Thomas Reid.

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

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erectas, at ulterior! exercitio luerit opus ad utrum-
que distinguendum. Aliquid simile aliqnando in
juvene propter cataractam congenitam coeco mihi
observare licuit. Hie ex paupercula familia rustica
ortus, statim post partum utramque pupillam habuit
obscuratam; probabiliter membrana pupillaris crassa
et opaca erat. I'ro incurabiH habitus nullam cura-
tionem habuit. '-anus excrevit, sed plane coecus;
omni lumine orbus, in scholas missus lepidi ingenii
sigiia dedit. Anno aetitis circiter decimo septimo,
nescio ex qua causa gravissima ophthalmia corripitur
cum tumure palpebrarum et acerbo dolore. In hoc
statu aliqualis medicatio adhibit a est. Observarunt
parentes lucera ab eo fugi, a luce rlolores crescere
Post aliquot hebdamades febris et ophthalmia de-
crescunt; cum summoejus stupore aliqualemluminis
usuram nanciscitur. Omit to scribere plures memora-
biles hujus visionis conditiones, nam ab eo tempore
frequenter, et semper admiraus, eurn conspexi Hoc
unum, quod ad rem facit, addo ; imagines in oculo
or 1 as penitus ei novas fuisse. Ab initio non paiieba-
tur sibi persi iaderi, reliquos homines erectos incedere,
putabat hominum capita sui ipsius petlibus esse ob-
yersa. Similiter arbores et ohjecta omnia ratione sui
inversa esse. Colorum diversitate vehementer delec-
tabatur, quorum -mllum conceptual habuerat Nam
quamdui coecus erat, si quid de rubro aut alio colore
audiverat, id comparaveratcum sensationibus gustus.
Kubmm sibi praesentaverat esse aliquid quasi dulce,
nigrum cum amarore c<imparaverat Successive sibi
imagines has formabat, et, ut reliqui ho-
rn ncs. In hoc hoinine nullae imagines visivae prac
extiteiunt, neque in organo, ncque in cerebro, cujus
nu la passio aut mutatio facta erat, Aliquot annis
post, hie juvenis, non sine meo dolore, phtliisicus mo-
riobatur." — P. 54.

(C Treatise of the Eye," pointed out,* as a
primary law of our nature. That a visible
object appears in the direction of a right
line perpendicular to the retina at that
point where its image is painted. If lines
drawn from the centre of the eye to all
parts of the reii/ia be perpendicular to it, as
they must be very nearly, this coincides
with the law we have mentioned, and is the
same in other words. In order, therefore,
that we may have a more distinct notion
of this law of our constitution, we may
observe —

1. That we can give no reason why the
retina is, of all parts of the body, the only
one on which pictures made by the rays of
light cause vision ; and, therefore, we must
resolve this solely into a law of our consti-
tution. We may form such pictures by
means of optical glasses, upon the hand, or
upon any other part of the body ; but they
are not felt, nor do they produce anything
like vision. A picture upon the retina is as
little felt as one upon the hand ; hut it pro-
duces vision, for no other reason that we
know, but because it is destined by the
wisdom of nature to this purpose. The
vibrations of the air strike upojor the eye,
the palate, and the olfactory membrane,
with the same force as upon the membrani
tympani of the ear. The impression they
make upon the last produces the sensation
of sound ; but their impression upon any of
the former produces no sensation at all.
This may be extended to all the senses,
whereof each hath its peculiar laws, accord-
ing to which the impressions made upon the
organ of that sense, produce sensations or
perceptions in the mind, that cannot be
produced by impressions made upon any
other organ.

