Thomas Reid.

The works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters online

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erect, to be a deduction of reason, drawn from
certain premises : whereas it seems to be an
immediate perception. And, secondly, Be-
cause the premises from which all mankind
are supposed to draw this conclusion, never
entered into the minds of the far greater
part, but are absolutely unknown to them.
We have no feeling or perception of the
pictures upon the retina, and as little surely

* This inverted picture is seen if we take the eye
of an ox i for example, and cut away the posterior
part of the sclerotica and choroid ; but, without this
preparation, it is apparent in the eyes of albino ani-
mals, of the owl, &c, in which the hard coat and
ctio • aid arc semi-diaphanous. — H.



of the position of them. In order to see
objects erect, according to the principles
of Kepler or Des Cartes, we must previ-
ously know that the rays of light come
from the object to the eye in straight lines ;
we must know that the rays from different
points of the object cross one another
before they form the pictures upon the
retina i and, lastly, we must know that these
pictures are really inverted. Now, although
all these things are true, and known to
philosophers, yet they are absolutely un-
known to the far greatest part of mankind :
nor is it possible that they who are abso-
lutely ignorant of them, should reason from
them, and build conclusions upon them.
Since, therefore, visible objects appear erect
to the ignorant as well as to the learned,
this cannot be a conclusion drawn from
premises which never entered into the minds
of the ignorant. We have indeed had oc-
casion to observe many instances of con-
clusions drawn, either by means of original
principles, or by habit, from premises which
pass through the mind very quickly, and
which are never made the objects of re-
flection ; but surely no man will conceive
it possible to draw conclusions fron pre-
mises which never entered into the mind at

Bishop Berkeley having justly rejected
this solution, gives one founded upon his
own principles ; wherein he is followed by
the judicious Dr Smith, in his " Optics;"
and this we shall next explain and examine.

That ingenious writer conceives the ideas
of sight to be altogether unlike those of
touch. And, since the notions we have of
an object by these different senses have no
similitude, we can learn only by experience
how one sense will be affected, by what, in
a certain manner, affects the other. Figure,
position, and even number, in tangible
objects, are ideas of touch ; and, although
there is no similitude between these and
the ideas of sight, yet we learn by expe-
rience, that a triangle affects the sight in
such a manner, and that a square affects it
in such another manner — hence we judge
that which affects it in the first manner, to
be a triangle, and that which affects it in
the second, to be a square. In the same
way, finding, from experience, that an object
in an erect position affects the eye in one
manner, and the same object in an inverted
position affects it in another, we learn to
judge, by the manner in which the eye is
affected, whether the object is erect or in-
verted. In a word, visible ideas, according
to this author, are signs of the tangible ;
and the mind passeth from the sign to the
thing signified, not by means of any simi-
litude between the one and other, nor by
any natural principle, but by having found
them constantly conjoined in experience, as

the sounds of a language are with the things
they signify : so that, if the images upon
the retina had been always erect, they
would have shewn the objects erect, in the
manner as they do now that they are in-
verted — nay, if the visible idea which we
now have from an inverted object, had been
associated from the beginning with the erect
position of that object, it would have signi-
fied an erect position, as readily as it now
signifies an inverted one. And, if the vis-
ible appearance of two shillings had been
found connected from the beginning with
the tangible idea of one shilling, that ap-
pearance would as naturally and readily
have signified the unity of the object as now
it signifies its duplicity.

This opinion is, undoubtedly, very inge-
nious ; and, if it is just, serves to resolve
not only the phsenomenon now under con-
sideration, but likewise that which we shall
next consider — our seeing objects single
with two eyes.

