THE PERCEPTION OF SPACE.
As adult thinkers we have a definite and apparently instantaneous
knowledge of the sizes, shapes, and distances of the things amongst
which we live and move; and we have moreover a practically definite
notion of the whole great infinite continuum of real space in which the
world swings and in which all these things are located. Nevertheless it
seems obvious that the baby's world is vague and confused in all these
respects. How does our definite knowledge of space grow up? This is one
of the quarrelsome problems in psychology. This chapter must be so brief
that there will be no room for the polemic and historic aspects of the
subject, and I will state simply and dogmatically the conclusions which
seem most plausible to me.
=The quality of voluminousness= exists in all sensations, just as
intensity does. We call the reverberations of a thunder-storm more
voluminous than the squeaking of a slate-pencil; the entrance into a
warm bath gives our skin a more massive feeling than the prick of a pin;
a little neuralgic pain, fine as a cobweb, in the face, seems less
extensive than the heavy soreness of a boil or the vast discomfort of a
colic or a lumbago; and a solitary star looks smaller than the noonday
sky. Muscular sensations and semicircular-canal sensations have volume.
Smells and tastes are not without it; and sensations from our inward
organs have it in a marked degree.
Repletion and emptiness, suffocation, palpitation, headache, are
examples of this, and certainly not less spatial is the consciousness we
have of our general bodily condition in nausea, fever, heavy
drowsiness, and fatigue. Our entire cubic content seems then sensibly
manifest to us as such, and feels much larger than any local pulsation,
pressure, or discomfort. Skin and retina are, however, the organs in
which the space-element plays the most active part. Not only does the
maximal vastness yielded by the retina surpass that yielded by any other
organ, but the intricacy with which our attention can subdivide this
vastness and perceive it to be composed of lesser portions
simultaneously coexisting alongside of each other is without a parallel
elsewhere. The ear gives a greater vastness than the skin, but is
considerably less able to subdivide it. The _vastness, moreover, is as
great in one direction as in another_. Its dimensions are so vague that
in it there is no question as yet of surface as opposed to depth;
'volume' being the best short name for the sensation in question.
_Sensations of different orders are roughly comparable with each other
as to their volumes._ Persons born blind are said to be surprised at the
largeness with which objects appear to them when their sight is
restored. Franz says of his patient cured of cataract: "He saw
everything much larger than he had supposed from the idea obtained by
his sense of touch. Moving, and especially living, objects appeared very
large." Loud sounds have a certain enormousness of feeling. 'Glowing'
bodies as Hering says, give us a perception "which seems _roomy_
(_raumhaft_) in comparison with that of strictly surface-color. A
glowing iron looks luminous through and through, and so does a flame."
The interior of one's mouth-cavity feels larger when explored by the
tongue than when looked at. The crater of a newly-extracted tooth, and
the movements of a loose tooth in its socket, feel quite monstrous. A
midge buzzing against the drum of the ear will often seem as big as a
butterfly. The pressure of the air in the tympanic cavity upon the
membrane gives an astonishingly large sensation.
_The voluminousness of the feeling seems to bear very little relation to
the size of the organ that yields it._ The ear and eye are
comparatively minute organs, yet they give us feelings of great volume.
The same lack of exact proportion between size of feeling and size of
organ affected obtains within the limits of particular sensory organs.
An object appears smaller on the lateral portions of the retina than it
does on the fovea, as may be easily verified by holding the two
forefingers parallel and a couple of inches apart, and transferring the
gaze of one eye from one to the other. Then the finger not directly
looked at will appear to shrink. On the skin, if two points kept
equidistant (blunted compass-or scissors-points, for example) be drawn
along so as really to describe a pair of parallel lines, the lines will
appear farther apart in some spots than in others. If, for example, we
draw them across the face, the person experimented upon will feel as if
they began to diverge near the mouth and to include it in a well-marked
ellipse.
[Illustration: FIG. 65 (after Weber).
The dotted lines give the real course of the points, the continuous
lines the course as felt.]
NOW MY FIRST THESIS IS THAT THIS EXTENSITY, _discernible in each and
every sensation, though more developed in some than in others_, IS THE
ORIGINAL SENSATION OF SPACE, out of which all the exact knowledge about
space that we afterwards come to have is woven by processes of
discrimination, association, and selection.
