An Essay Towards a New Theory of Vision
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George Berkeley >> An Essay Towards a New Theory of Vision
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133. Now, if a square surface perceived by touch be of the same sort with
a square surface perceived by sight, it is certain the blind man here
mentioned might know a square surface as soon as he saw it: it is no more
but introducing into his mind by a new inlet an idea he has been already
well acquainted with. Since, therefore, he is supposed to have known by
his touch that a cube is a body terminated by square surfaces, and that a
sphere is not terminated by square surfaces: upon the supposition that a
visible and tangible square differ only IN NUMERO it follows that he
might know, by the unerring mark of the square surfaces, which was the
cube, and which not, while he only saw them. We must therefore allow
either that visible extension and figures are specifically distinct from
tangible extension and figures, or else that the solution of this problem
given by those two thoughtful and ingenious men is wrong.
134. Much more might be laid together in proof of the proposition I have
advanced: but what has been said is, if I mistake not, sufficient to
convince anyone that shall yield a reasonable attention: and as for those
that will not be at the pains of a little thought, no multiplication of
words will ever suffice to make them understand the truth, or rightly
conceive my meaning.
135. I cannot let go the above-mentioned problem without some reflexion
on it. It hath been evident that a man blind from his birth would not, at
first sight, denominate anything he saw by the names he had been used to
appropriate to ideas of touch, VID. sect. 106. Cube, sphere, table are
words he has known applied to things perceivable by touch, but to things
perfectly intangible he never knew them applied. Those words in their
wonted application always marked out to his mind bodies or solid things
which were perceived by the resistance they gave: but there is no
solidity, no resistance or protrusion, perceived by sight. In short, the
ideas of sight are all new perceptions, to which there be no names
annexed in his mind: he cannot therefore understand what is said to him
concerning them: and to ask of the two bodies he saw placed on the table,
which was the sphere, which the cube? were to him a question downright
bantering and unintelligible; nothing he sees being able to suggest to
his thoughts the idea of body, distance, or in general of anything he had
already known.
136. It is a mistake to think the same thing affects both sight and
touch. If the same angle or square which is the object of touch be also
the object of vision, what should hinder the blind man at first sight
from knowing it? For though the manner wherein it affects the sight be
different from that wherein it affected his touch, yet, there being
beside his manner or circumstance, which is new and unknown, the angle or
figure, which is old and known, he cannot choose but discern it.
137. Visible figure and extension having been demonstrated to be of a
nature entirely different and heterogeneous from tangible figure and
extension, it remains that we inquire concerning. Now that visible motion
is not of the same sort with tangible motion seems to need no farther
proof, it being an evident corollary from what we have shown concerning
the difference there is between visible and tangible extension: but for a
more full and express proof hereof we need only observe that one who had
not yet experienced vision would not at first sight know motion. Whence
it clearly follows that motion perceivable by sight is of a sort distinct
from motion perceivable by touch. The antecedent I prove thus: by touch
he could not perceive any motion but what was up or down, to the right or
left, nearer or farther from him; besides these and their several
varieties or complications, it is impossible he should have any idea of
motion. He would not therefore think anything to be motion, or give the
name motion to any idea which he could not range under some or other of
those particular kinds thereof. But from sect. 95 it is plain that by the
mere act of vision he could not know motion upwards or downwards, to the
right or left, or in any other possible direction. From which I conclude
he would not know motion at all at first sight. As for the idea of motion
in abstract, I shall not waste paper about it, but leave it to my reader
to make the best he can of it. To me it is perfectly unintelligible.
138. The consideration of motion may furnish a new field for inquiry: but
since the manner wherein the mind apprehends by sight the motion of
tangible objects, with the various degrees thereof, may be easily
collected from what hath been said concerning the manner wherein that
sense doth suggest their various distances, magnitudes, and situations, I
shall not enlarge any farther on this subject, but proceed to consider
what may be alleged, with greatest appearance of reason, against the
proposition we have shown to be true. For where there is so much
prejudice to be encountered, a bare and naked demonstration of the truth
will scarce suffice. We must also satisfy the scruples that men may raise
in favour of their preconceived notions, show whence the mistake arises,
how it came to spread, and carefully disclose and root out those false
persuasions that an early prejudice might have implanted in the mind.