It is evident that, in this solution, it is
supposed that we do not originally, and
previous to acquired habits, see things
either erect or inverted, of one figure or
another, single or double ; but learn, from
experience, to judge of their tangible posi-
tion, figure, and number, by certain visible

Indeed, it must be acknowledged to be
extremely difficult to distinguish the imme-
diate and natural objects of sight, from
the conclusions which we have been ac-
customed from infancy to draw from them.
Bishop Berkeley was the first that attempted
to distinguish the one from the other, and
to trace out the boundary that divides them.
And if, in doing so, he hath gone a little to
the right hand or to the left, this might be
expected in a subject altogether new, and
of the greatest subtilty. The nature of
vision hath received great light from this
distinction ; and many phsenomena in
optics, which before appeared altogether
unaccountable, have been clearly and dis-
tinctly resolved by it. It is natural, and
almost unavoidable, to one who hath made
an important discovery in philosophy, to
carry it a little beyond its sphere, and to
apply it to the resolution of phsenomena
which do not fall within its province. Even
the great Newton, when he had discovered
the universal law of gravitation, and ob-
served how many of the phaenomena of
nature depend upon this, and other laws of
attraction and repulsion, could not help ex-
pressing his conjecture, that all the phseno-
mena of the material world depend upon
attracting and repelling forces in the par-
ticles of matter. And I suspect that the
ingenious Bishop of Cloyne, having found
so many phrenomena of vision reducible to
the constant association of the ideas of sight



and touch, carried this principle a little be-
yond its just limits.

In order to judge as well as we can
whether it is so, let us suppose such a blind
man as Dr Saunderson, having all the
knowledge and abilities which a blind man
may have, suddenly made to see perfectly.
Let us suppose him kept from all opportu-
nities of associating his ideas of sight with
those of touch, until the former become a
little familiar ; and the first surprise, occa-
sioned by objects so new, being abated, he
has time to canvass them, and to compare
them, in his mind, with the notions which
he formerly had by touch ; and, in particu-
lar, to compare, in his mind, that visible
extension which his eyes present, with the
extension in length and breadth with which
lie was before acquainted.

We have endeavoured to prove, that a
blind man may form a notion of the visible
extension and figure of bodies, from the |
i elation which it bears to their tangible I
extension and figure. Much more, when this
visible extension and figure are presented
to his eye, will he be able to compare thein
with tangible extension and figure, and to
perceive that the one has length and breadth
as well as the other ; that the one may be
bounded by lines, either straight or curve,
as well as the other. And, therefore, he
will perceive that there may be visible as
well as tangible circles, triangles, quadri-
lateral and multilateral figures. And, al-
though the visible figure is coloured, and
the tangible is not, they may, notwithstand-
ing, have the same figure ; as two objects
of touch may have the same figure, although
one is hot and the other cold.

We have demonstrated, that the proper-
ties of visible figures differ from those of
the plain figures which they represent ; but
it was observed, at the same time, that
when the object is so small as to be seen
distinctly at one view, and is placed directly
before the eye, the difference between the
visible and the tangible figure is too small
to be perceived by the senses. Thus, it is
true, that, of every visible triangle, the
three angles are greater than two right
angles ; whereas, in a plain triangle, the
three angles are equal to two right angles ;
hut when the visible triangle is small, its
three angles will be so nearly equal to two
right angles, that the sense cannot discern
the difference. In like manner, the circum-
ferences of unequal visible circles are not,
but those of plain circles are, in the ratio of
their diameters ; yet, in small visible circles,
the circumferences are very nearly in the
ratio of their diameters ; and the diameter
bears the same ratio to the circumference,
as in a plain circle, very nearly.

Hence it appears, that small visible
figures (and such only can be seen distinctly

at one view) have not only a resemblance
to the plain tangible figures which have the
name name, but are to all sense the same :
so that, if Dr Saunderson had been made to
see, and had attentively viewed the figures
of the first book of Euclid, he might, by
thought and consideration, without touching
them, have found out that they were the
very figures he was before so well . ac-
quainted with by touch.

When plain figures are seen obliquely,
their visible figure differs more from the
tangible ; and the representation which is
made to the eye, of solid figures, is still
more imperfect ; because visible extension
hath not three, but two dimensions only.
Yet, as it cannot be said that an exact pic-
ture of a man hath no resemblance of the
nmn, or that a perspective view of a house
hath no resemblance of the house, so it
cannot be said, with any propriety, that the
visible figure of a man or of a house hath
no resemblance of the objects which they

Bishop Berkeley therefore proceeds upon
a capital mistake, in supposing that there is
no resemblance betwixt the extension, figure,
and position which we see, and that which
we perceive by touch.

