OF THE PERCEPTION OF SPACE.
1. ACCORDING to the views above explained, certain of the
impressions of our senses convey to us the perception of
objects as existing in space; inasmuch as by the
constitution of our minds we cannot receive those
impressions otherwise than in a certain form, involving such
a manner of existence. But the question deserves to be
asked, _What_ are the impressions of sense by which we thus
become acquainted with space and its relations? And as we
have seen that this idea of space implies an act of the mind
as well as an impression on the sense, what manifestations
do we find of this activity of the mind, in our observation
of the external world?
It is evident that sight and touch are the senses by which
the relations of space are perceived, principally or
entirely. It does not appear that an odour, or a feeling of
warmth or cold, would, independently of experience, suggest
to us the conception of a space surrounding us. But when we
_see_ objects, we see that they are extended and occupy
space; when we _touch_ them, we feel that they are in a
space in which we also are. We have before our eyes any
object, for instance, a board covered with geometrical
diagrams; and we distinctly perceive, by vision, those lines
of which the relations are the subjects of our mathematical
reasoning. Again, we see before us a solid object, a cubical
box for instance; we see that it is within reach; we stretch
out the hand and perceive by the touch that it has sides,
edges, corners, which we had already perceived by vision. {118}
2. Probably most persons do not generally apprehend that
there is any material difference in these two cases;--that
there are any different acts of mind concerned in perceiving
by sight a mathematical diagram upon paper, and a solid cube
lying on a table. Yet it is not difficult to show that, in
the latter case at least, the perception of the shape of the
object is not immediate. A very little attention teaches us
that there is an act of judgment as well as a mere
impression of sense requisite, in order that we may see any
solid object. For there is no visible appearance which is
inseparably connected with solidity. If a picture of a cube
be rightly drawn, in perspective and skilfully shaded, the
impression upon the sense is the same as if it were a real
cube. The picture may be mistaken for a solid object. But it
is clear that, in this case, the solidity is given to the
object by an act of mental judgment. All that is seen is
outline and shade, figures and colours on a flat board. The
solid angles and edges, the relation of the faces of the
figure by which they form a cube, are matters of inference.
This, which is evident in the case of the pictured cube, is
true in all vision whatever. We see a scene before us on
which are various figures and colours, but the eye cannot
see more. It sees length and breadth, but no third
dimension. In order to know that there are solids, we must
infer as well as see. And this we do readily and constantly;
so familiarly, indeed, that we do not perceive the
operation. Yet we may detect this latent process in many
ways; for instance, by attending to cases in which the habit
of drawing such inferences misleads us. Most persons have
experienced this delusion in looking at a scene in a
theatre, and especially that kind of scene which is called a
_diorama_, when the interior of a building is represented.
In these cases, the perspective representations of the
various members of the architecture and decoration impress
us almost irresistibly with the conviction that we have
before us a space of great extent and complex form, instead
of a flat painted canvass. Here, at least, the space is our
own creation; but yet here, it is {119} manifestly created
by the same act of thought as if we were really in the
palace or the cathedral of which the halls and aisles thus
seem to inclose us. And the act by which we thus create
space of three dimensions out of visible extent of length
and breadth, is constantly and imperceptibly going on. We
are perpetually interpreting in this manner the language of
the visible world. From the appearances of things which we
directly see, we are constantly inferring that which we
cannot directly see,--their distance from us, and the
position of their parts.
3. The characters which we thus interpret are various. They
are, for instance, the visible forms, colours, and shades of
the parts, understood according to the maxims of
perspective; (for of perspective every one has a practical
knowledge, as every one has of grammar;) the effort by which
we fix both our eyes on the same object, and adjust each eye
to distinct vision; and the like. The right interpretation
of the information which such circumstances give us
respecting the true forms and distances of things, is
gradually learned; the lesson being begun in our earliest
infancy, and inculcated upon us every hour during which we
use our eyes. The completeness with which the lesson is
mastered is truly admirable; for we forget that our
conclusion is obtained indirectly, and mistake a judgment on
evidence for an intuitive perception. We see the breadth of
the street, as clearly and readily as we see the house on
the other side of it; and we see the house to be square,
however obliquely it be presented to us. This, however, by
no means throws any doubt or difficulty on the doctrine that
in all these cases we do interpret and infer. The rapidity
of the process, and the unconsciousness of the effort, are
not more remarkable in this case than they are when we
understand the meaning of the speech which we hear, or of
the book which we read. In these latter cases we merely hear
noises or see black marks; but we make, out of these
elements, thought and feeling, without being aware of the
act by which we do so. And by an exactly similar process we
see a variously-coloured {120} expanse, and collect from it
a space occupied by solid objects. In both cases the act of
interpretation is become so habitual that we can hardly stop
short at the mere impression of sense.
