virtue of the slowness with which
retinal excitements die away, the entire series of points will for an
instant be visible as an after-image, displaying the above peculiarity
of form according to its situation.[166] These radiating lines are
neither regular nor invariable in the same person, nor, probably,
equally curved in different individuals. We are incessantly drawing
them between the fovea and every point of the field of view. Objects
remain in their peripheral indistinctness only so long as they are
unnoticed. The moment we attend to them they grow distinct through one
of these motions--which leads to the idea prevalent among uninstructed
persons that we see distinctly all parts of the field of view at once.
_The result of this incessant tracing of radii is that whenever a local
sign P is awakened by a spot of light falling upon it, it recalls
forthwith, even though the eyeball be unmoved, the local signs of all
the other points which lie between P and the fovea._ It recalls them in
imaginary form, just as the normal reflex movement would recall them
in vivid form; and with their recall is given a consciousness more or
less faint of the whole line on which they lie. In other words, no
ray of light can fall on any retinal spot without the local sign of
that spot revealing to us, by recalling the line of its most habitual
associates, its direction and distance from the centre of the field.
The fovea acts thus as the origin of a system of polar co-ordinates,
in relation to which each and every retinal point has through an
incessantly-repeated process of association its distance and direction
determined. Were _P_ alone illumined and all the rest of the field dark
we should still, even with motionless eyes, know whether _P_ lay high
or low, right or left, through the _ideal streak_, different from all
other streaks, which _P_ alone has the power of awakening.[167]
And with this we can close the first great division of our subject. We
have shown that, within the range of every sense, experience takes
_ab initio_ the spatial form. We have also shown that in the cases
of the retina and skin every sensible total may be subdivided by
discriminative attention into sensible parts, which are also spaces,
and into relations between the parts, these being sensible spaces
too. Furthermore, we have seen (in note 167) that different parts,
once discriminated, necessarily fall into a determinate order, both
by reason of definite gradations in their quality, and by reason of
the fixed order of time-succession in which movements arouse them. But
in all this nothing has been said of the comparative _measurement_ of
one sensible space-total against another, or of the way in which, by
summing our divers simple sensible space-experiences together, we end
by constructing what we regard as the unitary, continuous, and infinite
objective Space of the real world. To this more difficult inquiry we
next pass.
THE CONSTRUCTION OF 'REAL' SPACE.
The problem breaks into two subordinate problems.
(1) _How is the subdivision and measurement of the several sensorial
spaces completely effected?_ and
(2) _How do their mutual addition and fusion and reduction to the same
scale, in a word, how does their synthesis, occur?_
I think that, as in the investigation just finished, we found ourselves
able to get along without invoking any data but those that pure
sensibility on the one hand, and the ordinary intellectual powers of
discrimination and recollection on the other, were able to yield;
so here we shall emerge from our more complicated quest with the
conviction that all the facts can be accounted for on the supposition
that no other mental forces have been at work save those we find
everywhere else in psychology: sensibility, namely, for the data; and
discrimination, association, memory, and choice for the rearrangements
and combinations which they undergo.
1. _The Subdivision of the Original Sense-spaces._
How are spatial subdivisions brought to consciousness? in other
words, How does spatial discrimination occur? The general subject of
discrimination has been treated in a previous chapter. Here we need
only inquire what are the conditions that make spatial discrimination
so much finer in sight than in touch, and in touch than in hearing,
smell, or taste.
_The first great condition is, that different points of the surface
shall differ in the quality of their immanent sensibility,_ that
is, that each shall carry its special local-sign. If the skin felt
everywhere exactly alike, a foot-bath could be distinguished from a
total immersion, as being smaller, but never distinguished from a
wet face. The local-signs are indispensable; two points which have
the same local-sign will always be felt as the same point. We do
not judge them two unless we have discerned their sensations to be
different.[168] Granted none but homogeneous irritants, that organ
would then distinguish the greatest multiplicity of irritants--would
count most stars or compass-points, or best compare the size of two
wet surfaces--whose local sensibility was the least even. A skin whose
sensibility shaded rapidly off from a focus, like the apex of a boil,
would be better than a homogeneous integument for spatial perception.
The retina, with its exquisitely sensitive fovea, has this peculiarity,
and undoubtedly owes to it a great part of the minuteness with which
we are able to subdivide the total bigness of the sensation it yields.
On its periphery the local differences do not shade off very rapidly,
and we can count there fewer subdivisions.
_But these local differences of feeling, so long as the surface is
unexcited from without, are almost null._ I cannot feel them by a pure
mental act of attention unless they belong to quite distinct parts
of the body, as the nose and the lip, the finger-tip and the ear;
their contrast needs the reinforcement of outward excitement to be
felt. In the spatial muchness of a colic--or, to call it by the more
spacious-sounding vernacular, of a 'bellyache'--one can with difficulty
distinguish the north-east from the south-west corner, but can do
so much more easily if, by pressing one's finger against the former
region, one is able to make the pain there more intense.
_The local differences require then an adventitious sensation,
superinduced upon them, to awaken the attention._ After the attention
has once been awakened in this way, it may continue to be conscious of
the unaided difference; just as a sail on the horizon may be too faint
for us to notice until someone's finger, placed against the spot, has
pointed it out to us, but may then remain visible after the finger has
been withdrawn. But all this is true only on condition that separate
points of the surface may be _exclusively_ stimulated. If the whole
surface at once be excited from without, and homogeneously, as, for
example, by immersing the body in salt water, local discrimination
is not furthered. The local-signs, it is true, all awaken at once;
but in such multitude that no one of them, with its specific quality,
stands out in contrast with the rest. If, however, a single extremity
be immersed, the contrast between the wet and dry parts is strong,
and, at the surface of the water especially, the local-signs attract
the attention, giving the feeling of a ring surrounding the member.
Similarly, two or three wet spots separated by dry spots, or two
or three hard points against the skin, will help to break up our
consciousness of the latter's bigness. In cases of this sort, where
points receiving an identical kind of excitement are, nevertheless,
felt to be locally distinct, and the objective irritants are also
judged multiple,--e.g., compass-points on skin or stars on retina,--the
ordinary explanation is no doubt just, and we judge the outward causes
to be multiple because we have discerned the local feelings of their
sensations to be different.
_Capacity for partial stimulation is thus the second condition favoring
discrimination._ A sensitive surface which has to be excited in all its
parts at once can yield nothing but a sense of undivided largeness.
This appears to be the case with the olfactory, and to all intents and
purposes with the gustatory, surfaces. Of many tastes and flavors, even
simultaneously presented, each affects the totality of its respective
organ, each appears with the whole vastness given by that organ, and
appears interpenetrated by the rest.[169]
I should have been willing some years ago to name without hesitation a
third condition of discrimination--saying it would be most developed
in that organ which is susceptible of the _most various qualities_ of
feeling. The retina is unquestionably such an organ. The colors and
shades it perceives are infinitely more numerous than the diversities
of skin-sensation. And it can feel at once white and black, whilst the
ear can in nowise so feel sound and silence. But the late researches
of Donaldson, Blix, and Goldscheider,[170] on specific points for
heat, cold, pressure, and pain in the skin; the older ones of Czermak
(repeated later by Klug in Ludwig's laboratory), showing that a hot and
a cold compass-point are no more easily discriminated as two than two
of equal temperature; and some unpublished experiments of my own--all
disincline me to make much of this condition now.[171] There is,
however, one quality of sensation which is particularly exciting, and
that is the _feeling of motion over any of our surfaces_. The erection
of this into a separate elementary quality of sensibility is one of the
most recent of psychological achievements, and is worthy of detaining
us a while at this point.