We may further observe, that Bishop
Berkeley's system, with regard to material
things, must have made him see this ques-
tion, of the erect appearance of objects, in
a very different light from that in which it ap-
pears to those who do not adopt his system.

In his theory of vision, he seems indeed
to allow, that there is an external material
world : but he believed that this external
world is tangible only, and not visible ; and
that the visible world, the proper object of
sight, is not external, but in the mind. If
this is supposed, he that affirms that he
eees things erect and not inverted, affirms
that there is a top and a bottom, a right
and a left in the mind. Now, I confess I
am not so well acquainted with the topo-
graphy of the mind, as to be able to affix
a meaning to these words when applied
to it.

We shall therefore allow, that, if visible
objects were not external, but existed only
in the mind, they could have no figure, or
position, or extension ; and that it would be
absurd to affirm, that they are seen either
erect or inverted, or that there is any re-
semblance between them and the objects of
touch. But when we propose the question,
why objects are seen erect and not in-
verted, we take it for granted, that we are
not in Bishop Berkeley's ideal world, but
in that world which men who yield to the
dictates of common sense, believe them-
selves to inhabit. We take it for granted,
that the objects both of sight and touch,
arc external, and have a certain figure, and



a certain position with regard to one another,
and with regard to our bodies, whether we
perceive it or not.

When I hold my walking-cane upright
in my hand, and look at it, I take it for
granted that I see and handle the same
individual object. When I say that I feel
it erect, my meaning is, that I feel the
head directed from the horizon, and the
point directed towards it ; and when I say
that I see it erect, I mean that I see it with
the head directed from the horizon, and
the point towards it. I conceive the hori-
zon as a fixed object both of sight and touch,
with relation to which, objects are said to
be high or low, erect or inverted ; and when
the question is asked, why I see the ob-
ject erect, and not inverted, it is the same
as if you should ask, why I see it in that
position which it really hath, or why the
eye shews the real position of objects, and
doth not shew them in an inverted posi-
tion, as they are seen by a common astro-
nomical telescope, or as their pictures are
seen upon the relina of an eye when it is

Section XII.


It is impossible to give a satisfactory an-
swer to this question, otherwise than by
pointing out the laws of nature which take
place in vision ; for by these the phseno-
mena of vision must be regulated.

Therefore, I answer, First, That, by a
law of nature, the rays of light proceed from
every point of the object to the pupil of
the eye, in straight lines ; Secondly, That,
by the laws of nature, the rays coming
from any one point of the object to the va-
rious parts of the pupil, are so refracted as
to meet again in one point of the retina ;
and the rays from different points of the
object, first crossing each other,* and then
proceeding to as many different points of
the retina, form an inverted picture of the

So far the principles of optics carry
us ; and experience further assures us, that,
if there is no such picture upon the retina,
there is no vision ; and that such as the
picture on the retina is, such is the appear-

« It is marvellous how widely both natural philo-
sophers and physiologists are at variance with regard
*'i the point of the eye at which the rays cross each
other. Some place this point in the cornea — some
in the region of the pupil— some in the centre of the
crystalline — and some in the vitreous humour.
Rrccnt experiments, instituted for the purpose of
determining its locality, and still unknown in this
country, place it tiehind the crystalline lens. This
is f- und to be at once the crossing point, both of the
rays of light and of the line of visible direction, and
the turning point on wlncl' the eye rolls. — H. _ .

ance of the object, in colour and figure,
distinctness or indistinctness, brightness or

It is evident, therefore, that the pictures
upon the retina are, by the laws of nature,
a mean of vision ; but in what way they
accomplish their end, we are totally igno-
rant. Philosophers conceive, that the im-
pression made on the retina by the rays of
light, is communicated to the optic nerve,
and by the optic nerve conveyed to some
part of the brain, by them called the senso-
rium ; and that the impression thus conveyed
to the sensorium is immediately perceived
by the mind, which is supposed to reside
there. But we know nothing of the seat of
the soul : and we are so far from perceiving
immediately what is transacted in the brain,
that of all parts of the human body we know
least about it. It is indeed very probable,
that the optic nerve is an instrument of
vision no less necessary than the retina ;
and that some impression is made upon it,
by means of the pictures on the retina.
But of what kind this impression is, we know