2. We may observe, that the laws of per-
ception, by the different senses, are very
different, not only in respect of the nature
of the objects perceived by them, but like-
wise in respect of the notices they give us
of the distance and situation of the object.
In all of them the object is conceived-j- to
be external, and to have real existence, in-
dependent of our perception : but in one,
the distance, figure, and situation of the
object, are all presented to the mind ; in
another, the figure and situation, but not
the distance ; and in others, neither figure,
situation, nor distance. In vain do we at-
tempt to account for these varieties in the
manner of perception by the different

« Porterfield did not first point this out; on the con-
trary, it was a common, if not the common doctrine
at the time he wrote. See below, the first note of
§ xviii.— H.

t The common sense of mankind assures us that
the object of sense, is not merely conceived to be ex-
ternal, butpercciWdin its externality ; that we know
the Non-Ego, not merely mediately, by a represents,
tion in the Ego, but immediately, as existing thr-ugh
only as existing in relation io our organs. — H.



senses, from principles of anatomy or na-
tural philosophy. They must at last be
resolved into the will of our Maker, who
intended that our powers of perception
should have certain limits, and adapted the
organs of perception, and the laws of na-
ture by which they operate, to his wise pur-

When we hear an unusual sound, the
sensation indeed is in the mind, but we
know that there is something external that
produced this sound. At the same time, our
hearing does not inform us whether the
sounding body is near or at a distance, in
this direction or that ; and therefore we look
round to discover it.

If any new phsenomenon appears in the
heavens, we see exactly its colour, its ap-
parent place, magnitude, and figure ; but
we see not its distance. It may be in the
atmosphere, it may be among the planets,
or it may be in the sphere of the fixed stars,
for anything the eye can determine.

The testimony of the sense of touch
reaches only to objects that are contiguous
to the organ, but, with regard to them, is
more precise and determinate. When we
feel a body with our hand, we know the
figure, distance, and position of it, as well
as whether it is rough or smooth, hard or
soft, hot or cold.

The sensations of touch, of seeing, and
hearing, are all in the mind, and can have
no existence but when they are perceived.
How do they all constantly and invariably
suggest the conception and belief of external
objects, which exist whether they are per-
ceived or not ? No philosopher can give
any other answer to this, but that such is
the constitution of our nature. How do we
know that the object of touch is at the
finger's end, and nowhere else ? — that the
object of sight is in such a direction from
the eye, and in no other, but may be at any
distance ?*— and that the object of hearing
may be at any distance,* and in any direc-
tion ? Not by custom surely — not by rea-
soning, or comparing ideas — but by the con-
stitution of our nature. How do we per-
ceive visible objects in the direction of right
lines perpendicular to that part of the retina
on which the rays strike, while we do not
perceive the objects of hearing in lines per-
pendicular to the membrana tympani upon
which the vibrations of the air strike ? Be-
cause such are the laws of our nature. How
do we know the parts of our bodies affected
by particular pains ? Not by experience
or by reasoning, but by the constitution of
nature. The sensation of pain is, no doubt,
in the mind, and cannot be said to have any
relation, from its own nature, to any part

* Tt has been previously noticed, that in no sense
does the mind perceive any ditant or mediate ob-
ject— H.

of the body ; but this sensation, by our con-
stitution, gives a perception of some parti-
cular part of the body, whose disorder causes
the uneasy sensation. If it were not so, a
man who never before felt either the gout
or the toothache, when he is first seized with
the gout in his toe, might mistake it for
the toothache.

Every sense, therefore, hath its peculiar
laws and limits, by the constitution of our
nature ; and one of the laws of sight is, that
we always see an object in the direction of
a right line, passing from its image on the
retina through the centre of the eye.

3. Perhaps some readers will imagine
that it is easier, and will answer the pur-
pose as well, to conceive a law of nature,
by which we shall always see objects in
the place in which they are, and in their
true position, without having recourse to
images on the retina, or to the optical centre
of the eye.

To this I answer, that nothing can be a
law of nature which is contrary to fact.
The laws of nature are the most general
facts we can discover in the operations of
nature. Like other facts, they are not to
be hit upon by a happy conjecture, but
justly deduced from observation ; like other
general facts, they are not to be drawn from
a few particulars, but from a copious, pa-
tient, and cautious induction. That we see
things always in their true place and posi-
tion, is not fact ; and therefore it can be no
law of nature. In a plain mirror, I see
myself, and other things, in places very
different from those they really occupy."
And so it happens in every instance where-
in the rays coming from the object are
either reflected or refracted before falling
upon the eye. Those who know anything
of optics, know that, in all such cases, the
object is seen in the direction of a line
passing from the centre of the eye, to the
point where the rays were last reflected
or refracted ; and that upon this all the
powers of the telescope and microscope