_The Sensation of Motion over Surfaces._
_The feeling of motion_ has generally been assumed by physiologists to
be impossible until the positions of _terminus a quo_ and _terminus
ad quem_ are severally cognized, and the successive occupancies of
these positions by the moving body are perceived to be separated by
a distinct interval of time.[172] As a matter of fact, however, we
cognize only the very slowest motions in this way. Seeing the hand
of a clock at XII and afterwards at VI, we judge that it has moved
through the interval. Seeing the sun now in the east and again in the
west, I infer it to have passed over my head. But we can only _infer_
that which we already generically know in some more direct fashion,
and it is experimentally certain that we have the feeling of motion
given us as a direct and simple _sensation_. Czermak long ago pointed
out the difference between seeing the motion of the second-hand of a
watch, when we look directly at it, and noticing the fact of its having
altered its position when we fix our gaze upon some other point of the
dial-plate. In the first case we have a specific quality of sensation
which is absent in the second. If the reader will find a portion of
his skin--the arm, for example--where a pair of compass-points an inch
apart are felt as one impression, and if he will then trace lines
a tenth of an inch long on that spot with a pencil-point, he will
be distinctly aware of the point's motion and vaguely aware of the
direction of the motion. The perception of the motion here is certainly
not derived from a pre-existing knowledge that its starting and ending
points are separate positions in space, because positions in space ten
times wider apart fail to be discriminated as such when excited by
the dividers. It is the same with the retina. One's fingers when cast
upon its peripheral portions cannot be counted--that is to say, the
five retinal tracts which they occupy are not distinctly apprehended
by the mind as five separate positions in space--and yet the slightest
_movement_ of the fingers is most vividly perceived as movement and
nothing else. It is thus certain that our sense of movement, being
so much more delicate than our sense of position, cannot possibly be
derived from it. _A curious observation by Exner_[173] completes the
proof that movement is a primitive form of sensibility, by showing it
to be much more delicate than our sense of succession in time. This
very able physiologist caused two electric sparks to appear in rapid
succession, one beside the other. The observer had to state whether
the right-hand one or the left-hand one appeared first. When the
interval was reduced to as short a time as 0.044'' the discrimination
of temporal order in the sparks became impossible. But Exner found
that if the sparks were brought so close together in space that their
irradiation-circles overlapped, the eye then felt their flashing as if
it were the motion of a single spark from the point occupied by the
first to the point occupied by the second, and the time-interval might
then be made as small as 0.015'' before the mind began to be in doubt
as to whether the apparent motion started from the right or from the
left. On the skin similar experiments gave similar results.
_Vierordt, at almost the same time,_[174] _called attention to certain
persistent illusions, amongst which are these:_ If another person
gently trace a line across our wrist or finger, the latter being
stationary, it will feel to us as if the member were moving in the
opposite direction to the tracing point. If, on the contrary, we
move our limb across a fixed point, it will be seen as if the point
were moving as well. If the reader will touch his forehead with his
forefinger kept motionless, and then rotate the head so that the skin
of the forehead passes beneath the finger's tip, he will have an
irresistible sensation of the latter being itself in motion in the
opposite direction to the head. So in abducting the fingers from each
other; some may move and the rest be still still, but the still ones
will feel as if they were actively separating from the rest. These
illusions, according to Vierordt, are survivals of a primitive form of
perception, when motion was felt as such, but ascribed to the whole
content of consciousness, and not yet distinguished as belonging
exclusively to one of its parts. When our perception is fully developed
we go beyond the mere relative motion of thing and ground, and can
ascribe absolute motion to one of these components of our total
object, and absolute rest to another. When, in vision for example,
the whole background moves together, we think that it is ourselves or
our eyes which are moving; and any object in the foreground which may
move relatively to the background is judged by us to be still. But
primitively this discrimination cannot be perfectly made. The sensation
of the motion spreads over all that we see and infects it. Any relative
motion of object and retina both makes the object seem to move, and
makes us feel ourselves in motion. Even now when our whole object moves
we still get giddy; and we still see an apparent motion of the entire
field of view, whenever we suddenly jerk our head and eyes or shake
them quickly to and fro. Pushing our eyeballs gives the same illusion.
We _know_ in all these cases what really happens, but the conditions
are unusual, so our primitive sensation persists unchecked. So it does
when clouds float by the moon. We _know_ the moon is still; but we
_see_ it move even faster than the clouds. Even when we slowly move our
eyes the primitive sensation persists under the victorious conception.
If we notice closely the experience, we find that any object towards
which we look appears moving to meet our eye.
But the most valuable contribution to the subject is the paper of G.
H. Schneider,[175] who takes up the matter zoologically, and shows by
examples from every branch of the animal kingdom that movement is the
quality by which animals most easily attract each other's attention.
The instinct of 'shamming death' is no shamming of death at all, but
rather a paralysis through fear, which saves the insect, crustacean,
or other creature from being _noticed at all_ by his enemy. It is
parallelled in the human race by the breath-holding stillness of the
boy playing 'I spy,' to whom the seeker is near; and its obverse side
is shown in our involuntary waving of arms, jumping up and down, and
so forth, when we wish to attract someone's attention at a distance.
Creatures 'stalking' their prey and creatures hiding from their
pursuers alike show how immobility diminishes conspicuity. In the
woods, if we are quiet, the squirrels and birds will actually touch us.
Flies will light on stuffed birds and stationary frogs.[176] On the
other hand, the tremendous shock of feeling the thing we are sitting
on begin to move, the exaggerated start it gives us to have an insect
unexpectedly pass over our skin, or a cat noiselessly come and snuffle
about our hand, the excessive reflex effects of tickling, etc., show
how exciting the sensation of motion is _per se_. A kitten cannot help
pursuing a moving ball. Impressions too faint to be cognized at all are
immediately felt if they move. A fly sitting is unnoticed,--we feel it
the moment it crawls. A shadow may be too faint to be perceived. As
soon as it moves, however, we see it. Schneider found that a shadow,
with distinct outline, and directly fixated, could still be perceived
when moving, although its objective strength might be but half as great
as that of a stationary shadow so faint as just to disappear. With a
blurred shadow in indirect vision the difference in favor of motion was
much greater--namely, 13.3:40.7. If we hold a finger between our closed
eyelid and the sunshine we shall not notice its presence. The moment we
move it to and fro, however, we discern it. Such visual perception as
this reproduces the conditions of sight among the radiates.[177]
Enough has now been said to show that _in the education of spatial
discrimination the motions of impressions across sensory surfaces must
have been the principal agent_ in breaking up our consciousness of the
surfaces into a consciousness of their parts. Even to-day the main
function of the peripheral regions of our retina is that of sentinels,
which, when beams of light move over them, cry 'Who goes there?' and
call the fovea to the spot. Most parts of the skin do but perform
the same office for the finger-tips. Of course finger-tips and fovea
leave _some_ power of direct perception to marginal retina and skin
respectively. But it is worthy of note that such perception is best
developed on the skin of the most movable parts (the labors of Vierordt
and his pupils have well shown this); and that in the blind, whose skin
is exceptionally discriminative, it seems to have become so through
the inveterate habit which most of them possess of twitching and
moving it under whatever object may touch them, so as to become better
acquainted with the conformation of the same. Czermak was the first to
notice this. It may be easily verified. Of course _movement of surface
under object is (for purposes of stimulation) equivalent to movement
of object over surface._ In exploring the shapes and sizes of things
by either eye or skin the movements of these organs are incessant and
unrestrainable. Every such movement draws the points and lines of the
object across the surface, imprints them a hundred times more sharply,
and drives them home to the attention. The immense part thus played by
movements in our perceptive activity is held by many psychologists[178]
to prove that the muscles are themselves the space-perceiving organ.
Not surface-sensibility, but 'the muscular sense,' is for these writers
the original and only revealer of objective extension. But they have
all failed to notice with what peculiar intensity muscular contractions
call surface-sensibilities into play, and that the mere discrimination
of impressions (quite apart from any question of measuring the space
between them) largely depends on the mobility of the surface upon which
they fall.[179]
2. _The Measurement of the sense-spaces against each other._
What precedes is all we can say in answer to the problem of
discrimination. Turn now to that of measurement of the several spaces
against each other, that being the first step in our constructing out
of our diverse space-experiences the one space we believe in as that of
the real world.
