An Essay Towards a New Theory of Vision

G >> George Berkeley >> An Essay Towards a New Theory of Vision

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78. This phenomenon of the horizontal moon is a clear instance of the
insufficiency of lines and angles for explaining the way wherein the mind
perceives and estimates the magnitude of outward objects. There is
nevertheless a use of computation by them in order to determine the
apparent magnitude of things, so far as they have a connexion with, and
are proportional to, those other ideas or perceptions which are the true
and immediate occasions that suggest to the mind the apparent magnitude
of things. But this in general may, I think, be observed concerning
mathematical computation in optics: that it can never be very precise and
exact since the judgments we make of the magnitude of external things do
often depend on several circumstances, which are not proportionable to,
or capable of being defined by, lines and angles.

79. From what has been said we may safely deduce this consequence; to
wit, that a man born blind and made to see would, at first opening of his
eyes, make a very different judgment of the magnitude of objects
intromitted by them from what others do. He would not consider the ideas
of sight with reference to, or as having any connexion with, the ideas of
touch: his view of them being entirely terminated within themselves, he
can no otherwise judge them great or small than as they contain a greater
or lesser number of visible points. Now, it being certain that any
visible point can cover or exclude from view only one other visible
point, it follows that whatever object intercepts the view of another
hath an equal number of visible points with it; and consequently they
shall both be thought by him to have the same magnitude. Hence it is
evident one in those circumstances would judge his thumb, with which he
might hide a tower or hinder its being seen, equal to that tower, or his
hand, the interposition whereof might conceal experimental means the
firmament from his view, equal to the firmament: how great an inequality
soever there may in our apprehensions seem to be betwixt those two
things, because of the customary and close connexion that has grown up in
our minds between the objects of sight and touch; whereby the very
different and distinct ideas of those two senses are so blended and
confounded together as to be mistaken for one and the same thing; out of
which prejudice we cannot easily extricate ourselves.

80. For the better explaining the nature of vision, and setting the
manner wherein we perceive magnitudes in a due light, I shall proceed to
make some observations concerning matters relating thereto, whereof the
want of reflexion, and duly separating between tangible and visible
ideas, is apt to create in us mistaken and confused notions. And FIRST, I
shall observe that the MINIMUM VISIBILE is exactly equal in all beings
whatsoever that are endowed with the visive faculty. No exquisite
formation of the eye, no peculiar sharpness of sight, can make it less in
one creature than in another; for it not being distinguishable into
parts, nor in any wise a consisting of them, it must necessarily be the
same to all. For suppose it otherwise, and that the MINIMUM VISIBILE of a
mite, for instance, be less than the MINIMUM VISIBILE of a man: the
latter therefore may by detraction of some part be made equal to the
former: it doth therefore consist of parts, which is inconsistent with
the notion of a MINIMUM VISIBILE or point.

81. It will perhaps be objected that the MINIMUM VISIBILE of a man doth
really and in itself contain parts whereby it surpasses that of a mite,
though they are not perceivable by the man. To which I answer, the
MINIMUM VISIBILE having (in like manner as all other the proper and
immediate objects of sight) been shown not to have any existence without
the mind of him who sees it, it follows there cannot be any pan of it
that is not actually perceived, and therefore visible. Now for any object
to contain distinct visible parts, and at the same time to be a MINIMUM
VISIBILE, is a manifest contradiction.

82. Of these visible points we see at all times an equal number. It is
every whit as great when our view is contracted and bounded by near
objects as when it is extended to larger and remoter. For it being
impossible that one MINIMUM VISIBILE should obscure or keep out of sight
mote than one other, it is a plain consequence that when my view is on
all sides bounded by the walls of my study see just as many visible
points as I could, in case that by the removal of the study-walls and all
other obstructions, I had a full prospect of the circumjacent fields,
mountains, sea, and open firmament: for so long as I am shut up within
the walls, by their interposition every point of the external objects is
covered from my view: but each point that is seen being able to cover or
exclude from sight one only other corresponding point, it follows that
whilst my sight is confined to those narrow walls I see as many points,
or MINIMA VISIBILIA, as I should were those walls away, by looking on all
the external objects whose prospect is intercepted by them. Whenever
therefore we are said to have a greater prospect at one time than
another, this must be understood with relation, not to the proper and
immediate, but the secondary and mediate objects of vision, which, as
hath been shown, properly belong to the touch.