=The Construction of Real Space.=--To the babe who first opens his senses
upon the world, though the experience is one of vastness or extensity,
it is of an extensity within which no definite divisions, directions,
sizes, or distances are yet marked out. Potentially, the room in which
the child is born is subdivisible into a multitude of parts, fixed or
movable, which at any given moment of time have definite relations to
each other and to his person. Potentially, too, this room taken as a
whole can be prolonged in various directions by the addition to it of
those farther-lying spaces which constitute the outer world. But
actually the further spaces are unfelt, and the subdivisions are
undiscriminated, by the babe; the chief part of whose education during
his first year of life consists in his becoming acquainted with them and
recognizing and identifying them in detail. This process may be called
that of the _construction of real space_, as a newly apprehended object,
out of the original chaotic experiences of vastness. It consists of
several subordinate processes:
First, the total object of vision or of feeling at any time _must have
smaller objects definitely discriminated within it_;
Secondly, _objects seen or tasted must be identified with objects felt,
heard_, etc., and _vice versa_, so that _the same 'thing'_ may come to
be recognized, although apprehended in such widely differing ways;
Third, the total extent felt at any time must be conceived as
_definitely located in the midst of the surrounding extents of which the
world consists_;
Fourth, these objects _must appear arranged in definite order_ in the
so-called three dimensions; and
Fifth, their relative sizes must be perceived--in other words, _they
must be measured_.
Let us take these processes in regular order.
1) =Subdivision or Discrimination.=--Concerning this there is not much to
be added to what was set forth in Chapter XIV. Moving parts, sharp
parts, brightly colored parts of the total field of perception 'catch
the attention' and are then discerned as special objects surrounded by
the remainder of the field of view or touch. That when such objects are
discerned apart they should appear as thus surrounded, must be set down
as an ultimate fact of our sensibility of which no farther account can
be given. Later, as one partial object of this sort after another has
become familiar and identifiable, the attention can be caught by more
than one at once. We then see or feel a number of distinct objects
alongside of each other in the general extended field. The
'alongsideness' is in the first instance vague--it may not carry with it
the sense of definite directions or distances--and it too must be
regarded as an ultimate fact of our sensibility.
2) =Coalescence of Different Sensations into the Same 'Thing.'=--When two
senses are impressed simultaneously we tend to identify their objects as
_one thing_. When a conductor is brought near the skin, the snap heard,
the spark seen, and the sting felt, are all located together and
believed to be different aspects of one entity, the 'electric
discharge.' The space of the seen object fuses with the space of the
heard object and with that of the felt object by an ultimate law of our
consciousness, which is that 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 clears is held to be the same with the place at which the
others appear._ This is the first and great '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. The sensation
chosen to be essentially the thing 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.
3) =The Sense of the Surrounding World.=--_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_. We can usually recover anything lost from our sight
by moving our eyes back in its direction; and it is through these
constant changes that 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 we may get to regard them as synonymous; and, empty space
then meaning for us mere _room for movement_, we may, if we are
psychologists, readily but erroneously assign to the 'muscular sense'
the chief rôle in perceiving extensiveness at all.
4) =The Serial Order of Locations.=--The muscular sense _has_ much to do
with defining the _order of position_ of things seen, felt, or heard. We
look at a point; another point upon the retina's margin catches our
attention, and in an instant we turn the fovea upon it, letting its
image successively fall upon all the points of the intervening retinal
line. The line thus traced so rapidly by the second point is itself a
visual object, with the first and second point at its respective ends.
It _separates_ the points, which become _located by its length_ with
reference to each other. If a third point catch the attention, more
peripheral still than the second point, then a still greater movement of
the eyeball and a continuation of the line will result, the second point
now appearing _between_ the first and third. Every moment of our life,
peripherally-lying objects are drawing lines like this between
themselves and other objects which they displace from our attention as
we bring them to the centre of our field of view. Each peripheral
retinal point comes in this way to _suggest_ a line at the end of which
it lies, a line which a possible movement will trace; and even the
motionless field of vision ends at last by signifying a system of
positions brought out by possible movements between its centre and all
peripheral parts.
It is the same with our skin and joints. By moving our hand over objects
we trace lines of direction, and new impressions arise at their ends.
The 'lines' are sometimes on the articular surfaces, sometimes on the
skin as well; in either case they give a definite order of arrangement
to the successive objects between which they intervene. Similarly with
sounds and smells. With our heads in a certain position, a certain sound
or a certain smell is most distinct. Turning our head makes this
experience fainter and brings another sound, or another smell, to its
maximum. The two sounds or smells are thus separated by the movement
located at its ends, the movement itself being realized as a sweep
through space whose value is given partly by the semicircular-canal
feeling, partly by the articular cartilages of the neck, and partly by
the impressions produced upon the eye.
By such general principles of action as these everything looked at,
felt, smelt, or heard comes to be located in a more or less definite
position relatively to other collateral things either actually presented
or only imagined as possibly there. I say 'collateral' things, for I
prefer not to complicate the account just yet with any special
consideration of the 'third dimension,' distance, or depth, as it has
been called.
3) =The Measurement of Things in Terms of Each Other.=--Here 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 the tongue larger than it
feels to the finger or eye, 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._
Two surfaces of skin superposed on each other are felt simultaneously,
and by the law laid down on p. 339 are judged to occupy an identical
place. Similarly of our hand, when seen and felt at the same time by its
resident sensibility.