The first thing that seems evident is that we have no _immediate_
power of comparing together with any accuracy the extents revealed by
different sensations. Our mouth-cavity feels indeed to itself smaller,
and to the tongue larger, than it feels to the finger or eye, our
tympanic membrane feels larger than our finger-tip, our lips feel
larger than a surface equal to them on our thigh. So much comparison is
immediate; but it is vague; and for anything exact we must resort to
other help.
_The great agent in comparing the extent felt by one sensory surface
with that felt by another, is superposition--superposition of one
surface upon another, and superposition of one outer thing upon many
surfaces._ Thus are exact equivalencies and common measures introduced,
and the way prepared for numerical results.
Could we not superpose one part of our skin upon another, or one object
on both parts, we should hardly succeed in coming to that knowledge of
our own form which we possess. The original differences of bigness of
our different parts would remain vaguely operative, and we should have
no certainty as to how much lip was equivalent to so much forehead, how
much finger to so much back.
But with the power of exploring one part of the surface by another
we get a direct perception of cutaneous equivalencies. The primitive
differences of bigness are overpowered when we feel by an immediate
sensation that a certain length of thigh-surface is in contact with
the entire palm and fingers. And when a motion of the opposite
finger-tips draws a line first along this same length of thigh and
then along the whole of the hand in question, we get a new manner of
measurement, less direct but confirming the equivalencies established
by the first. In these ways, by superpositions of parts and by tracing
lines on different parts by identical movements, a person deprived of
sight can soon learn to reduce all the dimensions of his body to a
homogeneous scale. By applying the same methods to objects of his own
size or smaller, he can with equal ease make himself acquainted with
their extension stated in terms derived from his own bulk, palms, feet,
cubits, spans, paces, fathoms (armspreads), etc. In these reductions it
is to be noticed that _when the resident sensations of largeness of two
opposed surfaces conflict, one of the sensations is chosen as the true
standard and the other treated as illusory. Thus an empty tooth-socket
is believed to be_ really smaller than the finger-tip which it will not
admit, although it may _feel_ larger; and in general it may be said
that the hand, as the almost exclusive organ of palpation, gives its
own magnitude to the other parts, instead of having its size determined
by them. In general, it is, as Fechner says, the extent felt by the
more sensitive part to which the other extents are reduced.[180]
But even though exploration of one surface by another were impossible,
we could always measure our various surfaces against each other by
applying the same extended object first to one and then to another.
We should of course have the alternative of supposing that the object
itself waxed and waned as it glided from one place to another (cf.
above, p. 141); but the principle of simplifying as much as possible
our world would soon drive us out of that assumption into the easier
one that objects as a rule keep their sizes, and that most of our
sensations are affected by errors for which a constant allowance must
be made.
In the retina there is no reason to suppose that the bignesses of
two impressions (lines or blotches) falling on different regions are
primitively felt to stand in any exact mutual ratio. It is only when
the impressions come from the _same object_ that we judge their sizes
to be the same. And this, too, only when the relation of the object
to the eye is believed to be on the whole unchanged. When the object
by moving changes its relations to the eye the sensation excited by
its image even on the same retinal region becomes so fluctuating
that we end by ascribing no absolute import whatever to the retinal
space-feeling which at any moment we may receive. So complete does
this overlooking of retinal magnitude become that it is next to
impossible to compare the visual magnitudes of objects at different
distances without making the experiment of superposition. We cannot
say beforehand how much of a distant house or tree our finger will
cover. The various answers to the familiar question, How large is the
moon?--answers which vary from a cartwheel to a wafer--illustrate
this most strikingly. The hardest part of the training of a young
draughtsman is his learning to feel directly the retinal (i.e.
primitively sensible) magnitudes which the different objects in the
field of view subtend. To do this he must recover what Ruskin calls
the 'innocence of the eye'--that is, a sort of childish perception of
stains of color merely as such, without consciousness of what they mean.
With the rest of us this innocence is lost. _Out of all the visual
magnitudes of each known object we have selected one as the_ REAL _one
to think of, and degraded all the others to serve as its signs_. This
'real' magnitude is determined by æsthetic and practical interests.
It is that which we get when the object is at the distance most
propitious for exact visual discrimination of its details. This is
the distance at which we hold anything we are examining. Farther than
this we see it too small, nearer too large. And the larger and the
smaller feeling vanish in the act of suggesting this one, their more
important _meaning_. As I look along the dining-table I overlook the
fact that the farther plates and glasses _feel_ so much smaller than
my own, for I _know_ that they are all equal in size; and the feeling
of them, which is a present sensation, is eclipsed in the glare of the
knowledge, which is a merely imagined one.
If the inconsistencies of sight-spaces _inter se_ can thus be reduced,
of course there can be no difficulty in equating sight-spaces
with spaces given to touch. In this equation it is probably the
touch-feeling which prevails as real and the sight which serves as
sign--a reduction made necessary not only by the far greater constancy
of felt over seen magnitudes, but by the greater practical interest
which the sense of touch possesses for our lives. As a rule, things
only benefit or harm us by coming into direct contact with our skin:
sight is only a sort of anticipatory touch; the latter is, in Mr.
Spencer's phrase, the 'mother-tongue of thought,' and the handmaid's
idiom must be translated into the language of the mistress before it
can speak clearly to the mind.[181]
Later on we shall see that the feelings excited in the joints when a
limb moves are used as signs of the path traversed by the extremity.
But of this more anon. As for the equating of sound-, smell-, and
taste-volumes with those yielded by the more discriminative senses,
they are too vague to need any remark. It may be observed of pain,
however, that its size has to be reduced to that of the normal tactile
size of the organ which is its seat. A finger with a felon on it, and
the pulses of the arteries therein, both 'feel' larger than we believe
they really 'are.'
It will have been noticed in the account given that _when two sensorial
space-impressions, believed to come from the same object, differ, then_
THE ONE MOST INTERESTING, _practically or æsthetically_, IS JUDGED
TO BE THE TRUE ONE. This law of interest holds throughout--though
a permanent interest, like that of touch, may resist a strong but
fleeting one like that of pain, as in the case just given of the felon.
3. _The Summation of the Sense-spaces._
Now for the next step in our construction of real space: _How are the
various sense-spaces added together into a consolidated and unitary
continuum?_ For they are, in man at all events, incoherent at the start.
* * * * *
Here again the first fact that appears is that _primitively our
space-experiences form a chaos, out of which we have no immediate
faculty for extricating them. Objects of different sense-organs,
experienced together, do not in the first instance appear either inside
or alongside or far outside of each other, neither spatially continuous
nor discontinuous, in any definite sense of these words._ The same
thing is almost as true of objects felt by different parts of the same
organ before discrimination has done its finished work. The most we
can say is that all our space-experiences together form an _objective
total_ and that this _objective total_ is vast.
Even now the space inside our mouth, which is so intimately known and
accurately measured by its inhabitant the tongue, can hardly be said
to have its internal directions and dimensions known in any exact
relation to those of the larger world outside. It forms almost a little
world by itself. Again, when the dentist excavates a small cavity in
one of our teeth, we feel the hard point of his instrument scraping,
in distinctly differing directions, a surface which seems to our
sensibility vaguely larger than the subsequent use of the mirror tells
us it 'really' is. And though the directions of the scraping differ
so completely _inter se_, not one of them can be identified with the
particular direction in the outer world to which it corresponds. The
space of the tooth-sensibility is thus really a little world by itself,
which can only become congruent with the outer space-world by farther
experiences which shall alter its bulk, identify its directions, fuse
its margins, and finally imbed it as a definite part within a definite
whole. And even though every joint's rotations should be felt to vary
_inter se_ as so many differences of direction in a common room;
even though the same were true of diverse tracings on the skin, and
of diverse tracings on the retina respectively, it would still not
follow that feelings of direction, on these different surfaces, are
intuitively comparable among each other, or with the other directions
yielded by the feelings of the semi-circular canals. It would not
follow that we should immediately judge the relations of them all to
each other in one space-world.
If with the arms in an unnatural attitude we 'feel' things, we are
perplexed about their shape, size, and position. Let the reader lie on
his back with his arms stretched above his head, and it will astonish
him to find how ill able he is to recognize the geometrical relations
of objects placed within reach of his hands. But the geometrical
relations here spoken of are nothing but identities recognized between
the directions and sizes perceived in this way and those perceived in
the more usual ways. The two ways do not fit each other intuitively.