83. The visive faculty considered with reference to its immediate objects
may be found to labour of two defects. FIRST, in respect of the extent or
number of visible points that are at once perceivable by it, which is
narrow and limited to a certain degree. It can take in at one view but a
certain determinate number of MINIMA VISIBILIA, beyond which it cannot
extend its prospect. Secondly, our sight is defective in that its view is
not only narrow, but also for the most part confused: of those things
that we take in at one prospect we can see but a few at once clearly and
unconfusedly: and the more we fix our sight on any one object, by so much
the darker and more indistinct shall the rest appear.

84. Corresponding to these two defects of sight, we may imagine as many
perfections, to wit, 1ST, that of comprehending in one view a greater
number of visible points. 2DLY, of being able to view them all equally
and at once with the utmost clearness and distinction. That those
perfections are not actually in some intelligences of a different order
and capacity from ours it is impossible for us to know.

85. In neither of those two ways do microscopes contribute to the
improvement of sight; for when we look through a microscope we neither
see more visible points, nor are the collateral points more distinct than
when we look with the naked eye at objects placed in a due distance. A
microscope brings us, as it were, into a new world: it presents us with a
new scene of visible objects quite different from what we behold with the
naked eye. But herein consists the most remarkable difference, to wit,
that whereas the objects perceived by the eye alone have a certain
connexion with tangible objects, whereby we are taught to foresee what
will ensue upon the approach or application of distant objects to the
parts of our own body, which much conduceth to its preservation, there is
not the like connexion between things tangible and those visible objects
that are perceived by help of a fine microscope.

86. Hence it is evident that were our eyes turned into the nature of
microscopes, we should not be much benefited by the change; we should be
deprived of the forementioned advantage we at present receive by the
visive faculty, and have left us only the empty amusement of seeing,
without any other benefit arising from it. But in that case, it will
perhaps be said, our sight would be endued with a far greater sharpness
and penetration than it now hath. But it is certain from what we have
already shown that the MINIMUM VISIBILE is never greater or lesser, but
in all cases constantly the same: and in the case of microscopical eyes I
see only this difference, to wit, that upon the ceasing of a certain
observable connexion betwixt the divers perceptions of sight and touch,
which before enabled us to regulate our actions by the eye, it would now
be rendered utterly unserviceable to that purpose.

87. Upon the whole it seems that if we consider the use and end of sight,
together with the present state and circumstances of our being, we shall
not find any great cause to complain of any defect or imperfection in it,
or easily conceive how it could be mended. With such admirable wisdom is
that faculty contrived, both for the pleasure and convenience of life.

88. Having finished what I intended to say concerning the distance and
magnitude of objects, I come now to treat of the manner wherein the mind
perceives by sight their situation. Among the discoveries of the last
age, it is reputed none of the least that the manner of vision hath been
more clearly explained than ever it had been before. There is at this day
no one ignorant that the pictures of external objects are painted on the
RETINA, or fund of the eye: that we can see nothing which is not so
painted: and that, according as the picture is more distinct or confused,
so also is the perception we have of the object: but then in this
explication of vision there occurs one mighty difficulty. The objects are
painted in an inverted order on the bottom of the eye: the upper part of
any object being painted on the lower part of the eye, and the lower part
of the object on the upper part of the eye: and so also as to right and
left. Since therefore the pictures are thus inverted, it is demanded how
it comes to pass that we see the objects erect and in their natural
posture?