139. FIRST, therefore, it will be demanded how visible extension and
figures come to be called by the same name with tangible extension and
figures, if they are not of the same kind with them? It must be something
more than humour or accident that could occasion a custom so constant and
universal as this, which has obtained in all ages and nations of the
world, and amongst allranks of men, the learned as well as the
illiterate.
140. To which I answer, we can no more argue a visible and tangible
square to be of the same species from their being called by the same
name, than we can that a tangible square and the monosyllable consisting
of six letters whereby it is marked are of the same species because they
are both called by the same name. It is customary to call written words
and the things they signify by the same name: for words not being
regarded in their own nature, or otherwise than as they are marks of
things, it had been superfluous, and beside the design of language, to
have given them names distinct from those of the things marked by them.
The same reason holds here also. Visible figures are the marks of
tangible figures, and from sect. 59 it is plain that in themselves they
are little regarded, or upon any other score than for their connexion
with tangible figures, which by nature they are ordained to signify. And
because this language of nature doth not vary in different ages or
nations, hence it is that in all times and places visible figures are
called by the same names as the respective tangible figures suggested by
them, and not because they are alike or of the same sort with them.
141. But, say you, surely a tangible square is liker to a visible square
than to a visible circle: it has four angles and as many sides: so also
has the visible square: but the visible circle has no such thing, being
bounded by one uniform curve without right lines or angles, which makes
it unfit to represent the tangible square but very fit to represent the
tangible circle. Whence it clearly follows that visible figures are
patterns of, or of the same species with, the respective tangible figures
represented by them: that they are like unto them, and of their own
nature fitted to represent them, as being of the same sort: and that they
are in no respect arbitrary signs, as words.
142. I answer, it must be acknowledged the visible square is fitter than
the visible circle to represent the tangible square, but then it is not
because it is liker, or more of a species with it, but because the
visible square contains in it several distinct parts, whereby to mark the
several distinct corresponding parts of a tangible square, whereas the
visible circle doth not. The square perceived by touch hath four
distinct, equal sides, so also hath it four distinct equal angles. It is
therefore necessary that the visible figure which shall be most proper to
mark it contain four distinct equal parts corresponding to the four sides
of the tangible square, as likewise four other distinct and equal parts
whereby to denote the four equal angles of the tangible square. And
accordingly we see the visible figures contain in them distinct visible
parts, answering to the distinct tangible parts of the figures signified
or suggested by them.
143. But it will not hence follow that any visible figure is like unto,
or of the same species with, its corresponding tangible figure, unless it
be also shown that not only the number but also the kind of the parts be
the same in both. To illustrate this, I observe that visible figures
represent tangible figures much after the same manner that written words
do sounds. Now, in this respect words are not arbitrary, it not being
indifferent what written word stands for any sound: but it is requisite
that each word contain in it so many distinct characters as there are
variations in the sound it stands for. Thus the single letter A is proper
to mark one simple uniform sound; and the word ADULTERY is accommodated
to represent the sound annexed to it, in the formation whereof there
being eight different collisions or modifications of the air by the
organs of speech, each of which produces a difference of sound, it was
fit the word representing it should consist of as many distinct
characters, thereby to mark each particular difference or part of the
whole sound. And yet nobody, I presume, will say the single letter a, or
the word ADULTERY, are like unto, or of the same species with, the
respective sounds by them represented. It is indeed arbitrary that, in
general, letters of any language represent sounds at all: but when that
is once agreed, it is not arbitrary what combination of letters shall
represent this or that particular sound. I leave this with the reader to
pursue, and apply it in his own thoughts.