Shall we say, then, that it is a law of
nature, that the object is seen in the direc-
tion which the rays have when they fall
on the eye, or rather in the direction con-
trary to that of the rays when they fall
upon the eye ? No. This is not true ;
and therefore it is no law of nature. For
the rays, from any one point of the object,
come to all parts of the pupil ; and there-
fore must have different directions : but we
see the object only in one of these direc-
tions — to wit, in the direction of the raj s
that come to the centre of the eye. And
this holds true, even when the rays that
should pass through the centre are stopped,

* This is a very inaccurate statement. In a
mirror 1 do not sec myself, Ac,— H.

J 60


and the object is seen by rays that pass at a
distance from the centre.*

Perhaps it may still be imagined, that,
although we are not made so as to see ob-
jects always in their true place, nor so as to
see them precisely in the direction of the
rays when they fall upon the cornea ; yet
we may be so made as to see the object
in the direction which the rays have when
they fall upon the retina, after they have un-
dergone all their refractions in the eye —
that is, in the direction in which the rays
pass from the crystalline to the retina. But
neither is this true ; and consequently it is
no law of our constitution. In order to
see that it is not true, we must conceive all
the rays that pass from the crystalline to
one point of the retina, as forming a small
cone, whose base is upon the back of the
crystalline, and whose vertex is a point of
the retina. It is evident that the rays which
form the picture in this point, have various
directions, even after they pass the crystal-
line : yet the object is seen only in one of
these directions — to wit, in the direction of
the rays that come from the centre of the
eye. Nor is this owing to any particular
virtue in the central rays, or in the centre
itself; for the central rays may be stopped.
When they are stopped, the image will be
formed upon the same point of the retina as
before, by rays that are not central, nor have
the same direction which the central rays
had : and in this case the object is seen in the
same direction as before, although there
are now no rays coming in that direction.*

From this induction we conclude, That
our seeing an object in that particular di-
rection in which we do see it, is not owing to
any law of nature by which we are made to
see it in the direction of the rays, either be-
fore their refractions in the eye, or after,
but to a law of our nature, by which we
see the object in the direction of the right
line that passeth from the picture of the
object upon the retina to the centre of the

* But still we always see in the direction of a line
made up of the directions of all the rays of the pencil,
and this line necessarily coincides with the direction
of the central ray, even where that ray itself is inter,
cepted; for the central line would still be the me-
dium of allthe lines of the various divergent or con.
vergent rays in the pencil — H.

t It is incorrect to say that "we see the object,"
(meaning the thing from which the rays come
by emanation or reflection, but which is unknown
and incognizalile by sight,) and so forih. It would
be more correct to describe vision— a perception, by
which we take immediate cognizance of light in re-
lation to our organ — (hat is, as diffused and figured
upen the retina, undrr various modifications of de-
gree and kind, {brightness and colour,)— and likewise
as falling on it in a particular direction. The image
on the retina is not itself an object of visual percep-
tion. It is only to be regarded as the complement of
those points, or of that sensitive surface, on which
the rays impinge, and with which they enter into re-
lntion. The total object of visual perception is thus
neither the rays in themselves, nor the organ in it-
self, but the rays and thf living organ in rer^TOcitj :

The facts upon which 1 ground this in-
duction, are taken from some curious ex-
periments of Scheiner, in his u Fundamen-
tum Opticum," quoted by Dr Porterfield,
and confirmed by his experience. I have
also repeated these experiments, and found
them to answer. As they are easily made,
and tend to illustrate and confirm the law
of nature I have mentioned, I shall recite
them as briefly and distinctly as I can.