How lax the connection between the system of visual and the system
of tactile directions is in man, appears from the facility with
which microscopists learn to reverse the movements of their hand in
manipulating things on the stage of the instrument. To move the slide
to the _seen_ left they must draw it to the _felt_ right. But in a very
few days the habit becomes a second nature. So in tying our cravat,
shaving before a mirror, etc., the right and left sides are inverted,
and the directions of our hand movements are the opposite of what they
seem. Yet this never annoys us. Only when by accident we try to tie
the cravat of another person do we learn that there are two ways of
combining sight and touch perceptions. Let any one try for the first
time to write or draw while looking at the image of his hand and paper
in a mirror, and he will be utterly bewildered. But a very short
training will teach him to undo in this respect the associations of his
previous lifetime.
Prisms show this in an even more striking way. If the eyes be armed
with spectacles containing slightly prismatic glasses with their bases
turned, for example, towards the right, every object looked at will be
apparently translocated to the left; and the hand put forth to grasp
any such object will make the mistake of passing beyond it on the left
side. But less than an hour of practice in wearing such spectacles
rectifies the judgment so that no more mistakes are made. In fact the
new-formed associations are already so strong, that when the prisms are
first laid aside again the opposite error is committed, the habits of a
lifetime violated, and the hand now passed to the right of every object
which it seeks to touch.
The primitive chaos thus subsists to a great degree through life so
far as our immediate sensibility goes. We feel our various objects
and their bignesses, together or in succession; but so soon as it
is a question of the order and relations of many of them at once
our intuitive apprehension remains to the very end most vague and
incomplete. Whilst we are attending to one, or at most to two or
three objects, all the others _lapse_, and the most we feel of them
is that they still linger on the outskirts and can be caught again by
turning in a certain way. Nevertheless _throughout all this confusion
we conceive of a world spread out in a perfectly fixed and orderly
fashion, and we believe in its existence. The question is: How do
this conception and this belief arise? How is the chaos smoothed and
straightened out?_
* * * * *
Mainly by two operations: Some of the experiences are apprehended to
exist out- and alongside of each other, and others are apprehended
to interpenetrate each other, and to occupy the same room. In this
way what was incoherent and irrelative ends by being coherent and
definitely related; nor is it hard to trace the principles, by which
the mind is guided in this arrangement of its perceptions, in detail.
In the first place, following the great intellectual law of economy,
we simplify, unify, and identify as much as we possibly can. _Whatever
sensible data can be attended to together we locate together. Their
several extents seem one extent. The place at which each appears is
held to be the same with the place at which the others appear. They
become, in short, so many properties of_ ONE AND THE SAME REAL THING.
This is the first and great commandment, the fundamental 'act' by which
our world gets spatially arranged.
In this _coalescence in a 'thing_,' one of the coalescing sensations
is held to _be_ the thing, the other sensations are taken for its more
or less accidental _properties_, or modes of appearance.[182] The
sensation chosen to be the thing essentially is the most constant and
practically important of the lot; most often it is hardness or weight.
But the hardness or weight is never without tactile bulk; and as we
can always see something in our hand when we feel something there, we
equate the bulk felt with the bulk seen, and thenceforward this common
bulk is also apt to figure as of the essence of the 'thing.' Frequently
a shape so figures, sometimes a temperature, a taste, etc.; but for the
most part temperature, smell, sound, color, or whatever other phenomena
may vividly impress us simultaneously with the bulk felt or seen,
figure among the accidents. Smell and sound impress us, it is true,
when we neither see nor touch the thing; but they are strongest when we
see or touch, so we locate the _source_ of these properties within the
touched or seen space, whilst the properties themselves we regard as
overflowing in a weakened form into the spaces filled by other things.
_In all this, it will be observed, the sense-data whose spaces coalesce
into one are yielded by different sense-organs._ Such data have no
tendency to displace each other from consciousness, but can be attended
to together all at once. Often indeed they vary concomitantly and
reach a maximum together. We may be sure, therefore, that the general
rule of our mind is to locate IN _each other all_ sensations which are
associated in simultaneous experience, and do not interfere with each
other's perception.[183]
_Different impressions on the same sense-organ_ do interfere with each
other's perception, and cannot well be attended to at once. Hence _we
do not locate them in each other's spaces, but arrange them in a serial
order of exteriority, each alongside of the rest, in a space larger
than that which any one sensation brings._ This larger space, however,
is an object of conception rather than of direct intuition, and bears
all the marks of being constructed piecemeal by the mind. The blind
man forms it out of tactile, locomotor, and auditory experiences,
the seeing man out of visual ones almost exclusively. As the visual
construction is the easiest to understand, let us consider that first.
* * * * *
Every single visual sensation or 'field of view' is limited. To get a
new field of view for our object the old one must disappear. But the
disappearance may be only partial. Let the first field of view be A B
C. If we carry our attention to the limit C, it ceases to be the limit,
and becomes the centre of the field, and beyond it appear fresh parts
where there were none before:[184] A B C changes, in short, to C D E.
But although the parts A B are lost to sight, yet their image abides in
the memory; and if we think of our first object A B C as having existed
or as still existing at all, we must think of it as it was originally
presented, namely, as spread out from C in one direction just as C D E
is spread out in another. A B and D E can never coalesce in one place
(as they could were they objects of different senses) because they can
never be perceived at once: we must lose one to see the other. So (the
letters standing now for 'things') we get to conceive of the successive
fields of things after the analogy of the several things which we
perceive in a single field. They must be out- and alongside of each
other, and we conceive that their juxtaposed spaces must make a larger
space. A B C + C D E must, in short, be imagined to exist in the form of
A B C D E or not imagined at all.
We can usually recover anything lost from sight by moving our attention
and our eyes back in its direction; and through these constant changes
every field of seen things comes at last to be thought of as always
having a fringe of _other things possible to be seen_ spreading in all
directions round about it. Meanwhile the movements concomitantly with
which the various fields alternate are also felt and remembered; and
gradually (through association) this and that movement come in our
thought to suggest this or that extent of fresh objects introduced.
Gradually, too, since the objects vary indefinitely in kind, we
abstract from their several natures and think separately of their mere
extents, of which extents the various movements remain as the only
constant introducers and associates. More and more, therefore, do we
think of movement and seen extent as mutually involving each other,
until at last (with Bain and J. S. Mill) we may get to regard them
as synonymous, and say, "What is the _meaning of the word extent_,
unless it be possible movement?"[185] We forget in this conclusion that
(whatever intrinsic extensiveness the movements may appear endowed
with), that seen spreadoutness which is the pattern of the abstract
extensiveness which we imagine came to us originally from the retinal
sensation.
The muscular sensations of the eyeball _signify_ this sort of visible
spreadoutness, just as this visible spreadoutness may come in later
experience to _signify_ the 'real' bulks, distances, lengths and
breadths known to touch and locomotion.[186] To the very end, however,
in us seeing men, the quality, the nature, the _sort of thing we mean_
by extensiveness, would seem to be the sort of feeling which our
retinal stimulations bring.
* * * * *
In one deprived of sight the principles by which the notion of real
space is constructed are the same. Skin-feelings take in him the place
of retinal feelings in giving the quality of lateral spreadoutness,
as our attention passes from one extent of them to another, awakened
by an object sliding along. Usually the moving object is our hand; and
feelings of movement in our joints invariably accompany the feelings
in the skin. But the feeling of the skin is what the blind man _means_
by his skin; so the size of the skin-feelings stands as the absolute
or real size, and the size of the joint-feelings becomes a sign of
these. Suppose, for example, a blind baby with (to make the description
shorter) a blister on his toe, exploring his leg with his finger-tip
and feeling a pain shoot up sharply the instant the blister is touched.
