Thomas Reid.

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

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The great Newton first gave an example
to philosophers, which always ought to be,
but rarely hath been followed, by distin-
guishing his conjectures from his conclu-
sions, and putting the former by themselves,
in the modest form of queries. This is fair
and legal ; but all other philosophical traf-
fic in conjecture ought to be held contra-
band and illicit. Indeed, his conjectures
have commonly more foundation in fact,
and more verisimilitude, than the dogma-
tical theories of most other philosophers ;
and, therefore, we ought not to omit that
which he hath offered concerning the cause
of our seeing objects single with two eyes,
in the 15th query annexed to his "Optics."

" Are not the species of objects seen
with both eyes, united where the optic
nerves meet before they come into the brain,
the fibres on the right side of both nerves
uniting there, and after union going thence
into the brain in the nerve which is on the
right side of the head, and the fibres on the
left side of both nerves uniting in the same
place, and after union going into the brain
in the nerve which is on the left side of the
head, and these two nerves meeting iu the
brain in such a mnnner that their fibres .

make but one entire species or picture, half
of which on the right side of the sensorium
comes from the right side of both eyes
through the right side of both optic nerves,
to the place where the nerves meet, and
from thence on the right side of the head
into the brain, and the other half on the
left side of the sensorium comes, in like
manner, from the left side of both eyes ?
For the optic nerves of such animals as
look the same way with both eyes (as men,
dogs, sheep, oxen, &c.) meet before they
come into the brain ; but the optic nerves
of such animals as do not look the same
way with both eyes (as of fishes, and of the
chameleon) do not meet, if I am rightly in-

I beg leave to distinguish this query into
two, which are of very different natures ;
one being purely anatomical, the other re-
lating to the carrying species or pictures of
visible objects to the sensorium.

The first question is, Whether the fibres
coming from corresponding points of the
two retina do not unite at the place where
the optic nerves meet, and continue united
from thence to the brain ; so that the right
optic nerve, after the meeting of the two
nerves, is composed of the fibres coming
from the right side of both retina, and the
left, of the fibres coming from the left side
of both retina ?

This is undoubtedly a curious and rational
question ; because, if we could find ground
from anatomy to answer it in the affirm-
ative, it would lead us a step forward in
discovering the cause of the correspondence
and sympathy which there is between cer-
tain points of the two retina. For, although
we know not what is the particular function
of the optic nerves, yet it is probable that
some impression made upon them, and
communicated along their fibres, is neces-
sary to vision ; and, whatever be the nature
of this impression, if two fibres are united
into one, an impression made upon one of
them, or upon both, may probably produce
the same effect. Anatomists think it a
sufficient account of a sympathy between
two parts of the body, when they are served
by branches of the same nerve ; we should,
therefore, look upon it as an important dis-
covery in anatomy, if it were found that the
same nerve sent branches to the corre-
sponding points of the retina.

But hath any such discovery been made ?
No, not so much as in one subject, as far as
I can learn ; but, in several subjects, the
contrary seems to have been discovered.
Dr Porterfield hath given us two cases at
length from Vesalius, and one from Ctesal-
pinus, wherein the optic nerves, after touch .
ing one another as usual, appeared to be
reflected back to the same side whence
the* came without any mixture of theii



fibres. Each of these persons had lost an
eye some time before his death, and the
optic nerve belonging to that eye was
shrunk, so that it could be distinguished
from the other at the place where they met.
Another case, which the same author gives
from Vesalius, is still more remarkable;
for in it the optic nerves did not touch at
all ; and yet, upon inquiry, those who were
most familiar with the person in his life-
time, declared that he never complained of
any defect of sight, or of his seeing objects
double. Diemerbroeck tells us, that Aqua-
pendens [ab Aquapendente] and Valverda
likewise affirm, that they have met with
subjects wherein the optic nerves' did not

As these observations were made before
Sir Isaac Newton put this query, it is un-
certain whether he was ignorant of them,
or whether he suspected some inaccu-
racy in them, and desired that the matter
might be more carefully examined. But,
from the following passage of the most
accurate Winslow, it does not appear that
later observations have been more favour-
able to his conjecture. " The union of
these (optic) nerves, by the small curva-
tures of their, is very difficult to be
unfolded in human bodies. This union is
commonly found to be very close ; but, in
some subjects, it seems to be no more than
a strong adhesion — in others, to be partly
made by an intersection or crossing of fibres.
They have been found quite separate ; and,
in other subjects, one of them has been
found to be very much altered both in size
and colour through its whole passage, the
other remaining in its natural state."

When we consider this conjecture of Sir
Isaac Newton by itself, it appears more
ingenious, and to have more verisimilitude,
than anything that has been offered upon
the subject ; and we admire the caution
and modesty of the author, in proposing it
only as a subject of inquiry : but when we
compare it with the observations of anato-
mists which contradict it,+ we are naturally

*■ See Meckel's " Pathologische Anatomie," I., p.
399.— H.