Experiment 1. Let a very small object,
such as the head of a pin, well illuminated,
be fixed at such a distance from the eye as
to be beyond the nearest limit and within
the farthest limit of distinct vision. For a
young eye, not near-sighted, the object may
be placed at the distance of eighteen inches.
Let the eye be kept steadily in one place, and
take a distinct view of the object. We
know, from the principles of optics, that
the rays from any one point of this object,
whether they pass through the centre of the
eye, or at any distance from the centre
which the breadth of the pupil will permit,
do all unite again in one point of the retina.
We know, also, that these rays have differ-
ent directions, both before they fall upon
the eye, and after they pass through the

Now, we can see the object by any one
small parcel of these rays, excluding the
rest, by looking through a small pin-hole in
a card. Moving this pin-hole over tne
various parts of the pupil, we can see the
object, first by the rays that pass above the
centre of the eye, then by the central rays,
then by the rays that pass below the centre,
and in like manner by the rays that pass on
the right and left of the centre. Thus, we
view this object, successively, by rays that
are central, and by rays that are not central ;
by rays that have different directions, and
are variously inclined to each other, both
when they fall upon the cornea, and when
they fall upon the retina; but always by
rays which fall upon the same point of the
retina. And what is the event ? It is this —
that the object is seen in the same individual
direction, whether seen by all these rays to-
gether, or by any one parcel of them.

Experiment 2. Let the object above
mentioned be now placed within the nearest
limit of distinct vision — that is, for an eye
that is not near-sighted, at the distance of

this organ is not, however, to be viewed as merely
the retina, but as the whole tract of nervous fibre
pertaining to the sense. In an act of vision, so
also in the other sensitive acts, i am thus con-
scious, (the word should not be restricted to self-
consciousness,) or immediately cognizant, not nnlv
of the affections of self, but of the phenomena of
something different from self, both, however, always
in relation to each other. According, as in differ-
ent senses, the subjective or the objective element
preponderates, we have s nsation or perception, the
secondary or the p)ima>y qualities of matter ; dis-
tinctions which are thus identified and carried up
into a general mw. But of this again H.



four or five inches. We know that, in this
case, the raya coining from one point of the
object do not meet in one point of the retina,
but spread over a small circular spot of it ;
the central rays occupying the centre of this
circle, the rays that pass above the centre
occupying the upper part of the circular spot,
and so of the rest. And we know that the
object is, in this case, seen confused ; every
point of it being seen, not in one, but in
various directions. To remedy this confu-
sion, we look at the object through the pin-
hole, and, while we move the pin-hole over
the various parts of the pupil, the object
does not keep its place, but seems to move in
a contrary direction.

It is here to be observed, that, when the
pin-hole is carried upwards over the pupil,
the picture of the object is carried upwards
upon the retina, and the object, at the same
time, seems to move downwards, so as to be
always in the right line, passing from the
picture through the centre of the eye. It is
likewise to be observed, that the rays which
form the upper and the lower pictures upon
the retina do not cross each other, as in or-
dinary vision; yet, still, the higher picture
shews the object lower, and the lower pic-
ture shews Jhe objeet higher, in the same
manner as when the rays cross each other.
Whence we may observe, by the way, that
this phsenomenon of our seeing objects in a
position contrary to that of their pictures
upon the retina, does not depend upon the
crossing of the rays, as Kepler and Des
Cartes conceived.

Experiment 3. Other things remaining
as in the last experiment, make thre^ pin-
holes in a straight line, so near that the rays
coming from the object through all the holes
may enter the pupil at the same time. In
this case, we have a very curious phsenome-
non ; for the object is seen triple with one
eye. And if you make more holes within
the breadth of the pupil, you will see as many
objects as there are holes. However, we
shall suppose them only three — one on the
right, one in the middle, and one on the left ;
in which case you see three objects standing
in a line from right to left.

It is here to be observed, that there are
three pictures on the retina ; that on the
left being formed by the rays which pass
on the left df the eye's centre, the middle
picture being formed by the central rays,
and the right-hand picture by the rays
which pass on the right of the eye's centre.
It is farther to be observed, that the object
which appears on the right, is not that
which is seen through the hole on the right,
but that which is seen through the hole on
the left; and, in like manner, the left-
hand object is seen through the hole on
the right, as is easily proved by covering
the holes successively : so that, whatever

is the direction of the rays which form the
right-hand and left-hand pictures, still the
right-hand picture shews a left-hand object,
and the left-hand picture shews a right-
hand object.