89. In answer to this difficulty we are told that the mind, perceiving an
impulse of a ray of light on the upper part of the eye, considers this
ray as coming in a direct line from the lower part of the object; and in
like manner tracing the ray that strikes on the lower part of the eye, it
is directed to the upper part of the object. Thus in the adjacent figure,
C, the lower point of the object ABC, is projected on C the upper part of
the eye. So likewise the highest point A is projected on A the lowest
part of the eye, which makes the representation CBA inverted: but the
mind considering the stroke that is made on C as coming in the straight
line CC from the lower end of the object; and the stroke or impulse on a
as coming in the line AA from the upper end of the object, is directed to
make a right judgment of the situation of the object ABC, notwithstanding
the picture of it is inverted. This is illustrated by conceiving a blind
man who, holding in his hands two sticks that cross each other, doth with
them touch the extremities of an object, placed in a perpendicular
situation. It is certain this man will judge that to be the upper part of
the object which he touches with the stick held in the undermost hand,
and that to be the lower part of the object which he touches with the
stick in his uppermost hand. This is the common explication of the erect
appearance of objects, which is generally received and acquiesced in,
being (as Mr. Molyneux tells us [Diopt. par. 2. c. 7. P. 289.]) 'allowed
by all men as satisfactory'.

90. But this account to me does not seem in any degree true. Did I
perceive those impulses, decussations, and directions of the rays of
light in like manner as hath been set forth, then indeed it would not be
altogether void of probability. And there might be some pretence for the
comparison of the blind man and his cross sticks. But the case is far
otherwise. I know very well that I perceive no such thing. And of
consequence I cannot thereby make an estimate of the situation of
objects. I appeal to anyone's experience, whether he be conscious to
himself that he thinks on the intersection made by the radious [SIC]
pencils, or pursues the impulses they give in right lines, whenever he
perceives by sight the position of any object? To me it seems evident
that crossing and tracing of the rays is never thought on by children,
idiots, or in truth by any other, save only those who have applied
themselves to the study of optics. And for the mind to judge of the
situation of objects by those things without perceiving them, or to
perceive them without knowing it, is equally beyond my comprehension. Add
to this that the explaining the manner of vision by the example of cross
sticks and hunting for the object along the axes of the radious pencils,
doth suppose the proper objects of sight to be perceived at a distance
from us, contrary to what hath been demonstrated.

91. It remains, therefore, that we look for some other explication of
this difficulty: and I believe it not impossible to find one, provided we
examine it to the bottom, and carefully distinguish between the ideas of
sight and touch; which cannot be too oft inculcated in treating of
vision: but more especially throughout the consideration of this affair
we ought to carry that distinction in our thoughts: for that from want of
a right understanding thereof the difficulty of explaining erect vision
seems chiefly to arise.

92. In order to disentangle our minds from whatever prejudices we may
entertain with relation to the subject in hand, nothing seems more
apposite than the taking into our thoughts the case of one born blind,
and afterwards, when grown up, made to see. And though, perhaps, it may
not be an easy task to divest ourselves entirely of the experience
received from sight, so as to be able to put our thoughts exactly in the
posture of such a one's, we must, nevertheless, as far as possible,
endeavour to frame true conceptions of what might reasonably be supposed
to pass in his mind.

93. It is certain that a man actually blind, and who had continued so
from his birth, would by the sense of feeling attain to have ideas of
upper and lower. By the motion of his hand he might discern the situation
of any tangible object placed within his FI reach. That part on which he
felt himself supported, or towards which he perceived his body to
gravitate, he would term lower, and the contrary to this upper; and
accordingly denominate whatsoever objects he touched.

94. But then, whatever judgments he makes concerning the situation of
objects are confined to those only that are perceivable by touch. All
those things that are intangible and of a spiritual nature, his thoughts
and desires, his passions, and in general all the modifications of the
soul, to these he would never apply the terms UPPER and LOWER, except
only in a metaphorical sense. He may, perhaps, by way of allusion, speak
of high or low thoughts: but those terms in their proper signification
would never be applied to anything that was not conceived to exist
without the mind. For a man born blind, and remaining in the same state,
could mean nothing else by the words HIGHER and LOWER than a greater or
lesser distance from the earth; which distance he would measure by the
motion or application of his hand or some other part of his body. It is
therefore evident that all those things which, in respect of each other,
would by him be thought higher or lower, must be such as were conceived
to exist without his mind, in the ambient space.