In these identifications and reductions of the many to the one it must
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.
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
might of course at first suppose that the object itself waxed and waned
as it glided from one place to another (cf. above, Fig. 65); 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 at
first felt to stand in any exact mutual ratio. But if the impressions
come from the _same object_, then we might judge their sizes to be just
the same. This, however, 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.
_It is the same with shape as with size._ Almost all the visible shapes
of things are what we call perspective 'distortions.' Square table-tops
constantly present two acute and two obtuse angles; circles drawn on our
wall-papers, our carpets, or on sheets of paper, usually show like
ellipses; parallels approach as they recede; human bodies are
foreshortened; and the transitions from one to another of these altering
forms are infinite and continual. Out of the flux, however, one phase
always stands prominent. It is the form the object has when we see it
easiest and best: and that is when our eyes and the object both are in
what may be called _the normal position_. In this position our head is
upright and our optic axes either parallel or symmetrically convergent;
the plane of the object is perpendicular to the visual plane; and if the
object is one containing many lines, it is turned so as to make them, as
far as possible, either parallel or perpendicular to the visual plane.
In this situation it is that we compare all shapes with each other; here
every exact measurement and every decision is made.
=Most sensations are signs to us of other sensations whose space-value is
held to be more real.= _The thing as it would appear to the eye if it
were in the normal position is what we think of_ whenever we get one of
the other optical views. Only as represented in the normal position do
we believe we see the object as it _is_; elsewhere, only as it seems.
Experience and custom soon teach us, however, that the seeming
appearance passes into the real one by continuous gradations. They teach
us, moreover, that seeming and being may be strangely interchanged. Now
a real circle may slide into a seeming ellipse; now an ellipse may, by
sliding in the same direction, become a seeming circle; now a
rectangular cross grows slant-legged; now a slant-legged one grows
rectangular.
Almost any form in oblique vision may be thus a derivative of almost any
other in 'primary' vision; and we must learn, when we get one of the
former appearances, to translate it into the appropriate one of the
latter class; we must learn of what optical 'reality' it is one of the
optical signs. Having learned this, we do but obey that law of economy
or simplification which dominates our whole psychic life, when we think
exclusively of the 'reality' and ignore as much as our consciousness
will let us the 'sign' by which we came to apprehend it. The signs of
each probable real thing being multiple and the thing itself one and
fixed, we gain the same mental relief by abandoning the former for the
latter that we do when we abandon mental images, with all their
fluctuating characters, for the definite and unchangeable _names_ which
they suggest. The selection of the several 'normal' appearances from out
of the jungle of our optical experiences, to serve as the real sights of
which we shall think, has thus some analogy to the habit of thinking in
words, in that by both we substitute terms few and fixed for terms
manifold and vague.
If an optical sensation can thus be a mere sign to recall another
sensation of the same sense, judged more real, _a fortiori_ can
sensations of one sense be signs of realities which are objects of
another. Smells and tastes make us believe the _visible_ cologne-bottle,
strawberry, or cheese to be there. Sights suggest objects of touch,
touches suggest objects of sight, etc. In all this substitution and
suggestive recall the only law that holds good is that in general the
most _interesting_ of the sensations which the 'thing' can give us is
held to represent its real nature most truly. It is a case of the
selective activity mentioned on p. 170 ff.
=The Third Dimension or Distance.=--This service of sensations as mere
signs, to be ignored when they have evoked the other sensations which
are their significates, was noticed first by Berkeley in his new theory
of vision. He dwelt particularly on the fact that the signs were not
_natural_ signs, but properties of the object merely _associated by
experience_ with the more real aspects of it which they recall. The
tangible 'feel' of a thing, and the 'look' of it to the eye, have
absolutely no point in common, said Berkeley; and if I think of the look
of it when I get the feel, or think of the feel when I get the look,
that is merely due to the fact that I have on so many previous occasions
had the two sensations at once. When we open our eyes, for example, we
think we see how far off the object is. But this feeling of distance,
according to Berkeley, cannot possibly be a retinal sensation, for a
point in outer space can only impress our retina by the single dot which
it projects 'in the fund of the eye,' and this dot is the same for _all_
distances. Distance from the eye, Berkeley considered not to be an
optical object at all, but an object of _touch_, of which we have
optical signs of various sorts, such as the image's apparent magnitude,
its 'faintness' or 'confusion,' and the 'strain' of accommodation and
convergence. By distance being an object of 'touch,' Berkeley meant that
our notion of it consists in ideas of the amount of muscular movement of
arm or legs which would be required to place our hand upon the object.
Most authors have agreed with Berkeley that creatures unable to move
either their eyes or limbs would have no notion whatever of distance or
the third dimension.