4. But yet there are various ways in which we may satisfy
ourselves that these two parts of the process of seeing
objects are distinct. To separate these operations is
precisely the task which the artist has to execute, in
making a drawing of what he sees. He has to recover the
consciousness of his real and genuine sensations, and to
discern the lines of objects as they appear. This at first
he finds difficult; for he is tempted to draw what he knows
of the forms of visible objects, and not what he sees: but
as he improves in his art, he learns to put on paper what he
sees only, separated from what he infers, in order that thus
the inference, and with it a conception like that of the
reality, may be left to the spectator. And thus the natural
process of vision is the habit of seeing that which cannot
be seen; and the difficulty of the art of drawing consists
in learning not to see more than is visible.
5. But again; even in the simplest drawing we exhibit
something which we do not see. However slight is our
representation of objects, it contains something which we
create for ourselves. For we draw an _outline_. Now an
outline has no existence in nature. There are no visible
lines presented to the eye by a group of figures. We
separate each figure from the rest, and the boundary by
which we do this is the outline of the figure; and the like
may be said of each member of every figure. A painter of our
own times has made this remark in a work upon his art[4\2]:
'The effect which natural objects produce upon our sense of
vision is that of a number of parts, or distinct masses of
form and colour, and not of lines. But when we endeavour to
represent by painting the objects which are before us, or
which invention supplies to our minds, {121} the first and
the simplest means we resort to is this picture, by which we
separate the form of each object from those that surround
it, marking its boundary, the extreme extent of its
dimensions in every direction, as impressed on our vision:
and this is termed drawing its outline.'
[Note 4\2: Phillips _On Painting._]
6. Again, there are other ways in which we see clear
manifestations of the act of thought by which we assign to
the parts of objects their relations in space, the
impressions of sense being merely subservient to this act.
If we look at a medal through a glass which inverts it, we
see the figures upon it become concave depressions instead
of projecting convexities; for the light which illuminates
the nearer side of the convexity will be transferred to the
opposite side by the apparent inversion of the medal, and
will thus imply a hollow in which the side nearest the light
gathers the shade. Here our decision as to which part is
nearest to us, has reference to the side from which the
light comes. In other cases the decision is more
spontaneous. If we draw black outlines, such as represent
the edges of a cube seen in perspective, certain of the
lines will cross each other; and we may make this cube
appear to assume two different positions, by determining in
our own mind that the lines which belong to one end of the
cube shall be understood to be before or to be behind those
which they cross. Here an act of the will, operating upon
the same sensible image, gives us two cubes, occupying two
entirely different positions. Again, many persons may have
observed that when a windmill in motion at a distance from
us, (so that the outline of the sails only is seen,) stands
obliquely to the eye, we may, by an effort of thought, make
the obliquity assume one or the other of two positions; and
as we do this, the sails, which in one instance appear to
turn from right to left, in the other case turn from left to
right. A person a little familiar with this mental effort,
can invert the motion as often as he pleases, so long as the
conditions of form and light do not offer a manifest
contradiction to either position. {122}.
Thus we have these abundant and various manifestations of
the activity of the mind, in the process by which we collect
from vision the relations of solid space of three
dimensions. But we must further make some remarks on the
process by which we perceive mere visible figure; and also,
on the mode in which we perceive the relations of space by
the touch; and first, of the latter subject.
7. The opinion above illustrated, that our sight does not
give us a direct knowledge of the relations of solid space,
and that this knowledge is acquired only by an inference of
the mind, was first clearly taught by the celebrated Bishop
Berkeley[5\2], and is a doctrine now generally assented to
by metaphysical speculators.
[Note 5\2: _Theory of Vision._]
But does the sense of _touch_ give us directly a knowledge
of space? This is a question which has attracted
considerable notice in recent times; and new light has been
thrown upon it in a degree which is very remarkable, when we
consider that the philosophy of perception has been a
prominent subject of inquiry from the earliest times. Two
philosophers, advancing to this inquiry from different
sides, the one a metaphysician, the other a physiologist,
have independently arrived at the conviction that the long
current opinion, according to which we acquire a knowledge
of space by the sense of touch, is erroneous. And the
doctrine which they teach instead of the ancient errour, has
a very important bearing upon the principle which we are
endeavouring to establish,--that our knowledge of space and
its properties is derived rather from the active operations
than from the passive impressions of the percipient mind.