Experiment 4. It is easy to see how the
two last experiments may be varied, by
placing the object beyond the farthest limit
of distinct vision. In order to make this
experiment, I looked at a candle at the dis-
tance of ten feet, and put the eye of my
spectacles behind the card, that the rays
from the same point of the object might
meet and cross each other, before they
reached the retina. In this case, as in the
former, the candle was seen triple through
the three pin-holes ; but the candle on the
right was seen through the hole on the
right ; and, on the contrary, the left-hand
candle was seen through the hole on the
left. In this experiment it is evident,
from the principles of optics, that the rays
forming the several pictures on the retina
cross each other » little before they reach
the retina ; and, therefore, the left-hand
picture is formed by the rays which pass
through the hole on the right : so that the
position of the pictures is contrary to that
of the holes by which they are formed ; and,
therefore, is also contrary to that of their
objects — as we have found it to be in the
former experiments.

These experiments exhibit several un-
common phenomena, that regard the appa-
rent place, and the direction of visible
objects from the eye ; phenomena that
seem to be most contrary to the common
rules of vision. When we look at the same
time through three holes that are in a right
line, and at certain distances from each
other, we expect that the objects seen
through them should really be, and should
appear^to be, at a distance from each other.
Yet, by the first experiment, we may,
through three such holes, see the same
object, and the same point of that object ;
and through all the three it appears in the
same individual place and direction.

When the rays of light come from the
object in right lines to the eye, without
any reflection, inflection, or refraction, we
expect that the object should appear in its
real and proper direction from - the eye ;
and so it commonly does. But in the
second, third, and fourth experiments, we
see the object in a direction which is not
its true and real direction from the eye,
although the rays come from the object to
the eye, without any inflection, reflection,
or refraction. ',

When both the object and the eye are
fixed without the least motion, and the
medium unchanged, we expect that the
objeet should appear to rest, and keep the
same place. "Yet, in the second and fourth



experiments, when both the eye and the ob-
ject are at rest, and the medium unchanged,
we make the object appear to move upwards
or downwards, or in any direction we please.

When we look, at the same time and
irith the same eye, through holes that stand
in a line from right to left, we expect
that the object seen through the left-
hand hole should appear on the left, and the
object seen through the right-hand hole
should appear on the right. Yet, in the third
experiment, we find the direct contrary.

Although many instances occur in see-
ing the same object double with two eyes,
we always expect that it should appear
single when seen only by one eye. Yet, in
the second and fourth experiments, we have
instances wherein the same object may
appear double, triple, or quadruple to one
eye, without the help of a polyhedron or
multiplying glass.

All these extraordinary phsenomena, re-
garding the direction of visible objects from
the eye, as well as those that are common
and ordinary, lead us to that law of nature
which I have mentioned, and are the neces-
sary consequences of it. And, as there is
no probability that we shall ever be able to
give a reason why pictures upon the retina
make us see external objects, any more
than pictures upon the hand or upon the
cheek ; or, that we shall ever be able to
give a reason, why we see the object in the
direction of a line passing from its picture
through the centre of the eye, rather than
in any other direction — I am, therefore, apt
to look upon this law as a primary law of
our constitution.

To prevent being misunderstood, I beg
the reader to observe, that I do not mean
to affirm that the picture upon the retina
will make us see an object in the direction
mentioned, or in any direction, unless the
optic nerve, and the other more immediate
instruments of vision, be sound, and per-
form their function. We know not well
what is the office of the optic nerve, nor in
what manner it performs that office ; but
that it hath some part in the faculty of see-
ing, seems to be certain ; because, in an
amaurosis, which is believed to be a disorder
of the optic nerve, the pictures on the retina
are clear and distinct, and yet there is no

We know still less of the use and func-
tion of the choroid membrane ; but it seems
likewise to be necessary to vision : for it is
well known, that pictures upon that part of
the retina where it is not covered by the
choroid — I mean at the entrance of the
optic nerve — produce no vision, any more
than a picture upon the hand. • We ac-

* TCeid here adopts the theory of Mariotte, who first

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 37 of 114)