Thomas Reid.

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

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sensation of colour whatsoever, the organ of sight
amid have given us no in ormation, cither with re-
spect tojigures or to distances ; ano, of consequence,
would have been as useless to us, as if we had i>ecn
afflicted, from the moment of our birth, with a %utta
seiena."—£)iisertatio?i t &c, p. G6, note ; 2d w£



Tins is a question of some importance, in
order to our having a distinct notion of the
faculty of seeing : and to give all the light
to it we can, it is necessary to compare this
sense with other senses, and to make some
suppositions, by which we may be enabled
to distinguish things that are apt to be con-
founded, although they are totally dif-

There are three of our senses which give
us intelligence of things at a, distance :*
smell, hearing, and sight. In smelling and
<*n hearing, we have a sensation or impres-
sion upon the mind, which, by our consti-
tution, we conceive to be a sign of some-
thing external : but the position of this
external thing, with regard to the organ of
sense, is not presented to the mind along
with the sensation. When I hear the
sound of a coach, I could not, previous to
experience, determine whether the sounding
body was above or below, to the right hand
or to the left. So that the sensation, sug-
gests to me some external object as the
cause or occasion of it ; but it suggests not
the position of that object, whether it lies
in this direction or in that. The same
thing may be said with regard to smelling.
But the case is quite different with regard
to seeing. When I see an object, the ap-
pearance which the colour of it makes, may
be called the sensation, which suggests to
me some external thing as its cause ; but
it suggests likewise the individual direction
and position of this cause witfi regard to
the eye. I know it is precisely in such a
a direction, and in no other. At the same
time, I am not conscious of anything that
can be called sensation, but the sensation of
colour. The position of the coloured thing
is no sensation ; but it is by the laws of my
constitution presented to the mind along
with the colour, without any additional

Let us suppose that the eye were so con-
stituted that the rays coming from any one
point of the object were not, as they are in
our eyes, collected in one point of the
retina, but diffused over the whole : it is
evident to those who understand the struc-
ture of the eye, that such an eye as we have
supposed, would shew the colour of a body
as our eyes do, but that it would neither
shew figure nor position. The operation
of such an eye would be precisely similar
to that of hearing and smell ; it would give

The questions concerning the mutual dependence
of colour on extension, and of extension and figure
on colour, in perception and imagination, cannot be
dismissed in a foot-note. I shall endeavour, in Note
E, to shew that we can neither see nor imagine
colour apart from extension, nor extension and figure
apart from colour.— H.

* Properly speaking, nosen&e gives us a knowledge
of aught hut what is in immediate contact with its
organ. All else is something over and above percep-
tion — H.

no perception of figure or extension, but
merely of colour. Nor is the supposition
we have made altogether imaginary : for it
is nearly the case of most people who have
cataracts, whose crystalline, as Mr Chesel-
den observes, does not altogether exclude
the rays of light, but diffuses them over the
retina, so that such persons see things as
one does through a glass of broken gelly :
they perceive the colour, but nothing of
the figure or magnitude of objects.*"

Again, if we should suppose that smell
and sound were conveyed in right lines from
the objects, and that every sensation of
hearing and smell suggested the precise
direction or position of its object ; in this
case, the operations of hearing and smelling
would be similar to that of seeing : we
should smell and hear the figure of objects,
in the same sense as now we see it ; and
every smell and sound would be associated
with some figure in the imagination, as
colour is in our present state, -f-