Undoubtedly the persuasion that we acquire a knowledge of
form by the touch is very obviously suggested by our common
habits. If we wish to know the form of any body in the dark,
or to correct the impressions conveyed by sight, when we
suspect them to be false, we have only, it seems to us, at
least at first, to stretch forth the hand and touch the
object; and we learn its {123} shape with, no chance of
errour. In these cases, form appears to be as immediate a
perception of the sense of touch, as colour is of the sense
of sight.
8. But is this perception really the result of the passive
sense of touch merely? Against such an opinion Dr. Brown,
the metaphysician of whom I speak, urges[6\2] that the
feeling of touch alone, when any object is applied to the
hand, or any other part of the body, can no more convey the
conception of form or extension, than the sensation of an
odour or a taste can do, except we have already some
knowledge of the relative position of the parts of our
bodies; that is, except we are already in possession of an
idea of space, and have, in our minds, referred our limbs to
their positions; which is to suppose the conception of form
already acquired.
[Note 6\2: _Lectures_, Vol. i. p. 459, (1824).]
9. By what faculty then do we originally acquire our
conceptions of the relations of position? Brown answers by
the _muscular sense_; that is, by the conscious exertions of
the various muscles by which we move our limbs. When we feel
out the form and position of bodies by the hand, our
knowledge is acquired, not by the mere touch of the body,
but by perceiving the course the fingers must take in order
to follow the surface of the body, or to pass from one body
to another. We are conscious of the slightest of the
volitions by which we thus feel out form and place; we know
whether we move the finger to the right or left, up or down,
to us or from us, through a large or a small space; and all
these conscious acts are bound together and regulated in our
minds by an idea of an extended space in which they are
performed. That this idea of space is not borrowed from the
sight, and transferred to the muscular feelings by habit, is
evident. For a man born blind can feel out his way with his
staff, and has his conceptions of position determined by the
conditions of space, no less than one who has the use of his
eyes. And the muscular consciousness which reveals to us the
position of objects and parts of objects, {124} when we feel
them out by means of the hand, shows itself in a thousand
other ways, and in all our limbs: for our habits of
standing, walking, and all other attitudes and motions, are
regulated by our feeling of our position and that of
surrounding objects. And thus, we cannot touch any object
without learning something respecting its position; not that
the sense of touch directly conveys such knowledge; but we
have already learnt, from the muscular sense, constantly
exercised, the position of the limb which the object thus
touches.
10. The justice of this distinction will, I think, be
assented to by all persons who attend steadily to the
process itself, and might be maintained by many forcible
reasons. Perhaps one of the most striking evidences in its
favour is that, as I have already intimated, it is the
opinion to which another distinguished philosopher, Sir
Charles Bell, has been led, reasoning entirely upon
physiological principles. From his researches it resulted
that besides the nerves which convey the impulse of the will
from the brain to the muscle, by which every motion of our
limbs is produced, there is another set of nerves which
carry back to the brain a sense of the condition of the
muscle, and thus regulate its activity; and give us the
consciousness of our position and relation to surrounding
objects. The motion of the hand and fingers, or the
consciousness of this motion, must be combined with the
sense of touch properly so called, in order to make an inlet
to the knowledge of such relations. This consciousness of
muscular exertion, which he has called a sixth sense[7\2],
is our guide, Sir C. Bell shows, in the common practical
government of our motions; and he states that having given
this explanation of perception as a physiological doctrine,
he had afterwards with satisfaction seen it confirmed by Dr.
Brown's speculations.
[Note 7\2: _Bridgewater Treatise_, p. 195. _Phil. Trans._
1826, Pt. ii. p. 167.]
11. Thus it appears that our consciousness of the relations
of space is inseparably and fundamentally connected with our
own actions in space. We perceive {125} only while we act;
our sensations require to be interpreted by our volitions.
The apprehension of extension and figure is far from being a
process in which we are inert and passive. We draw lines
with our fingers; we construct surfaces by curving our
hands; we generate spaces by the motion of our arms. When
the geometer bids us form lines, or surfaces, or solids by
motion, he intends his injunction to be taken as
hypothetical only; we need only conceive such motions. But
yet this hypothesis represents truly the origin of our
knowledge; we perceive spaces by motion at first, as we
conceive spaces by motion afterwards:--or if not always by
actual motion, at least by potential. If we perceive the
length of a staff by holding its two ends in our two hands
without running the finger along it, this is because by
habitual motion we have already acquired a measure of the
distance of our hands in any attitude of which we are
conscious. Even in the simplest case, our perceptions are
derived not from the touch, but from the sixth sense; and
this sixth sense at least, whatever may be the case with the
other five, implies an active mind along with the passive sense.