There is not the least probability that
there is any picture or image of the ob-
ject either in the optic nerve or brain.
The pictures on the retina are formed by
the rays of light ; and, whether we suppose,
with some, that their impulse upon the re-
tina causes some vibration of the fibres of
the optic nerve, or, with others, that it
gives motion to some subtile fluid contained
in the nerve, neither that vibration nor
this motion can resemble the visible ob-
ject which is presented to the mind. Nor
is there any probability that the mind per-
ceives the pictures upon the retina. These
pictures are no more objects of our percep-
tion, than the brain is, or the optic nerve.
No man ever saw the pictures in his own
eye, nor indeed the pictures in the eye
of another, until it was taken out of the
head and duly prepared.

It is very strange, that philosophers, of
all ages, should have agreed in this notion,
that the images of external objects are con-
veyed by the organs of sense to the brain,
and are there perceived by the mind."
Nothing can be more unphilosophicaL For,
First, This notion hath no foundation in fact
and observation. Of all the organs of
sense, the eye only, as far as we can disco-
ver, forms any kind of image of its object ;
and the images formed by the eye are not
in the brain, but only in the bottom of the
eye ; nor are they at all perceived or felt
by the mind.-|- Secondly, It is as difficult

• Thi« statement in its unqualified universality is
altogether erroneous. — H.

t 1 his would renuue a second eye behind the
retina; which eye would also see the images bait.



to. conceive how the mind perceives images
in the brain, as how it perceives things
more distant. If any man will shew how
the mind may perceive images in the brain,
I will undertake to shew how it may per-
ceive the most distant objects ; for, if we
give eyes to the mind, to perceive what is
transacted at home in its dark chamber,
why may we not make these eyes a little
longer-sighted ? and then we shall have no
occasion for that unphilosophical fiction of
images in the brain. In a word, the man-
ner and mechanism of the mind's percep-
tion is quite beyond our comprehension ;
and this way of explaining it, by images in
the brain, seems to be founded upon very
gross notions of the mind and its opera-
tions ; as if the supposed images in the
brain, by a kind of contact, formed similar
impressions or images of objects upon the
mind, of which impressions it is supposed to
be conscious.

We have endeavoured to shew, through-
out the course of this inquiry, that the im-
pressions made upon the mind by means of
the five senses, have not the least resem-
blance to the objects of sense ; and, there-
fore, as we see no shadow of evidence that
there are any such images in the brain, so
we see no purpose, in philosophy, that the
supposition of them can answer. Since the
picture upon the retina, therefore, is neither
itself seen by the mind, nor produces any
impression upon the brain or sensorium,
which is seen by the mind, nor makes any
impression upon the mind that resembles
the object, it may still be asked, How this
picture upon the retina causes vision ?

Before we answer this question, it is pro-
per to observe, that, in the operations of the
mind, as well as in those of bodies, we must
often be satisfied with knowing that cer-
tain things are connected, and invariably
follow one another, without being able to
discover the chain that goes between them.
It is to such connections that we give the
name of laws of nature ; and when we say
that one thing produces another by a law
of nature, this signifies no more, but that
one thing, which we call in popular lan-
guage the cause, is constantly and invari-
ably followed by another, which we call the
effect ; and that we know not how they are
connected. Thus, we see it is a fact, that
bodies gravitate towards bodies; and that
this gravitation is regulated by certain
mathematical proportions, according to the
distances of the bodies from each other,
and their quantities of matter. Being un-
able to discover the cause of this gravita-
tion, and presuming that it is the immediate
operation, either of the Author of nature,

bb they are pictured on the concavity of that mem-
brane..— H.

or of some subordinate cause, which we
have not hitherto been able to reach, we
call it a law of nature. If any philoso-
pher should hereafter be so happy as to
discover the cause of gravitation, this can
only be done by discovering some more
general law of nature, of which the gravi-
tation of bodies is a necessary consequence.
In every chain of natural causes, the highest
link is a primary law of nature, and the
highest link which we can trace, by just
induction, is either this primary law of
nature, or a necessary consequence of it.
To trace out the laws of nature, by induc-
tion from the pheenomena of nature, is all
that true philosophy aims at, and all that it
can ever reach.