144. It must be confessed that we are not so apt to confound other signs
with the things signified, or to think them of the same species, as we
are visible and tangible ideas. But a little consideration will show us
how this may be without our supposing them of a like nature. These signs
are constant and universal, their connexion with tangible ideas has been
learnt at our first entrance into the world; and ever since, almost every
moment of our lives, it has been occurring to our thoughts, and fastening
and striking deeper on our minds. When we observe that signs are
variable, and of human institution; when we remember there was a time
they were not connected in our minds with those things they now so
readily suggest; but that their signification was learned by the slow
steps of experience: this preserves us from confounding them. But when we
find the same signs suggest the same things all over the world; when we
know they are not of human institution, and cannot remember that we ever
learned their signification, but think that at first sight they would
have suggested to us the same things they do now: all this persuades us
they are of the same species as the things respectively represented by
them, and that it is by a natural resemblance they suggest them to our
minds.
145. Add to this that whenever we make a nice survey of any object,
successively directing the optic axis to each point thereof, there are
certain lines and figures described by the motion of the head or eye,
which being in truth perceived by feeling, do nevertheless so mix
themselves, as it were, with the ideas of sight, that we can scarce think
but they appertain to that sense. Again, the ideas of sight enter into
the mind several at once, more distinct and unmingled than is usual in
the other senses beside the touch. Sounds, for example, perceived at the
same instant, are apt to coalesce, if I may so say, into one sound: but
we can perceive at the same time great variety of visible objects, very
separate and distinct from each other. Now tangible extension being made
up of several distinct coexistent parts, we may hence gather another
reason that may dispose us to imagine a likeness or an analogy between
the immediate objects of sight and touch. But nothing, certainly, doth
more contribute to blend and confound them together than the strict and
close connexion they have with each other. We cannot open our eyes but
the ideas of distance, bodies, and tangible figures are suggested by
them. So swift and sudden and unperceived is the transition from visible
to tangible ideas that we can scarce forbear thinking them equally the
immediate object of vision.
146. The prejudice which is grounded on these, and whatever other causes
may be assigned thereof, sticks so fast that it is impossible without
obstinate striving and labour of the mind to get entirely clear of it.
But then the reluctancy we find in rejecting any opinion can be no
argument of its truth to whoever considers what has been already shown
with regard to the prejudices we entertain concerning the distance,
magnitude, and situation of objects; prejudices so familiar to our minds,
so confirmed and inveterate, as they will hardly give way to the clearest
demonstration.
147. Upon the whole, I think we may fairly conclude that the proper
objects of vision constitute an universal language of the Author of
Nature, whereby we are instructed how to regulate our actions in order to
attain those things that are necessary to the preservation and well-being
of our bodies, as also to avoid whatever may be hurtful and destructive
of them. It is by their information that we are principally guided in all
the transactions and concerns of life. And the manner wherein they
signify and mark unto us the objects which are at a distance is the same
with that of languages and signs of human appointment, which do not
suggest the things signified by any likeness or identity of nature, but
only by an habitual connexion that experience has made us to observe
between them.
148. Suppose one who had always continued blind be told by his guide that
after he has advanced so many steps he shall come to the brink of a
precipice, or be stopped by a wall; must not this to him seem very
admirable and surprizing? He cannot conceive how it is possible for
mortals to frame such predictions as these, which to him would seem as
strange and unaccountable as prophesy doth to others. Even they who are
blessed with the visive faculty may (though familiarity make it less
observed) find therein sufficient cause of admiration. The wonderful art
and contrivance wherewith it is adjusted to those ends and purposes for
which it was apparently designed, the vast extent, number, and variety of
objects that are at once with so much ease and quickness and pleasure
suggested by it: all these afford subject for much and pleasing
speculation, and may, if anything, give us some glimmering analogous
prenotion of things which are placed beyond the certain discovery and
comprehension of our present state.