* Reid, as remarked by Mr Fearn, misinterprets
Cheselden in founding on the expressions of this
report, a proof of his own paradox, that-colour can
possibly be an object of vision, apait from extension.
There is no ground in that repot t for .such an
inference ; for it contains absolutely nothing to in.
validate, and much to support the doctrine — that,
though sensations of colour may be experienced
thiough the medium of an imperfect cataract, while
the .figures of external objects are intercepted or
broken down ; yet that, in these sensations, coloui
being diffused over the retina, must appear to uu
extended, and of an extension limited by the bound,
aries of that sensitive membrane itself. The relative
passage of Cheselden is as follows :— '* Though we
say of the gentleman couched between thirteen And
fourteen years of age, that he was blind, as we do
of all people who have ripe cataracts, yet they are
never so blind from that cause, but ihey can discern
day from night, and for the most part in a strong
light distinguish black, white, and scarlet; but the
light by which these perceptions are made, being le^.
in obliquely through the aqueous humour, or the
anterior surface of the crystalline, by which the rays
cannot be brought into a focus upon the retina, they
can discern in no other manner than a sound eye can
through a glass of broken jelly, where a great variety
oi surfaces so differently refract the light, that the
sevetal distinct pencils of rays cannot he collected by
the eye into their proper foci, wherefore the shape nf
an o jed in such a case cannot be at all disccn ed,
though the colour may And thus it was with this
young gentleman, who, though he knew these colours
asunder in a good light, yet, when he saw them afrer
he was couched, the faint ideas he had of them before,
were not sufficient for him to know hem by after-
wards, and therefore he did not think ihem the
same which he had before known by those names " —
There are also several statements in the repot t which
shew that thepatieniwas, on the recovery of distinct
vision, perfectly familiar with differences of visible
magnitude Sec NoteE. — H.

■f To render this supposition possible, we mud
not only change theobjective, but also the subjective
conditions of smell and hearing; for, with our or-
gans of these senses, and our nervous system in ge-
neral, constituted as they are at present, the resul*
would not be as a-sumed, even were the olfactory
effluvia and audible vibrations convejed in right
lines.from bodies to the nose and ear But to sup-
pose both subjective and objective conditions than .. ed
is to suppose new qualities and n^w senses altogether;
an hypothesis which would hardly serve the ourposc
of an illustration, a notiori.

A similar hypothesis and illustration ii 'to be
found in Condillac's " Trail e des Sensations;" but,



We have reason to believe, that the rays
of light make some impression upon the
retina; but we are not conscious of this
impression ; nor have anatomists or philo-
sophers been able to discover the nature and
effects of it ; whether it produces a vibra-
tion in the nerve, or the motion of some
subtile fluid contained in the nerve, or some-
thing different from either, to which we
cannot give a name. Whatever it is, we
shall call it the material impression ; remem-
bering carefully, that it is not an impression
upon the mind, but upon the body ; and
that it is no sensation, nor can resemble
sensation, any more than figure or motion
can resemble thought. Now, this material
impression, -made upon a particular point of
the retina, by the laws ot our constitution,
suggests two things to the mind — namely,
the colour and the position of some exter-
nal object. No man can give a reason why
the same material impression might not
have suggested sound, or smell, or either
of these, along with the position of the object.
That it should suggest colour and position,
and nothing else, we can resolve only into
our constitution, or the will of our Maker.
And since there is no necessary connection
between these two things suggested by this
material impression, it might, if it had so
pleased our Creator, have suggested one of
them without the other. Let us suppose,
therefore, since it plainly appears to be
possible, that our eyes had been so framed
as to suggest to us the position of the object,
without suggesting colour, or any other
quality : What is the consequence of this
supposition ? It is evidently this, that the
person endued with such an eye, would per-
ceive the visible figure of bodies, without
having any sensation or impression made
upon his mind. The figure he perceives is
altogether external ; and therefore cannot
be called an impression upon the mind,
without the grossest abuse of language. If
it should be said, that it is impossible to
perceive a figure, unless there be some im-
pression of it upon the mind, I beg leave
not to admit the impossibility of this without
some proof : and I can find none. Neither
can I conceive what is meant by an impres-
sion of figure upon the mind. I can conceive
an impression of figure upon wax, or upon
any body that is fit to receive it ; but an
impression of it upon the mind, is to me
quite unintelligible ; and, although I form
the most distinct conception of the figure, I
cannot, upon the strictest examination, find
any impression of it upon my mind.