There are laws of nature by which the
operations of the mind are regulated, there
are also laws of nature that govern the
material system ; and, as the latter are the
ultimate conclusions which the human
faculties can reach in the philosophy of
bodies, so the former are the ultimate con-
clusions we can reach in the philosophy of

To return, therefore, to the question
above proposed, we may see, from what
hath been just now observed, that it
amounts to this — By what law of nature is
a picture upon the retina the mean or
occasion of my seeing an external object of
the same figure and colour in a contrary
position, and in a certain direction from the

It will, without doubt, be allowed that
I see the whole object in the same manner
and by the same law by which I see any
one point of it. Now, I know it to be a
fact, that, in direct vision, I see every point
of the object in the direction of the right line
that passeth from the centre of the eye to
that point of the object. And I know,
likewise, from optics, that the ray of
light that comes to the centre of my
eye, passes on to the retina in the same
direction. Hence, it appears to be a fact,
that every point of the otiject is seen in the
direction of a right line passing from the
picture of that point on the retina, through
the centre of the eye. As this is a fact that
holds universally and invariably, it must
either be a law of nature, or the necessary
consequence of some more general law of
nature ; and, according to the just rules of
philosophising, we may hold it for a law of
nature, until some more general law be
discovered, whereof it is a necessary conse-
quence — which, I suspect, can never be

* A confirmation of this doctrine is drawn from
the cases of Cheselden and others, in which no men-
tal Inversion of the objects is noticed, and which had
it occur red, is too remarkable a phenomenon to have
been overlooked. It is, indeed, generally asserted ih.i'



Thus, we see that the phenomena of
vision lead us by the hand to a law of na-
ture, or a law of our constitution, of which
law, our seeing objects erect by inverted
images, is a necessary consequence. For
it necessarily follows, from the law we have
mentioned, that the object whose picture is
lowest on the retina must be seen in the
highest direction from the eye ; and that
the object whose picture is on the right of
the retina must be seen on the left; so
that, if the pictures had been erect in the
retina, we should have seen the object in-
verted. My chief intention in handling
this question, was to point out this law of
nature, which, as it is a part of the consti-
tution of the human mind, belongs properly
to the subject of this inquiry. For this
reason, I shall make some farther remarks
upon it, after doing justice to the ingenious
Dr Porterfield, who, long ago, in the
" Medical Essays," or, more lately, in his

such inversion has never been observed in any
patient, surgically restored to sight. I am aware,
however, of one oase of .in opposite purport. It is
mentioned, on his own nb-ervation, by a very intelli-
gent philosopher and physician, Professor Leiden-
fro-it of Duisburg ; and, as his rare worn — " Confes^-io
quid putet per Expericntiam didicisse de Mente
Humana," I79.J — is altogether unkn >wn in this
country, I .•■hall extract from it the whole passage: —
" Hae imagines fonnantur in orgnno, non in
cerebro. — Mutantur et pcrvcrtuntur ab organo laeso,
etiamsi illaesum maneat cerebrum. Non eas con-
natas habemus, sed exercitio continuato eas formare
discimus. Elegans exemplum habemus in evangelio
Marc. H. cf. loll. 9. Vir adultus a nativitate coecus,
et potentia miraculosa sancti servatorissubito curatua
priino actu visionis uteris distiuguere non poterat,
utrumne staturae, quas videtoat, homines eesent, an
arbores. Sine dubio jam antcenrationem sciverat ex
relatione aliorum,et ex inanuum suarnmexperientia,
tdin hominis quam stipitis arboreae staturas < sie

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 36 of 114)