149. I do not design to trouble myself with drawing corollaries from the
doctrine I have hitherto laid down. If it bears the test others may, so
far as they shall think convenient, employ their thoughts in extending it
farther, and applying it to whatever purposes it may be subservient to:
only, I cannot forbear making some inquiry concerning the object of
geometry, which the subject we have been upon doth naturally lead one to.
We have shown there is no such idea as that of extension in abstract, and
that there are two kinds of sensible extension and figures which are
entirely distinct and heterogeneous from each other. Now, it is natural
to inquire which of these is the object of geometry.
150. Some things there are which at first sight incline one to think
geometry conversant about visible extension. The constant use of the
eyes, both in the practical and speculative parts of that science, doth
very much induce us thereto. It would, without doubt, seem odd to a
mathematician to go about to convince him the diagrams he saw upon paper
were not the figures, or even the likeness of the figures, which make the
subject of the demonstration. The contrary being held an unquestionable
truth, not only by mathematicians, but also by those who apply themselves
more particularly to the study of logic; I mean, who consider the nature
of science, certainty, and demonstration: it being by them assigned as
one reason of the extraordinary clearness and evidence of geometry that
in this science the reasonings are free from those inconveniences which
attend the use of arbitrary signs, the very ideas themselves being copied
out and exposed to view upon paper. But, by the bye, how well this agrees
with what they likewise assert of abstract ideas being the object of
geometrical demonstration I leave to be considered.
151. To come to a resolution in this point we need only observe what hath
been said in sect. 59, 60, 61, where it is shown that visible extensions
in themselves are little regarded, and have no settled determinable
greatness, and that men measure altogether, by the application of
tangible extension to tangible extension. All which makes it evident that
visible extension and figures are not the object of geometry.
152. It is therefore plain that visible figure are of the same use in
geometry that words are: and the one may as well be accounted the object
of that science as the other, neither of them being otherwise concerned
therein than as they represent or suggest to the mind the particular
tangible figures connected with them. There is indeed this difference
between the signification of tangible figures by visible figures, and of
ideas by words: that whereas the latter is variable and uncertain,
depending altogether on the arbitrary appointment of men, the former is
fixed and immutably the same in all times and places. A visible square,
for instance, suggests to the mind the same tangible figure in Europe
that it doth in America. Hence it is that the voice of the Author of'
Nature which speaks to our eyes, is not liable to that misinterpretation
and ambiguity that languages of human contrivance are unavoidably subject
to.
153. Though what has been said may suffice to show what ought to be
determined with relation to the object of geometry, I shall nevertheless,
for the fuller illustration thereof, consider the case of an
intelligence, or unbodied spirit, which is supposed to see perfectly
well, i.e. to have a clear perception of the proper and immediate objects
of sight, but to have no sense of touch. Whether there be any such being
in Nature or no is beside my purpose to inquire. It sufficeth that the
supposition contains no contradiction in it. Let us now examine what
proficiency such a one may be able to make in geometry. Which speculation
will lead us more clearly to see whether the ideas of sight can possibly
be the object of that science.
154. FIRST, then, it is certain the aforesaid intelligence could have no
idea of a solid, or quantity of three dimensions, which followeth from
its not having any idea of distance. We indeed are prone to think that we
have by sight the ideas of space and solids, which ariseth from our
imagining that we do, strictly speaking, see distance and some parts of
an object at a greater distance than others; which hath been demonstrated
to be the effect of the experience we have had, what ideas of touch are
connected with such and such ideas attending vision: but the intelligence
here spoken of is supposed to have no experience of touch. He would not,
therefore, judge as we do, nor have any idea of distance, outness, or
profundity, nor consequently of space or body, either immediately or by
suggestion. Whence it is plain he can have no notion of those parts of
geometry which relate to the mensuration of solids and their convex or
concave surfaces, and contemplate the properties of lines generated by
the section of a solid. The conceiving of any part whereof is beyond the
reach of his faculties.