95. Whence it plainly follows that such a one, if we suppose him made to
see, would not at first sight think anything he saw was high or low,
erect or inverted; for it hath been already demonstrated in sect. 41 that
he would not think the things he perceived by sight to be at any distance
from him, or without his mind. The objects to which he had hitherto been
used to apply the terms UP and DOWN, HIGH and LOW, were such only as
affected or were some way perceived by his couch: but the proper objects
of vision make a new set of ideas, perfectly distinct and different from
the former, and which can in no sort make themselves perceived by touch.
There is, therefore, nothing at all that could induce him to think those
terms applicable to them: nor would he ever think it till such time as he
had observed their connexion with tangible objects, and the same
prejudice began to insinuate itself into his understanding, which from
their infancy had grown up in the understandings of other men.

96. To set this matter in a clearer light I shall make use of an example.
Suppose the above-mentioned blind person by his touch perceives a man to
stand erect. Let us inquire into the manner of this. By the application
of his hand to the several parts of a human body he had perceived
different tangible ideas, which being collected into sundry complex ones,
have distinct names annexed to them. Thus one combination of a certain
tangible figure, bulk, and consistency of parts is called the head,
another the hand, a third the foot, and so of the rest: all which complex
ideas could, in his understanding, be made up only of ideas perceivable
by touch. He had also by his touch obtained an idea of earth or ground,
towards which he perceives the parts of his body to have a natural
tendency. Now, by ERECT nothing more being meant than that perpendicular
position of a man wherein his feet are nearest to the earth, if the blind
person by moving his hand over the parts of the man who stands before him
perceives the tangible ideas that compose the head to be farthest from,
and those that compose the feet to be nearest to, that other combination
of tangible ideas which he calls earth, he will denominate that man
erect. But if we suppose him on a sudden to receive his sight, and that
he behold a man standing before him, it is evident in that case he would
neither judge the man he sees to be erect nor inverted; for he never
having known those terms applied to any other save tangible things, or
which existed in the space without him, and what he sees neither being
tangible nor perceived as existing without, he could not know that in
propriety of language they were applicable to it.

97. Afterwards, when upon turning his head or eyes up and down to the
right and left he shall observe the visible objects to change, and shall
also attain to know that they are called by the same names, and connected
with the objects perceived by touch; then indeed he will come to speak of
them and their situation, in the same terms that he has been used to
apply to tangible things; and those that he perceives by turning up his
eyes he will call upper, and those that by turning down his eyes he will
call lower.

98. And this seems to me the true reason why he should think those
objects uppermost that are painted on the lower part of his eye: for by
turning the eye up they shall be distinctly seen; as likewise those that
are painted on the highest part of the eye shall be distinctly seen by
turning the eye down, and are for that reason esteemed lowest; for we
have shown that to the immediate objects of sight considered in
themselves, he would not attribute the terms HIGH and LOW. It must
therefore be on account of some circumstances which are observed to
attend them: and these, it is plain, are the actions of turning the eye
up and down, which suggest a very obvious reason why the mind should
denominate the objects of sight accordingly high or low. And without this
motion of the eye, this turning it up and down in order to discern
different objects, doubtless ERECT, INVERSE, and other the like terms
relating to the position of tangible objects, would never have been
transferred, or in any degree apprehended to belong to the ideas of
sight: the mere act of seeing including nothing in it to that purpose;
whereas the different situations of the eye naturally direct the mind to
make a suitable judgment of the situation of objects intromitted by it.

99. Farther, when he has by experience learned the connexion there is
between the several ideas of sight and touch, he will be able, by the
perception he has of the situation of visible things in respect of one
another, to make a sudden and true estimate of the situation of outward,
tangible things corresponding to them. And thus it is he shall perceive
by sight the situation of external objects which do not properly fall
under that sense.

100. I know we are very prone to think that, if just made to see, we
should judge of the situation of visible things as we do now: but we are
also as prone to think that, at first sight, we should in the same way
apprehend the distance and magnitude of objects as we do now: which hath
been shown to be a false and groundless persuasion. And for the like
reasons the same censure may be passed on the positive assurance that
most men, before they have thought sufficiently of the matter, might have
of their being able to determine by the eye at first view, whether
objects were erect or inverse.