If wc suppose, last of all, that the eye
hath the power restored of perceiving colour,

as Mr Stewart observes, though thus anticipated,
there is no ground for thinking that Reid was
at all acquainted with the writings of the French phi-
losopher. — H.

I apprehend that it will be allowed, that
now it perceives figure in the very same
manner as before, with this difference only,
that colour is always joined with it.

In answer, therefore, to the question pro-
posed, there seems to be no sensation thai
is appropriated to visible figure, or whose
office it is to suggest it. It seems to be
suggested immediately by the material im-
pression upon the organ, of which we are
not conscious : and why may not a material
impressioii upon the rethia suggest visible
figure, as well as the material impression
made upon the hand, when we grasp a ball,
suggests real figure ? In the one case, one
and the san.e material impression, suggests
both colour and visible figure ; and in the
other case, one and the same material im-
pression suggests hardness, heat, or cold,
and real figure, all at the same time.

We shall conclude this section withan-
other question upon this subject. Since the
visible figure of bodies is a real and exter-
nal object to the eye, as their tangible figure
is to the touch, it may be asked, Whence
arises the difficulty of attending to the first,
and the facility of attending to the last ? It
is certain that the first is more frequently
presented to the eye, than the last is to the
touch ; the first is as distinct and deter-
minate an object as the last, and seems in
its own nature as proper for speculation.
Yet so little hath it been attended to, that
it never had a name in any language, until
Bishop Berkeley gave it that which we have
used after his example, to distinguish it
from the figure which is the object of touch.

The difficulty of attending to the visible
figure of bodies, and making it an object of
thought, appears so similar to that which
we find in attending to our sensations, that
both have probably like causes. Nature
intended the visible figure as a sign of the
tangible figure and situation of bodies, and
hath taught us, by a kind of instinct, to put
it always to this use. Hence it happens,
that the mind passes over it with a rapid
motion, to attend to the things signified by
it. It is as unnatural to the mind to stop
at the visible figure, and attend to it, as it
is to a spherical body to stop upon an in-
clined plane. There is an inward principle,
which constantly carries it forward, and
which cannot be overcome but by a contrary

There are other external things which-
nature intended for signs ; and we find
this common to them all. that the mind is
disposed to overlook them, and to attend
only to the things signified by them. Thus
there are certain modifications of the hu-
man face, which are natural signs of the
present disposition of the mind. Every
man understands the meaning of these signs,
but not one of a hundred ever attended to



the signs themselves, or knows anything
about them. Hence you may find many
an excellent practical physiognomist who
knows nothing of the proportions of a face,
nor can delineate or describe the expression
of any one passion.

An excellent painter or statuary can
tell, not only what are the proportions of a
good face, but what changes every passion
makes in it. This, however, is one of the
chief mysteries of his art, to the acquisition
of which infinite labour and attention, as well
as a happy genius, are required ; but when
he puts his art in practice, and happily ex-
presses a passion by its proper signs, every
one understands the meaning of these signs,
without art, and without reflection.

What has been said of painting, might
easily be applied to all the fine arts. The
difficulty in them all consists in knowing
and attending to those natural signs where-
of every man understands the meaning.

We pass from the sign to the thing sig-
nified, with ease, and by natural impulse ;
but to go backward from the thing signi-
fied to the sign, is a work of labour and
difficulty. Visible figure, therefore, being
intended by nature to be a sign, we pass on
immediately to the thing signified, and can-
not easily return to give any attention to
the sign.

Nothing shews more clearly our indis-
position to attend to visible figure and vi-
sible extension than this — that, although
mathematical reasoning is no less appli-
cable to them, than to tangible figure and
extension, yet they have entirely escaped
the notice of mathematicians. While that
figure and that extension which are objects
of touch, have been tortured ten thousand
ways for twenty centuries, and a very
noble system of science has been drawn
out of them, not a single proposition do
we find with regard to the figure and ex-
tension which are the immediate objects of
sight !