155. Farther, he cannot comprehend the manner wherein geometers describe
a right line or circle; the rule and compass with their use being things
of which it is impossible he should have any notion: nor is it an easier
matter for him to conceive the placing of one plane or angle on another,
in order to prove their equality: since that supposeth some idea of
distance or external space. All which makes it evident our pure
intelligence could never attain to know so much as the first elements of
plane geometry. And perhaps upon a nice inquiry it will be found he
cannot even have an idea of plane figures any more than he can of solids;
since some idea of distance is necessary to form the idea of a
geometrical plane, as will appear to whoever shall reflect a little on
it.
156. All that is properly perceived by the visive faculty amounts to no
more than colours, with their variations and different proportions of
light and shade. But the perpetual mutability and fleetingness of those
immediate objects of sight render them incapable of being managed after
the manner of geometrical figures; nor is it in any degree useful that
they should. It is true there are divers of them perceived at once, and
more of some and less of others: but accurately to compute their
magnitude and assign precise determinate proportions between things so
variable and inconstant, if we suppose it possible to be done, must yet
be a very trifling and insignificant labour.
157. I must confess men are tempted to think that flat or plane figures
are immediate objects of sight, though they acknowledge solids are not.
And this opinion is grounded on what is observed in painting, wherein (it
seems) the ideas immediately imprinted on the mind are only of planes
variously coloured, which by a sudden act of the judgment are changed
into solids. But with a little attention we shall find the planes here
mentioned as the immediate objects of sight are not visible but tangible
planes. For when we say that pictures are planes, we mean thereby that
they appear to the touch smooth and uniform. But then this smoothness and
uniformity, or, in other words, this planeness of the picture, is not
perceived immediately by vision: for it appeareth to the eye various and
multiform.
158. From all which we may conclude that planes are no more the immediate
object of sight than solids. What we strictly see are not solids, nor yet
planes variously coloured: they are only diversity of colours. And some
of these suggest to the mind solids, and other plane figures, just as
they have been experienced to be connected with the one or the other: so
that we see planes in the same way that we see solids, both being equally
suggested by the immediate objects of sight, which accordingly are
themselves denominated planes and solids. But though they are called by
the same names with the things marked by them, they are nevertheless of a
nature entirely different, as hath been demonstrated.
159. What hath been said is, if I mistake not, sufficient to decide the
question we proposed to examine, concerning the ability of a pure spirit,
such as we have described, to know GEOMETRY. It is, indeed, no easy
matter for us to enter precisely into the thoughts of such an
intelligence, because we cannot without great pains cleverly separate and
disentangle in our thoughts the proper objects of sight from those of
touch which are connected with them. This, indeed, in a complete degree
seems scarce possible to be performed: which will not seem strange to us
if we consider how hard it is for anyone to hear the words of his native
language pronounced in his ears without understanding them. Though he
endeavour to disunite the meaning from the sound, it will nevertheless
intrude into his thoughts, and he shall find it extreme difficult, if not
impossible, to put himself exactly in the posture of a foreigner that
never learned the language, so as to be affected barely with the sounds
themselves, and not perceive the signification annexed to them.
160. By this time, I suppose, it is clear that neither abstract nor
visible extension makes the object of geometry; the not discerning of
which may perhaps have created some difficulty and useless labour in
mathematics. Sure I am, that somewhat relating thereto has occurred to my
thoughts, which, though after the most anxious and repeated examination I
am forced to think it true, doth, nevertheless, seem so far out of the
common road of geometry, that I know not whether it may not be thought
presumption, if I should make it public in an age, wherein that science
hath received such mighty improvements by new methods; great part whereof,
as well as of the ancient discoveries, may perhaps lose their reputation,
and much of that ardour with which men study the abstruse and fine
geometry be abated, if what to me, and those few to whom I have imparted
it, seems evidently true, should really prove to be so.
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