The experiment gives him four different kinds of sensation--two of
them protracted, two sudden. The first pair are the movement-feeling
in the joints of the upper limb, and the movement-feeling on the skin
of the leg and foot. These, attended to together, have their extents
identified as one objective space--the hand moves through the same
space in which the leg lies. The second pair of objects are the pain
in the blister, and the peculiar feeling the blister gives to the
finger. Their spaces also fuse; and as each marks the end of a peculiar
movement-series (arm moved, leg stroked), the movement-spaces are
_emphatically_ identified with each other at _that_ end. Were there
other small blisters distributed down the leg, there would be a number
of these emphatic points; the movement-spaces would be identified, not
only as totals, but point for point.[187]
Just so with spaces beyond the body's limits. Continuing the
joint-feeling beyond the toe, the baby hits another object, which he
can still think of when he brings his hand back to its blister again.
That object at the end of that joint-feeling means a new place for him,
and the more such objects multiply in his experience the wider does the
space of his conception grow. If, wandering through the woods to-day by
a new path, I find myself suddenly in a glade which affects my senses
exactly as did another I reached last week at the end of a different
walk, I believe the two identical affections to present the same
persisting glade, and infer that I have attained it by two differing
roads. The spaces walked over grow congruent by their extremities;
though apart from the common sensation which those extremities give me,
I should be under no necessity of connecting one walk with another at
all. The case in no whit differs when shorter movements are concerned.
If, moving first one arm and then another, the blind child gets the
same kind of sensation upon the hand, and gets it again as often as he
repeats either process, he judges that he has touched the same object
by both motions, and concludes that the motions terminate in a common
place. From place to place marked in this way he moves, and adding the
places moved through, one to another, he builds up his notion of the
extent of the outer world. The seeing man's process is identical; only
his units, which may be successive bird's-eye views, are much larger
than in the case of the blind.
FEELINGS IN JOINTS AND FEELINGS IN MUSCLES.
1. _Feelings of Movement in Joints._
I have been led to speak of feelings which arise in joints. As these
feelings have been too much neglected in Psychology hitherto, in
entering now somewhat minutely into their study I shall probably at the
same time freshen the interest of the reader, which under the rather
dry abstractions of the previous pages may presumably have flagged.
When, by simply flexing my right forefinger on its metacarpal joint,
I trace with its tip an inch on the palm of my left hand, is my
feeling of the size of the inch purely and simply a feeling in the
skin of the palm, or have the muscular contractions of the right hand
and forearm anything to do with it? In the preceding pages I have
constantly assumed spatial sensibility to be an affair of surfaces.
At first starting, the consideration of the 'muscular sense' as a
space-measurer was postponed to a later stage. Many writers, of whom
the foremost was Thomas Brown, in his _Lectures on the Philosophy of
the Human Mind_, and of whom the latest is no less a Psychologist than
Prof. Delbœuf,[188] hold that the consciousness of active muscular
motion, aware of its own amount, is the _fons et origo_ of all spatial
measurement. It would seem to follow, if this theory were true, that
two skin-feelings, one of a large patch, one of a small one, possess
their difference of spatiality, not as an immediate element, but
solely by virtue of the fact that the large one, to get its points
_successively_ excited, demands more muscular contraction than the
small one does. Fixed associations with the several amounts of muscular
contraction required in this particular experience would thus explain
the apparent sizes of the skin-patches, which sizes would consequently
not be primitive data but derivative results.
_It seems to me that no evidence of the muscular measurements
in question exists;_ but that all the facts may be explained by
surface-sensibility, provided we take that of the joint-surfaces also
into account.
The most striking argument, and the most obvious one, which an
upholder of the muscular theory is likely to produce is undoubtedly
this fact: if, with closed eyes, we trace figures in the air with the
extended forefinger (the motions may occur from the metacarpal-, the
wrist-, the elbow-, or the shoulder-joint indifferently), what we are
_conscious of_ in each case, and indeed most acutely conscious of,
is the geometric path described by the finger-tip. Its angles, its
subdivisions, are all as distinctly felt as if seen by the eye; and yet
the surface of the finger-tip receives no impression at all.[189] But
with each variation of the figure, the muscular contractions vary, and
so do the feelings which these yield. Are not these latter the sensible
data that make us aware of the lengths and directions we discern in the
traced line?
Should we be tempted to object to this supposition of the advocate of
perception by muscular feelings, that we have _learned_ the spatial
significance of these feelings by reiterated experiences of _seeing_
what figure is drawn when each special muscular grouping is felt, so
that in the last resort the muscular space feelings would be derived
from retinal-surface feelings, our opponent might immediately hush us
by pointing to the fact that in persons born blind the phenomenon in
question is even more perfect than in ourselves.
If we suggest that the blind may have originally traced the figures on
the cutaneous surface of cheek, thigh, or palm, and may now remember
the specific figure which each present movement formerly caused the
skin-surface to perceive, he may reply that the delicacy of the motor
perception far exceeds that of most of the cutaneous surfaces; that,
in fact, we can feel a figure traced only in its differentials, so to
speak,--a figure which we merely _start_ to trace by our finger-tip, a
figure which, traced in the same way _on_ our finger-tip by the hand of
another, is almost if not wholly unrecognizable.
* * * * *
The champion of the muscular sense seems likely to be triumphant
_until we invoke the articular cartilages_, as internal surfaces whose
sensibility is called in play by every movement we make, however
delicate the latter may be.
To establish the part they play in our geometrizing, it is necessary
to review a few facts. It has long been known by medical practitioners
that, in patients with cutaneous anæsthesia of a limb, whose muscles
also are insensible to the thrill of the faradic current, a very
accurate sense of the way in which the limb may be flexed or extended
by the hand of another may be preserved.[190] On the other hand, we
may have this sense of movement impaired when the tactile sensibility
is well preserved. That the pretended feeling of outgoing innervation
can play in these cases no part, is obvious from the fact that the
movements by which the limb changes its position are passive ones,
imprinted on it by the experimenting physician. The writers who have
sought a _rationale_ of the matter have consequently been driven by way
of exclusion to assume the articular surfaces to be the seat of the
perception in question.[191]
_That the joint-surfaces are sensitive_ appears evident from the fact
that in inflammation they become the seat of excruciating pains, and
from the perception by everyone who lifts weights or presses against
resistance, that every increase of the force opposing him betrays
itself to his consciousness principally by the starting-out of new
feelings or the increase of old ones, in or about the joints. If the
structure and mode of mutual application of two articular surfaces be
taken into account, it will appear that, granting the surfaces to _be_
sensitive, no more favorable mechanical conditions could be possible
for the delicate calling of the sensibility into play than are realized
in the minutely graduated rotations and firmly resisted variations of
pressure involved in every act of extension or flexion. Nevertheless it
is a great pity that we have as yet no direct testimony, no expressions
from patients with healthy joints accidentally laid open, of the
impressions they experience when the cartilage is pressed or rubbed.
The first approach to direct evidence, so far as I know, is contained
in the paper of Lewinski,[192] published in 1879. This observer had a
patient the inner half of whose leg was anæsthetic. When this patient
stood up, he had a curious illusion about the position of his limb,
which disappeared the moment he lay down again: he thought himself
_knock-kneed_. If, as Lewinski says, we assume the inner half of the
joint to share the insensibility of the corresponding part of the
skin, then he _ought_ to feel, when the joint-surfaces pressed against
each other in the act of standing, the outer half of the joint most
strongly. But this is the feeling he would also get whenever it was
by any chance sought to force his leg into a knock-kneed attitude.
Lewinski was led by this case to examine the feet of certain ataxic
patients with imperfect sense of position. He found in every instance
that when the toes were flexed _and drawn upon_ at the same time (the
joint-surfaces drawn asunder) all sense of the amount of flexion
disappeared. On the contrary, when he pressed a toe _in_, whilst
flexing it, the patient's appreciation of the amount of flexion was
much improved, evidently because the artificial increase of articular
pressure made up for the pathological insensibility of the parts.
Since Lewinski's paper an important experimental research by A.