101. It will, perhaps, be objected co our opinion that a man, for
instance, being thought erect when his feet are next the earth, and
inverted when his head is next the earth, it doth hence follow that by
the mere act of vision, without any experience or altering the situation
of the eye, we should have determined whether he were erect or inverted:
for both the earth itself, and the limbs of the man who stands thereon,
being equally perceived by sight, one cannot choose seeing what part of
the man is nearest the earth, and what part farthest from it, i.e.
whether he be erect or inverted.

I02. To which I answer, the ideas which constitute the tangible earth and
man are entirely different from those which constitute the visible earth
and man. Nor was it possible, by virtue of the visive faculty alone,
without superadding any experience of touch, or altering the position of
the eye, ever to have known, or so much as suspected, there had been any
relation or connexion between them. Hence a man at first view would not
denominate anything he saw earth, or head, or foot; and consequently he
could not tell by the mere act of vision whether the head or feet were
nearest the earth: nor, indeed, would we have thereby any thought of
earth or man, erect or inverse, at all: which will be made yet more
evident if we nicely observe, and make a particular comparison between,
the ideas of both senses.

103. That which I see is only variety of light and colours. That which I
feel is hard or soft, hot or cold, rough or smooth. What similitude, what
connexion have those ideas with these? Or how is it possible that anyone
should see reason to give one and the same name to combinations of ideas
so very different before he had experienced their coexistence? We do not
find there is any necessary connexion betwixt this or that tangible
quality and any colour whatsoever. And we may sometimes perceive colours
where there is nothing to be felt. All which doth make it manifest that
no man, at first receiving of his sight, would know there was any
agreement between this or that particular object of his sight and any
object of touch he had been already acquainted with: the colours,
therefore, of the head would to him no more suggest the idea of head than
they would the idea of foot.

104. Farther, we have at large shown (VID. sect. 63 and 64) there is no
discoverable necessary connexion between any given visible magnitude and
any one particular tangible magnitude; but that it is entirely the result
of custom and experience, and depends on foreign and accidental
circumstances that we can by the perception of visible extension inform
ourselves what may be the extension of any tangible object connected with
it. Hence it is certain that neither the visible magnitude of head or
foot would bring along with them into the mind, at first opening of the
eyes, the respective tangible magnitudes of those parts.

105. By the foregoing section it is plain the visible figure of any part
of the body hath no necessary connexion with the tangible figure thereof,
so as at first sight to suggest it to the mind. For figure is the
termination of magnitude; whence it follows that no visible magnitude
having in its own nature an aptness to suggest any one particular
tangible magnitude, so neither can any visible figure be inseparably
connected with its corresponding tangible figure: so as of itself and in
a way prior to experience, it might suggest it to the understanding. This
will be farther evident if we consider that what seems smooth and round
to the touch may to sight, if viewed through a microscope, seem quite
otherwise.

106. From all which laid together and duly considered, we may clearly
deduce this inference. In the first act of vision no idea entering by the
eye would have a perceivable connexion with the ideas to which the names
EARTH, MAN, HEAD, FOOT, etc., were annexed in the understanding of a
person blind from his birth; so as in any sort to introduce them into his
mind, or make themselves be called by the same names, and reputed the
same things with them, as afterwards they come to be.

107. There doth, nevertheless, remain one difficulty, which perhaps may
seem to press hard on our opinion, and deserve not to be passed over: for
though it be granted that neither the colour, size, nor figure of the
visible feet have any necessary connexion with the ideas that compose the
tangible feet, so as to bring them at first sight into my mind, or make
me in danger of confounding them before I had been used to, and for some
time experienced their connexion: yet thus much seems undeniable, namely,
that the number of the visible feet being the same with that of the
tangible feet, I may from hence without any experience of sight
reasonably conclude that they represent or are connected with the feet
rather than the head. I say, it seems the idea of two visible feet will
sooner suggest to the mind the idea of two tangible feet than of one
head; so that the blind man upon first reception of the visive faculty
might know which were the feet or two, and which the head or one.

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