This opinion seems to me unjustifiable. I cannot get over the fact that
all our sensations are of _volume_, and that the primitive field of view
(however imperfectly distance may be discriminated or measured in it)
cannot be of something _flat_, as these authors unanimously maintain.
Nor can I get over the fact that distance, when I see it, is a genuinely
_optical feeling_, even though I be at a loss to assign any one
physiological process in the organ of vision to the varying degrees of
which the variations of the feeling uniformly correspond. It is awakened
by all the optical signs which Berkeley mentioned, and by more besides,
such as Wheatstone's binocular disparity, and by the parallax which
follows on slightly moving the head. When awakened, however, it seems
optical, and not heterogeneous with the other two dimensions of the
visual field.
The mutual equivalencies of the distance-dimension with the up-and-down
and right-to-left dimensions of the field of view can easily be settled
without resorting to experiences of touch. A being reduced to a single
eyeball would perceive the same tridimensional world which we do, if he
had our intellectual powers. For the _same moving things_, by
alternately covering different parts of his retina, would determine the
mutual equivalencies of the first two dimensions of the field of view;
and by exciting the physiological cause of his perception of depth in
various degrees, they would establish a scale of equivalency between the
first two and the third.
First of all, one of the sensations given by the object would be chosen
to represent its 'real' size and shape, in accordance with the
principles so lately laid down. One sensation would measure the 'thing'
present, and the 'thing' would measure the other sensations--the
peripheral parts of the retina would be equated with the central by
receiving the image of the same object. This needs no elucidation in
case the object does not change its distance or its front. But suppose,
to take a more complicated case, that the object is a stick, seen first
in its whole length, and then rotated round one of its ends; let this
fixed end be the one near the eye. In this movement the stick's image
will grow progressively shorter; its farther end will appear less and
less separated laterally from its fixed near end; soon it will be
screened by the latter, and then reappear on the opposite side, the
image there finally resuming its original length. Suppose this movement
to become a familiar experience; the mind will presumably react upon it
after its usual fashion (which is that of unifying all data which it is
in any way possible to unify), and consider it the movement of a
constant object rather than the transformation of a fluctuating one.
Now, the _sensation of depth_ which it receives during the experience is
awakened more by the far than by the near end of the object. But how
much depth? What shall measure its amount? Why, at the moment the far
end is about to be eclipsed, the difference of its distance from the
near end's distance must be judged equal to the stick's whole length;
but that length has already been seen and measured by a certain visual
sensation of breadth. _So we find that given amounts of the visual
depth-feeling become signs of given amounts of the visual
breadth-feeling, depth becoming equated with breadth. The measurement of
distance is, as Berkeley truly said, a result of suggestion and
experience. But visual experience alone is adequate to produce it, and
this he erroneously denied._
=The Part played by the Intellect in Space-perception.=--But although
Berkeley was wrong in his assertion that out of optical experience alone
no perception of distance can be evolved, he gave a great impetus to
psychology by showing how originally incoherent and incommensurable in
respect of their extensiveness our different sensations are, and how our
actually so rapid space-perceptions are almost altogether acquired by
education. Touch-space is one world; sight-space is another world. The
two worlds have no essential or intrinsic congruence, and only through
the 'association of ideas' do we know what a seen object signifies in
terms of touch. Persons with congenital cataracts relieved by surgical
aid, whose world until the operation has been a world of tangibles
exclusively, are ludicrously unable at first to name any of the objects
which newly fall upon their eye. "It might very well be _a horse_," said
the latest patient of this sort of whom we have an account, when a
10-litre bottle was held up a foot from his face.[45] Neither do such
patients have any accurate notion in motor terms of the relative
distances of things from their eyes. All such confusions very quickly
disappear with practice, and the novel optical sensations translate
themselves into the familiar language of touch. The facts do not prove
in the least that the optical sensations are not _spatial_, but only
that it needs a subtler sense for analogy than most people have, to
discern the _same_ spatial aspects and relations in them which
previously-known tactile and motor experiences have yielded.
=Conclusion.=--To sum up, the whole history of space-perception is
explicable if we admit on the one hand sensations with certain amounts
of extensity native to them, and on the other the ordinary powers of
discrimination, selection, and association in the mind's dealings with
them. The fluctuating import of many of our optical sensations, the
same sensation being so ambiguous as regards size, shape, locality, and
the like, has led many to believe that such attributes as these could
not possibly be the result of sensation at all, but must come from some
higher power of intuition, synthesis, or whatever it might be called.
But the fact that a present sensation can at any time become the sign of
a represented one judged to be more real, sufficiently accounts for all
the phenomena without the need of supposing that the quality of
extensity is created out of non-extensive experiences by a
super-sensational faculty of the mind.