Goldscheider[193] has appeared, which completely establishes our
point. This patient observer caused his fingers, arms, and legs
to be passively rotated upon their various joints in a mechanical
apparatus which registered both the velocity of movement impressed
and the amount of angular rotation. No active muscular contraction
took place. The minimal felt amounts of rotation were in all cases
surprisingly small, being much less than a single angular degree in
all the joints except those of the fingers. Such displacements as
these, the author says (p. 490), can hardly be detected by the eye.
The point of application of the force which rotated the limb made no
difference in the result. Rotations round the hip-joint, for example,
were as delicately felt when the leg was hung by the heel as when it
was hung by the thigh whilst the movements were performed. Anæsthesia
of the skin produced by induction-currents also had no disturbing
effect on the perception, nor did the various degrees of pressure of
the moving force upon the skin affect it. It became, in fact, all
the more distinct in proportion as the concomitant pressure-feelings
were eliminated by artificial anæsthesia. When the joints themselves,
however, were made artificially anæsthetic the perception of the
movement grew obtuse and the angular rotations had to be much increased
before they were perceptible. All these facts prove according to Herr
Goldscheider, that _the joint surfaces and these alone are the starting
point of the impressions by which the movements of our members are
immediately perceived._
Applying this result, which seems invulnerable, to the case of the
tracing finger-tip, we see that our perception of the latter gives
no countenance to the theory of the muscular sense. _We indubitably
localize the finger-tip at the successive points of its path by means
of the sensations which we receive from our joints._ But if this is so,
it may be asked, why do we feel the figure to be traced, not within the
joint itself, but in such an altogether different place? And why do we
feel it so much larger than it really is?
I will answer these questions by asking another: Why do we move our
joints at all? Surely to gain something more valuable than the insipid
joint-feelings themselves. And these more interesting feelings are in
the main produced upon the _skin_ of the moving part, or of some other
part over which it passes, or upon the eye. With movements of the
fingers we explore the configuration of all real objects with which
we have to deal, our own body as well as foreign things. Nothing
that interests us is located in the joint; everything that interests
us either _is_ some part of our skin, or is something that we see as
we handle it. The cutaneously felt and the seen extents come thus to
figure as the important things for us to concern ourselves with. Every
time the joint moves, even though we neither see, nor feel cutaneously,
the reminiscence of skin-events and sights which formerly coincided
with that extent of movement, ideally awaken as the movement's import,
and the mind drops the present sign to attend to the import alone. The
joint-sensation itself, as such, does not disappear in the process. A
little attention easily detects it, with all its fine peculiarities,
hidden beneath its vaster suggestions; so that really the mind has
two space-perceptions before it, congruent in form but different in
scale and place, either of which exclusively it may notice, or both at
once,--the joint-space which it _feels_ and the real space which it
_means_.
The joint-spaces serve so admirably as signs because of their capacity
for _parallel variation_ to all the peculiarities of external motion.
There is not a direction in the real world nor a ratio of distance
which cannot be matched by some direction or extent of joint-rotation.
Joint-feelings, like all feelings, are roomy. Specific ones are
contrasted _inter se_ as different directions are contrasted within
the same extent. If I extend my arm straight out at the shoulder, the
rotation of the shoulder-joint will give me one feeling of movement;
if then I sweep the arm forward, the same joint will give me another
feeling of movement. Both these movements are felt to happen in space,
and differ in specific quality. Why shall not the specificness of the
quality just consist in the feeling of a peculiar _direction?_[194] Why
may not the several joint-feelings _be_ so many perceptions of movement
in so many different directions? That we cannot explain why they
_should_ is no presumption that they _do_ not, for we never can explain
why any sense-organ should awaken the sensation it does.
But if the joint-feelings are directions and extents, standing in
relation to each other, the task of association in interpreting their
import in eye- or skin-terms is a good deal simplified. Let the
movement _bc_, of a certain joint, derive its absolute space-value
from the cutaneous feeling it is always capable of engendering;
then the longer movement _abcd_ of the same joint will be judged to
have a greater space-value, even though it may never have wholly
merged with a skin-experience. So of differences of direction: so
much joint-difference = so much skin-difference; therefore, more
joint-difference = more skin-difference. _In fact, the joint-feeling
can excellently serve as a map on a reduced scale, of a reality which
the imagination can identify at its pleasure with this or that sensible
extension simultaneously known in some other way._
When the joint-feeling in itself acquires an emotional interest,--which
happens whenever the joint is inflamed and painful,--the secondary
suggestions fail to arise, and the movement is felt where it is, and in
its intrinsic scale of magnitude.[195]
The localization of the joint-feeling in a space simultaneously known
otherwise (i.e. to eye or skin), is what is commonly called the
_extradition or eccentric projection of the feeling_. In the preceding
chapter I said a good deal on this subject; but we must now see a
little more closely just what happens in this instance of it. The
content of the joint-feeling, to begin with, is an object, and _is_ in
itself a place. For it to be _placed_, say _in the elbow_, the elbow as
seen or handled must already have become another object for the mind,
and with its place as thus known, the place which the joint-feeling
fills must coalesce. That the latter should be felt 'in the elbow' is
therefore a 'projection' of it into the place of another object as
much as its being felt in the finger-tip or at the end of a cane can
be. But when we say 'projection' we generally have in our mind the
notion of a _there_ as contrasted with a _here_. What is the _here_
when we say that the joint-feeling is _there_? The 'here' seems to be
the spot which the mind has chosen for its own post of observation,
usually some place within the head, but sometimes within the throat or
breast--not a rigorously fixed spot, but a region from any portion of
which it may send forth its various acts of attention. Extradition from
either of _these_ regions is the common law under which we perceive the
whereabouts of the north star, of our own voice, of the contact of our
teeth with each other, of the tip of our finger, of the point of our
cane on the ground, or of a movement in our elbow-joint.
But _for the distance between the 'here' and the 'there' to be felt,
the entire intervening space must be itself an object of perception._
The consciousness of this intervening space is the _sine quâ non_ of
the joint-feeling's projection to the farther end of it. When it is
filled by our own bodily tissues (as where the projection only goes as
far as the elbow or finger-tip) we are sensible of its extent alike by
our eye, by our exploring movements, and by the resident sensations
which fill its length. When it reaches beyond the limits of our body,
the resident sensations are lacking, but limbs and hand and eye suffice
to make it known. Let me, for example, locate a feeling of motion
coming from my elbow-joint in the point of my cane a yard beyond my
hand. Either I see this yard as I flourish the cane, and the seen end
of it then absorbs my sensation just as my seen elbow might absorb
it, or I am blind and imagine the cane as an object continuing my
arm, either because I have explored both arm and cane with the other
hand, or because I have pressed them both along my body and leg. If I
project my joint-feeling farther still, it is by a conception rather
than a distinct imagination of the space. I _think_: 'farther,' 'thrice
as far,' etc.; and thus get a symbolic image of a distant path at
which I point.[196] But the 'absorption' of the joint-feeling by the
distant spot, in whatever terms the latter may be apprehended, is
never anything but that coalescence into one 'thing' already spoken
of on page 184, of whatever different sensible objects interest our
attention at once.
2. _Feelings of Muscular Contraction._
Readers versed in psychological literature will have missed, in our
account thus far, the usual invocation of 'the muscular sense.' This
word is used with extreme vagueness to cover all resident sensations,
whether of motion or position, in our members, and even to designate
the supposed feeling of efferent discharge from the brain. We shall
later see good reason to deny the existence of the latter feeling. We
have accounted for the better part at least of the resident feelings of
motion in limbs by the sensibility of the articular surfaces. The skin
and ligaments also must have feelings awakened as they are stretched
or squeezed in flexion or extension. And I am inclined to think that
_the sensations of our contracting muscles themselves probably play
as small a part in building up our exact knowledge of space as any
class of sensations which we possess._ The muscles, indeed, play an
all-important part, but it is through the remote effect of their
contractions on other sensitive parts, not through their own resident
sensations being aroused. In other words, _muscular contraction is only
indirectly instrumental, in giving us space-perceptions, by its effects
on surfaces._ In skin and retina it produces a motion of the stimulus
upon the surface; in joints it produces a motion of the surfaces upon
each other--such motion being by far the most delicate manner of
exciting the surfaces in question. One is tempted to doubt whether the
muscular sensibility as such plays even a subordinate part as _sign_ of
these more immediately geometrical perceptions which are so uniformly
associated with it as effects of the contraction objectively viewed.