When the geometrician draws a diagram
with the most perfect accuracy — when he
keeps his eye fixed upon it, while he goes
through a long process of reasoning, and
demonstrates the relations of the several
parts of his figure— he does not consider
that the visible figure presented to his eye,
is only the representative of a tangible figure,
upon which all his attention is fixed ; he
does not consider that these two figures
have really different properties ; and that,
what he demonstrates to be true of the one,
is not true of the other.

This, perhaps, will seem so great a para-
dox, even to mathematicians, as to require
demonstration before it can be believed.
Nor is the demonstration at all difficult, if
the reader will have patience to enter but
a little into the mathematical consideration

of visible figure, which we shall call t/i6
geometry ofvisibles.

Section IX.


In this geometry, thedefinitions of a point ;
of a line, whether straight or curve ; of an
angle, whether acute, or right, or obtuse ;
and of a circle — are the same as in common
geometry. The mathematical reader will
easily enter into the whole mystery of this
geometry, if he attends duly to these few
evident principles.

1. Supposing the eye placed in the centre
of a sphere, every great circle of the sphere
will have the same appearance to the eye
as if it was a straight line ; for the curva-
ture of the circle being turned directly to-
ward the eye, is not perceived by it. And,
for the same reason, any line which is drawn
in the plane of a great circle of the sphere,
whether it be in reality straight or curve,
will appear straight to the eye.

2. Every visible right line will appear to
coincide with some great circle of the
sphere ; and the circumference of that great
circle, even when it is produced until it
returns into itself, will appear to be a con-
tinuation of the same visible right line, all
the parts of it being visibly in directum.
For the eye, perceiving only the position of
o , bjects~witlr regartTftTitself , and not their
dlstancepiviirsee those points in the same
visible pla ce which have the same position
witE'regard to the eye, how different soever
theiFmstances from it maybe. Now, since
a'plane'passing through the eye and a given
visible right line, will be the plane of some
great circle of the sphere, every point of the
visible right line will have the same position
as some point of the great circle ; therefore,
they will both have the same visible place,
and coincide to the eye; and the whole
circumference of the great circle, continued
even until 'it returns into itself, will appear
to be a continuation of the same visible
right line.

Hence it follows —

3. That every visible right line, when it
is continued in directum, as far as it may he
continued, will be represented by a great
circle of a sphere, in whose centre the eye
is placed. It follows —

4. That the visible angle comprehended
under two visible right lines, is equal to the
spherical angle comprehended under the
two great circles which are the representa-
tives of these visible lines. For, since the
visible lines appear to coincide with the

* Mow does this differ from a doctrine of Perspec-
tive ? — At any -ate. the notion is Berkeley's. Com.
pare" New Theoiy of Vision," \i, 153— 159.— H.



great circles, the visible angle compre-
hended under the former must be equal to
the visible angle comprehended under the
latter. But the visible angle comprehended
under the two great circles, when seen from
the centre, is of the same magnitude with
the spherical angle which they really com-
prehend, as mathematicians know ; therej- '
fore, the visible angle made by any two
visible lines is equal to the spherical angle-
made by the two great circles of the sphere
which are their representatives.

5. Hence it is evident, that every visible
right-lined triangle will coincide in all its
parts with some spherical triangle. The
sides of the one will appear equal to the
sides of the other, and the angles of the
one to the angles of the other, each to each ;
and, therefore, the whole of the one triangle
will appear equal to the whole of the other.
In ajvo_rd, to the eye they will be one and
the same, and have the same mathematii
properties. The properties, therefore,
visible right-lined triangles are not the same
with the properties of plain triangles, but
are the same with those of spherical tri-

6. Every lesser circle of the sphere will
appear , a circle to the eye, placed, as we
have supposed all along, in the centre of
the sphere ; and, on the other hand, every
visible circle will appear to coincide with
some lesser circle of the sphere.