12. Upon attentive consideration, it will be clear that a
large portion of the perceptions respecting space which
appear at first to be obtained by sight alone, are, in fact,
acquired by means of this sixth sense. Thus we consider the
visible sky as a single surface surrounding us and returning
into itself, and thus forming a hemisphere. But such a mode
of conceiving an object of vision could never have occurred
to us, if we had not been able to turn our heads, to follow
this surface, to pursue it till we find it returning into
itself. And when we have done this, we necessarily present
it to ourselves as a concave inclosure within which we are.
The sense of sight alone, without the power of muscular
motion, could not have led us to view the sky as a vault or
hemisphere. Under such circumstances, we should have
perceived only what was presented to the eye in one
position; and if different appearances had been presented in
succession, we could {126} not have connected them as parts
of the same picture, for want of any perception of their
relative position. They would have been so many detached and
incoherent visual sensations. The muscular sense connects
their parts into a whole, making them to be only different
portions of one universal scene[8\2].
[Note 8\2: It has been objected to this view that we might
obtain a conception of the sky as a hemisphere, by being
ourselves turned round, (as on a music-stool, for instance,)
and thus seeing in succession all parts of the sky. But this
assertion I conceive to be erroneous. By being thus turned
round, we should see a number of pictures which we should
put together as parts of a plane picture; and when we came
round to the original point, we should have no possible
means of deciding that it was the _same_ point: it would
appear only as a _repetition_ of the picture. That sight, of
itself, can give us only a plane picture, the doctrine of
Berkeley, appears to be indisputable; and, no less so, the
doctrine that it is the consciousness of our own action in
space which puts together these pictures so that they cover
the surface of a solid body. We can see length and breadth
with our eyes, but we must thrust out our arm towards the
flat surface, in order that we may, in our thoughts, combine
a third dimension with the other two.]
13. These considerations point out the fallacy of a very
curious representation made by Dr. Reid, of the convictions
to which man would be led, if he possessed vision without
the sense of touch. To illustrate this subject, Reid uses
the fiction of a nation whom he terms the _Idomenians_, who
have no sense except that of sight. He describes their
notions of the relations of space as being entirely
different from ours. The axioms of their geometry are quite
contradictory to our axioms. For example, it is held to be
self-evident among them that two straight lines which
intersect each other once, must intersect a second time;
that the three angles of any triangle are _greater_ than two
right angles; and the like. These paradoxes are obtained by
tracing the relations of lines on the surface of a concave
sphere, which surrounds the spectator, and on which all
visible appearances may be supposed to be presented to him.
But from what is said above it appears that the notion of
such a sphere, and such a connexion of visible objects which
are seen in different {127} directions, cannot be arrived at
by sight alone. When the spectator combines in his
conception the relations of long-drawn lines and large
figures, as he sees them by turning his head to the right
and to the left, upwards and downwards, he ceases to be an
Idomenian. And thus our conceptions of the properties of
space, derived through the exercise of one mode of
perception, are not at variance with those obtained in
another way; but all such conceptions, however produced or
suggested, are in harmony with each other; being, as has
already been said, only different aspects of the same idea.
14. If our perceptions of the position of objects around us
do not depend on the sense of vision alone, but on the
muscular feeling brought into play when we turn our head, it
will obviously follow that the same is true when we turn the
eye instead of the head. And thus we may learn the form of
objects, not by looking at them with a fixed gaze, but by
following the boundary of them with the eye. While the head
is held perfectly still, the eye can rove along the outlines
of visible objects, scrutinize each point in succession, and
leap from one point to another; each such act being
accompanied by a muscular consciousness which makes us aware
of the direction in which the look is travelling. And we may
thus gather information concerning the figures and places
which we trace out with the visual ray, as the blind man
learns the forms of things which he traces out with his
staff, being conscious of the motions of his hand.
15. This view of the mode in which the eye perceives
position, which is thus supported by the analogy of other
members employed for the same purpose, is further confirmed
by Sir Charles Bell by physiological reasons. He teaches us
that[9\2] when an object is seen we employ two senses: there
is an impression on the retina; but we receive also the idea
of position or relation in space, which it is not the office
of the retina to give, by our consciousness of the efforts
of the voluntary {128} muscles of the eye: and he has traced
in detail the course of the nerves by which these muscles
convey their information. The constant _searching_ motion of
the eye, as he terms it[10\2], is the means by which we
become aware of the position of objects about us.
[Note 9\2: _Phil. Trans._ 1823. On the Motions of the Eye.]