For this opinion many reasons can be assigned. First, it seems _a
priori_ improbable that such organs as muscles should give us feelings
whose variations bear any exact proportion to the spaces traversed
when they contract. As G. E. Müller says,[197] their sensory nerves
must be excited either chemically or by mechanical compression whilst
the contractions last, and in neither case can the excitement be
proportionate to the position into which the limb is thrown. The
chemical state of the muscle depends on the _previous_ work more
than on the actually present contraction; and the internal pressure
of it depends on the resistance offered more than on the shortening
attained. _The intrinsic muscular sensations are likely therefore to
be merely those of massive strain or fatigue, and to carry no accurate
discrimination with them of lengths of path moved through._
Empirically we find this probability confirmed by many facts. The
judicious A. W. Volkmann observes[198] that:
"Muscular feeling gives tolerably fine evidence as to the _existence_
of movement, but hardly any direct information about its extent or
direction. We are not aware that the contractions of a _supinator
longus_ have a wider range than those of a _supinator brevis_;
and that the fibres of a bipenniform muscle contract in opposite
directions is a fact of which the muscular feeling itself gives not
the slightest intimation. Muscle-feeling belongs to that class of
general sensations which tell us of our inner states, but not of outer
relations; it does not belong among the space-perceiving senses."
E. H. Weber in his article Tastsinn called attention to the fact that
muscular movements as large and strong as those of the diaphragm go on
continually without our perceiving them as motion.
G. H. Lewes makes the same remark. When we think of our muscular
sensations as movements in space, it is because we have ingrained with
them in our imagination a movement on a surface simultaneously felt.
"Thus whenever we breathe there is a contraction of the muscles of
the ribs and the diaphragm. Since we _see_ the chest expanding, we
know it as a movement and can only think of it as such. But the
diaphragm itself is not seen, and consequently by no one who is not
physiologically enlightened on the point is this diaphragm thought of
in movement. Nay, even when told by a physiologist that the diaphragm
moves at each breathing, every one who has not seen it moving
downward pictures it as an upward movement, because the chest moves
upward."[199]
A personal experience of my own seems strongly to corroborate this
view. For years I have been familiar, during the act of gaping, with a
large, round, smooth sensation in the region of the throat, a sensation
characteristic of gaping and nothing else, but which, although I had
often wondered about it, never suggested to my mind the motion of
anything. The reader probably knows from his own experience exactly
what feeling I mean. It was not till one of my students told me, that I
learned its objective cause. If we look into the mirror while gaping,
we see that at the moment we have this feeling the hanging palate
_rises_ by the contraction of its intrinsic muscles. The contraction of
these muscles and the compression of the palatine mucous membrane are
what occasion the feeling; and I was at first astonished that, coming
from so small an organ, it could appear so voluminous. Now the curious
point is this--that no sooner had I learned by the eye its objective
space-significance, than I found myself enabled mentally to _feel_ it
as a movement upwards of a body in the situation of the uvula. When I
now have it, my fancy _injects_ it, so to speak, with the image of the
rising uvula; and it _absorbs_ the image easily and naturally. In a
word, a muscular contraction gave me a sensation whereof I was unable
during forty years to interpret a motor meaning, of which two glances
of the eye made me permanently the master. To my mind no further proof
is needed of the fact that muscular contraction, merely as such, need
not be perceived directly as so much motion through space.
Take again the contractions of the muscles which make the eyeball
rotate. The feeling of these is supposed by many writers to play the
chief part in our perceptions of extent. The space seen between two
things _means_, according to these authors, nothing but the amount
of contraction which is needed to carry the _fovea_ from the first
thing to the second. But close the eyes and note the contractions in
themselves (even when coupled as they still are with the delicate
surface sensations of the eyeball rolling under the lids), and we
are surprised at finding how vague their space-import appears. Shut
the eyes and roll them, and you can with no approach to accuracy
tell the outer object which shall first be seen when you open them
again.[200] Moreover, if our eye-muscle-contractions had much to do
with giving us our sense of seen extent, we ought to have a natural
illusion of which we find no trace. Since the feeling in the muscles
grows disproportionately intense as the eyeball is rolled into an
extreme eccentric position, all places on the extreme _margin_ of
the field of view ought to appear farther from the centre than they
really are, for the fovea cannot get to them without an amount of this
feeling altogether in excess of the amount of actual rotation.[201]
When we turn to the muscles of the body at large we find the same
vagueness. Goldscheider found that the minimal perceived rotation
of a limb about a joint was no less when the movement was 'active'
or produced by muscular contraction than when it was 'passively'
impressed.[202] The consciousness of active movement became so blunt
when the joint (alone!) was made anæsthetic by faradization, that it
became evident that the feeling of contraction could never be used for
_fine_ discrimination of extents. And that it was not used for coarse
discriminations appeared clear to Goldscheider from certain other
results which are too circumstantial for me to quote in detail.[203]
His general conclusion is that we feel our movements exclusively in
our articular surfaces, and that our muscular contractions in all
probability hardly occasion this sort of perception at all.[204]
My conclusion is that the 'muscular sense' must fall back to the humble
position from which Charles Bell raised it, and no longer figure in
Psychology as the leading organ in space-perception which it has been
so long 'cracked up' to be.
* * * * *
Before making a minuter study of Space as apprehended by the eye,
we must turn to see what we can discover of space as known to the
blind. But as we do so, let us cast a glance upon the results of
the last pages, and ask ourselves once more whether the building up
of orderly space-perceptions out of primitive incoherency requires
any mental powers beyond those displayed in ordinary intellectual
operations. I think it is obvious--granting the spacial _quale_ to
exist in the primitive sensations--that discrimination, association,
addition, multiplication, and division, blending into generic images,
substitution of similars, selective emphasis, and abstraction from
uninteresting details, are quite capable of giving us all the
space-perceptions we have so far studied, without the aid of any
mysterious 'mental chemistry' or power of 'synthesis' to create
elements absent from the original data of feeling. It cannot be too
strongly urged in the face of mystical attempts, however learned,
that there is not a landmark, not a length, not a point of the
compass in real space which _is_ not some _one_ of our feelings,
either experienced directly as a presentation or ideally suggested
by another feeling which has come to serve as its sign. In degrading
some sensations to the rank of signs and exalting others to that of
realities signified, we smooth out the wrinkles of our first chaotic
impressions and make a continuous order of what was a rather incoherent
multiplicity. But the _content_ of the order remains identical with
that of the multiplicity--sensational both, through and through.
HOW THE BLIND PERCEIVE SPACE.
The blind man's construction of real space differs from that of the
seeing man most obviously in the larger part which synthesis plays
in it, and the relative subordination of analysis. The seeing baby's
eyes take in the whole room at once, and discriminative attention
must arise in him before single objects are visually discerned. The
blind child, on the contrary, must form his mental image of the room
by the addition, piece to piece, of parts which he learns to know
successively. With our eyes we may apprehend instantly, in an enormous
bird's-eye view, a landscape which the blind man is condemned to build
up bit by bit after weeks perhaps of exploration. We are exactly in
his predicament, however, for spaces which exceed our visual range. We
think the ocean as a whole by multiplying mentally the impression we
get at any moment when at sea. The distance between New York and San
Francisco is computed in days' journeys; that from earth to sun is so
many times the earth's diameter, etc.; and of longer distances still
we may be said to have no adequate mental image whatever, but only
numerical verbal symbols.