7. Moreover, the whole surface of the
sphere will represent the whole of visible
space ; for, since every visible point coin-
cides with some point of the surface of the
sphere, and has the same visible place, it
follows, that all the parts of the spherical
surface taken together, will represent all
possible visible places — that is, the whole of
visible space. And from this it follows, in
the last place —

8. That every visible figure will be repre-
sented by that part of the surface of the
sphere on which it might be projected, the
eye being in the centre. And every such
visible figure will bear the same ratio to the
whole of visible space, as the part of the
spherical surface which represents it, bears
to the whole spherical surface.

The mathematical reader, I hope, will
enter into these principles with perfect
facility, and will as easily perceive that the
following propositions with regard to visible
figure and space, which we offer only as a
specimen, may be mathematically demon-
strated from them, and are not less true nor
less evident than the propositions of Euclid,
with regard to tangible figures.

Prop. 1. Every right line being produced,
will at last return into itself.

2. A right line, returning into itself, is
the longest possible right line ; and all other
right lines bear a finite ratio to it.

3. A right line returning into itself,
divides the whole of visible space into two
equal parts, which will both be compre-
hended under this right line.

4. The whole of visible space bears a
finite ratio to any part of it.

5. Any twj^g|d]Jinej_bejn^profliified,
)vill TneefrTn two points, andmutually
bisect each other

• 6. If two lines be parallel — that is, every
where equally distant from each other —
they cannot both be straight.

7. Any right line being given, a point/
may be found, which is at the same dis-
tance from all the points of the given right
line. '

8. A circle may be parallel to a right i
line — that is, may be equally distant from I
it in all its parts. '

9. Right-lined triangles that are similar,
are also equal.

\ 10. Of every right-lined triangle, the /
hree arjg'es taken together, are greater
han two right angles. I

11. The angles of a right-lined triangle,
may all be right angles, or all obtuse angles.

12. Unequal circles are not as the
squares of their diameters, nor are their
circumferences in the ratio of their dia-

This small specimen of the geometry of
visibles, is intended to lead the reader to a
clear and distinct conception of the figure
and extension which is presented to the
mind by vision ; and to demonstrate the
truth of what we have affirmed above —
namely, that those figures and that exten-
sion which are the immediate objects of
sight, are not the figures and the extension
about which common geom etry is employed ;
that the geometrician, while he looks at his
diagram, and demonstrates a proposition,
hath a figure presented to his eye, which is
only a sign and representative of a tangible
figure ; that he gives not the least atten-
tion to the first, but attends only to the
last ; and that these two figures have differ-
ent properties, so that what he demon-
strates of the one, is not true of the

It deserves, however, to be remarked,
that, as a small part of a spherical surface
differs not sensibly from a plain surface,
so a small part of visible extension differs
very little from that extension in length
and breadth, which is the object of touch.
And it is likewise to be observed, that the
human eye is so formed, that an object
which is seen distinctly and at one view
can occupy but a small part of visible space ;
for we never see distinctly what is at a
considerable distance from the axis of the
eye ; and, therefore, when we would see a
large object at one view, the eye must be
at so great a distance, that the object



occupies but a small part of visible space.
From these two observations, it follows,
that plain figures which are seen at one
view, when their planes are not oblique, but
direct to the eye, differ little from the
visible figures which they present to the
eye. The several lines in the tangible
figure, have very nearly the same propor-
tion to each other as in the visible ; and
the angles of the one are very nearly, al-
though not strictly and mathematically,
equal to those of the other. Although,
therefore, we have found many instances
of natural signs which have no similitude
to the things signified, this is not the case
with regard to visible figure. It hath, in
all cases, such a similitude to the thing
signified by it, as a plan or profile hath to that
which it represents ; and, in some cases, the
sign and thing signified have to all sense the
same figure and the same proportions. If
we could find a, being endued with sight
only, without any other external sense,
and capable of reflecting and reasoning
upon what he sees, the notions and phi-
losophical speculations of such a being,
might assist us in the difficult task of
distinguishing the perceptions which we
have purely by sight, from those which de-
rive their origin from other senses. Let
us suppose such a being, and conceive,

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 34 of 114)