[Note 10\2: _Bridgewater Treatise_, p. 282. I have adopted,
in writing the above, the views and expressions of Sir
Charles Bell. The essential part of the doctrine there
presented is, that the eye constantly makes efforts to turn,
so that the image of an object to which our attention is
drawn, shall fall upon a certain particular point of the
retina; and that when the image falls upon any other point,
the eye turns away from this oblique into the direct
position. Other writers have maintained that the eye thus
turns not because the point on which the image falls in
direct vision is the most _sensible_ point, but that it is
the point of _greatest distinctness_ of vision. They urge
that a small star, which disappears when the eye is turned
full upon it, may often be seen by looking a little away
from it: and hence, they infer that the parts of the retina
removed from the spot of direct vision, are more sensible
than it is. The facts are very curious, however they be
explained, but they do not disturb the doctrine delivered in
the text.]
16. It is not to our present purpose to follow the
physiology of this subject; but we may notice that Sir C.
Bell has examined the special circumstances which belong to
this operation of the eye. We learn from him that the
particular point of the eye which thus traces the forms of
visible objects is a part of the retina which has been
termed the _sensible spot_; being that part which is _most
distinctly_ sensible to the impressions of light and colour.
This part, indeed, is not a spot of definite size and form,
for it appears that proceeding from a certain point of the
retina, the distinct sensibility diminishes on every side by
degrees. And the searching motion of the eye arises from the
desire which we instinctively feel of receiving upon the
sensible spot the image of the object to which the attention
is directed. We are uneasy and impatient till the eye is
turned so that this is effected. And as our attention is
transferred from point to point of the scene before us, the
eye, and this point of the eye in particular, travel along
with the thoughts; and the muscular sense, which tells us of
these movements of the organ of {129} vision, conveys to us
a knowledge of the forms and places which we thus
successively survey.
17. How much of activity there is in the process by which we
perceive the outlines of objects appears further from the
language by which we describe their forms. We apply to them
not merely adjectives of form, but verbs of motion. An
abrupt hill _starts_ out of the plain; a beautiful figure
has a _gliding_ outline. We have
The windy summit, wild and high,
Roughly _rushing_ on the sky.
These terms express the course of the eye as it follows the
lines by which such forms are bounded and marked. In like
manner another modern poet[11\2] says of Soracte, that it
From out the plain
_Heaves_ like a long-swept wave about to break,
And on the curl _hangs pausing_.
[Note 11\2: Byron, _Ch. Har._ vi. st. 75.]
Thus the muscular sense, which is inseparably connected with
an act originating in our own mind, not only gives us all
that portion of our perceptions of space in which we use the
sense of touch, but also, at least in a great measure,
another large portion of such perceptions, in which we
employ the sense of sight. As we have before seen that our
_knowledge_ of solid space and its properties is not
conceivable in any other way than as the result of a mental
act, governed by conditions depending on its own nature; so
it now appears that our _perceptions_ of visible figure are
not obtained without an act performed under the same
conditions. The sensations of touch and sight are
subordinated to an idea which is the basis of our
speculative knowledge concerning space and its relations;
and this same idea is disclosed to our consciousness by its
practically regulating our intercourse with the external world.
By considerations such as have been adduced and referred to,
it is proved beyond doubt, that in a great {130} number of
cases our knowledge of form and position is acquired from
the muscular sense, and not from sight directly:--for
instance, in all cases in which we have before us objects so
large and prospects so extensive that we cannot see the
whole of them in one position of the eye[12\2].
[Note 12\2: The expression in the first edition was 'large
objects and extensive spaces.' In the text as now given, I
state a definite size and extent, within which the sight by
itself can judge of position and figure.
The doctrine, that we require the assistance of the muscular
sense to enable us to perceive space of three dimensions, is
not at all inconsistent with this other doctrine, that
within the space which is seen by the fixed eye, we perceive
the relative positions of points directly by vision, and
that, consequently, we have a perception of _visible
figure_.
Sir Charles Bell has said, (_Phil. Trans._ 1823, p. 181,)
'It appears to me that the utmost ingenuity will be at a
loss to devise an explanation of that power by which the eye
becomes acquainted with the position and relation of
objects, if the sense of muscular activity be excluded which
accompanies the motion of the eyeball.' But surely we should
have no difficulty in perceiving the relation of the sides
and angles of a small triangle, placed before the eye, even
if the muscles of the eyeball were severed. This subject is
resumed b. iv. c. ii. sect. 11.]
We now quit the consideration of the properties of Space,
and consider the Idea of Time.
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