But the symbol will often give us the emotional effect of the
perception. Such expressions as the abysmal vault of heaven, the
endless expanse of ocean, etc., summarize many computations to the
imagination, and give the sense of an enormous horizon. So it seems
with the blind. They multiply mentally the amount of a distinctly felt
freedom to move, and gain the immediate sense of a vaster freedom
still. Thus it is that blind men are never without the consciousness of
their horizon. They all enjoy travelling, especially with a companion
who can describe to them the objects they pass. On the prairies they
feel the great openness; in valleys they feel closed in; and one has
told me that he thought few seeing people could enjoy the view from
a mountain-top more than he. A blind person on entering a house or
room immediately receives, from the reverberations of his voice and
steps, an impression of its dimensions, and to a certain extent of its
arrangement. The tympanic sense noticed on p. 140, _supra_, comes
in to help here, and possibly other forms of tactile sensibility not
yet understood. Mr. W. Hanks Levy, the blind author of 'Blindness and
the Blind' (London), gives the following account of his powers of
perception:
"Whether within a house or in the open air, whether walking or
standing still, I can tell, although quite blind, when I am opposite
an object, and can perceive whether it be tall or short, slender
or bulky. I can also detect whether it be a solitary object or a
continuous fence; whether it be a close fence or composed of open
rails; and often whether it be a wooden fence, a brick or stone wall,
or a quick-set hedge. I cannot usually perceive objects if much lower
than my shoulder, but sometimes very low objects can be detected.
This may depend on the nature of the objects, or on some abnormal
state of the atmosphere. The currents of air can have nothing to do
with this power, as the state of the wind does not directly affect
it; the sense of hearing has nothing to do with it, as when snow
lies thickly on the ground objects are more distinct, although the
footfall cannot be heard. I seem to perceive objects through the skin
of my face, and to have the impressions immediately transmitted to
the brain. The only part of my body possessing this power is my face;
this I have ascertained by suitable experiments. Stopping my ears
does not interfere with it, but covering my face with a thick veil
destroys it altogether. None of the five senses have anything to do
with the existence of this power, and the circumstances above named
induce me to call this unrecognized sense by the name of 'facial
perception.'... When passing along a street I can distinguish shops
from private houses, and even point out the doors and windows, etc.,
and this whether the doors be shut or open. When a window consists
of one entire sheet of glass, it is more difficult to discover than
one composed of a number of small panes. From this it would appear
that glass is a bad conductor of sensation, or at any rate of the
sensation specially connected with this sense. When objects below the
face are perceived, the sensation seems to come in an oblique line
from the object to the upper part of the face. While walking with a
friend in Forest Lane, Stratford, I said, pointing to a fence which
separated the road from a field, 'Those rails are not quite as high
as my shoulder.' He looked at them, and said they were higher. We,
however, measured, and found them about three inches lower than my
shoulder. At the time of making this observation I was about four feet
from the rails. Certainly in this instance facial perception was more
accurate than sight. When the lower part of a fence is brickwork, and
the upper part rails, the fact can be detected, and the line where the
two meet easily perceived. Irregularities in height, and projections
and indentations in walls, can also be discovered."
According to Mr. Levy, this power of seeing with the face is diminished
by a fog, but not by ordinary darkness. At one time he could tell when
a cloud obscured the horizon, but he has now lost that power, which
he has known several persons to possess who are totally blind. These
effects of aqueous vapor suggest immediately that fluctuations in the
heat radiated by the objects may be the source of the perception. One
blind gentleman, Mr. Kilburne, an instructor in the Perkins Institution
in South Boston, who has the power spoken of in an unusual degree,
proved, however, to have no more delicate a sense of temperature in
his face than ordinary persons. He himself supposed that his ears had
nothing to do with the faculty until a complete stoppage of them,
not only with cotton but with putty on top of it, by abolishing the
perception entirely, proved his first impression to be erroneous. Many
blind men say immediately that their ears are concerned in the matter.
Sounds certainly play a far more prominent part in the mental life
of the blind than in our own. In taking a walk through the country,
the mutations of sound, far and near, constitute their chief delight.
And to a great extent their imagination of distance and of objects
moving from one distant spot to another seems to consist in thinking
how a certain sonority would be modified by the change of place. It is
unquestionable that the semi-circular-canal feelings play a great part
in defining the points of the compass and the direction of distant
spots, in the blind as in us. We _start_ towards them by feelings
of this sort; and so many directions, so many different-feeling
starts.[205]
The only point that offers any theoretic difficulty is the prolongation
into space of the direction, after the start. We saw, ten pages back,
that for extradition to occur beyond the skin, the portion of skin
in question _and_ the space beyond must form a common object for
some other sensory surface. The eyes are for most of us this sensory
surface; for the blind it can only be other parts of the skin, coupled
or not with motion. But the mere gropings of the hands in every
direction must end by surrounding the whole body with a sphere of
felt space. And this sphere must become enlarged with every movement
of locomotion, these movements gaining their space-values from the
semi-circular-canal feelings which accompany them, and from the
farther and farther parts of large fixed objects (such as the bed, the
wainscoting, or a fence) which they bring within the grasp. It might
be supposed that a knowledge of space acquired by so many successive
discrete acts would always retain a somewhat jointed and so to speak,
granulated character. When we who are gifted with sight think of a
space too large to come into a single field of view, we are apt to
imagine it as composite, and filled with more or less jerky stoppings
and startings (think, for instance, of the space from here to San
Francisco), or else we reduce the scale symbolically and imagine how
much larger on a map the distance would look than others with whose
totality we are familiar.
I am disposed to believe, after interrogating many blind persons, that
the use of imaginary maps on a reduced scale is less frequent with them
than with the rest of us. Possibly the extraordinary changeableness
of the visual magnitudes of things makes this habit natural to us,
while the fixity of tactile magnitudes keeps them from falling into
it. (When the blind young man operated on by Dr. Franz was shown a
portrait in a locket, he was vastly surprised that the face could be
put into so small a compass: it would have seemed to him, he said,
as impossible as to put a bushel into a pint.) Be this as it may,
however, the space which each blind man feels to extend beyond his body
is felt by him as one smooth continuum--all trace of those muscular
startings and stoppings and reversals which presided over its formation
having been eliminated from the memory. It seems, in other words, a
generic image of the space-element common to all these experiences,
with the unessential particularities of each left out. In truth,
_where_ in this space a start or a stop may have occurred was quite
accidental. It may never occur just there again, and so the attention
lets it drops altogether. Even as long a space as that traversed in a
several-mile walk will not necessarily appear to a blind man's thought
in the guise of a series of locomotor acts. Only where there is some
distinct locomotor difficulty, such as a step to ascend, a difficult
crossing, or a disappearance of the path, will distinct locomotor
images constitute the idea. Elsewhere the space seems continuous, and
its parts may even all seem coexistent; though, as a very intelligent
blind friend once remarked to me, 'To think of such distances involves
probably more mental wear and tear and brain-waste in the blind than
in the seeing.' This seems to point to a greater element of successive
addition and construction in the blind man's idea.
Our own visual explorations go on by means of innumerable stoppings
and startings of the eyeballs. Yet these are all effaced from the
final space-sphere of our visual imagination. They have neutralized
each other. We can even distribute our attention to the right and left
sides simultaneously, and think of those two quarters of space as
coexistent. Does the smoothing out of the locomotor interruptions from
the blind man's tactile space-sphere offer any greater paradox? Surely
not. And it is curious to note that both in him and in us there is one
particular locomotor feeling that is apt to assert itself obstinately
to the last. We and he alike spontaneously imagine space as lying _in
front_ of us, for reasons too obvious to enumerate. If we think of the
space behind us, we, as a rule, have to _turn round_ mentally, and
in doing so the front space vanishes. But in this, as in the other
things of which we have been talking, individuals differ widely.
Some, in imagining a room, can think of all its six surfaces at once.
Others mentally turn round, or, at least, imagine the room in several
successive and mutually exclusive acts (cf. p. 54, above).
* * * * *
Sir William Hamilton, and J. S. Mill after him, have quoted approvingly
an opinion of Platner (an eighteenth-century philosopher) regarding the
space-perceptions of the blind. Platner says:
"The attentive observation of a person born blind... has convinced
me that the sense of touch by itself is altogether incompetent to
afford us the representation of extension and space.... In fact, to
those born blind, time serves instead of space. Vicinity and distance
mean in their mouths nothing more than the shorter or longer time ...
necessary to attain from some one feeling to some other."