+ Anatomists are now nearly agreed, that, in the
normal state, there is a partial decussation of the
human optic nerve. Soemmering, Treviranus, Ku-
dolphi, Johannes Mueller, Langenbeck, Magendie,
Mayo, &c, are paramount authority for the fact. I
dcnot know whether the observation has been made,
that the degree of decussation in different animals is
exactly in the inverse ratio of what we might have
been led, at first sight, theoretically to anticipate.^ In
proportion as the convergence .is complete—*'. e.,
where the axis of the field of vision of the severaTeyes
coincides with the axis of the field of vision common
to- both, as in men and apes — there we find the de-
cussation most partial and obscure ; whereas, in the
lower animals, in proportion as»we.find the fieldsof
the two eyes exclusive of each other, and where, conse-
quently, the necessity of bringing the two organs into
unison -might seem abolished, there, howevpr, we find
the crossing of the optic fibres complete. In fishes,
accordingly! it is distinct and isolated ; in birds, it taker

led to this reflection, That, if we trust to
the conjectures of men of the greatest
genius in the operations of nature, we have
only the chance of going wrong in an inge-
nious manner.

The second part of the query is, Whether
the two species of objects from the two eyes
are not, at the place where the optic nerves
meet, united into one species or picture,
half of which is carried thence to the sen.
sorium in the right optic nerve, and the
other half in the left ? and whether these
two halves are not so put together again at
the sensorium, as to make one species or
picture ?

Here it seems natural to put the previous
question, What reason have we to believe
that pictures of objects are at all carried to
the sensorium, either by the optic nerves,
or by any other nerves ? Is it not possible
that this great philosopher, as well as many
of a lower form, having been led into this
opinion at first by education, may have con-
tinued in it, because he never thought of
calling it in question ? I confess this was
my own case for a considerable part of my
life. But since I was led by accident to
think seriously what reason I had to believe
it, I could find none at all. It seems to be
a mere hypothesis, as much as the Indian
philosopher's elephant. I am not conscious
of any pictures of external objects in my
sensorium, any more than in my stomach :
the things which I perceive by my senses,
appear to be external, and not in any part
of the brain ; and my sensations, properly
so called, have no resemblance of external

The conclusion from all that hath been
said, in no less than seven sections, upon
our seeing objects single with two. eyes,
is this — That, by an original property
of human eyes, objects painted upon the
centres of the two retina, or upon points
similarly situate with regard to the centres,
appear in the same visible place ; that the
most plausible attempts to account for this
property of the eyes, have been unsuccess-
ful ; and, therefore, that it must be either
a primary law of our constitution, or the
consequence of some more general law,
which is not yet discovered.

We have now finished what we intended
to say, both of the visible appearances of
things to the eye, and of the laws of our
constitution by which those appearances

more the appearance of an interlacement ; in the
mammalia, that of a fusion of substance. A 6econd
consideration, however, reconciles theory and observ-
ation. Some, however, as Woolaston, make the
parallel motion of the eyes to be dependent on the
connection of the optic nerves ; and, besides experi.
ments, there are various pathological ca*es in favoui
of.-Magendie's opinion, that the fifth pair are the
nerves on, which the energies of sight, hearing,
smell, and' taste are proximately ana principally de.
pendent. — H.



arc exhibited. But it was observed, in the
beginning \A this chapter, that the visible
appearances of objects serve only as signs
of' their distance, magnitude, figure, and
other tangible qualities. The visible ap-
pearance is that which is presented to the
mind by nature, according to those laws of
our constitution which have been explained,
lint th^j thin;' 1 signified by that appearance,
id that which is presented to the mind by

When one speaks to us in a language
that is familiar, we hear certain sounds,
and this is all the effect that his discourse
has upon us by nature ; but by custom we
understand the meaning of these sounds ;
and, therefore, we fix our attention, not
upon the sounds, but upon the things sig-
nified by them. In like manner, we see
only the visible appearance of objects by
nature; but we learn by custom to inter-
pret these appearances, and to understand
their meaning. And when this visual
language is learned, and becomes familiar,
we attend only to the things signified ; and
cannot, without great difficulty, attend to
the signs by which they are presented. The
mind passes from one to the other so
rapidly and so familiarly, that no trace of
the sign is left in the memory, and we seem
immediately, and without the intervention
of any sign, to perceive the thing sig-

When I look at the apple-tree which
stands before my window, I perceive, at the
first glance, its distance and magnitude, the
roughness of its trunk, the disposition of
its branches, the figure of its leaves and
fruit. I seem to perceive all these things
immediately. The visible appearance which
presented them all to the mind, has entirely
escaped me ; I cannot, without great diffi-
culty, and painful abstraction, attend to it,
even when it stands before me. Yet it is
certain that this visible appearance only
is presented to my eye by nature, and that
I learned by custom to collect all the rest
from it. If I had never seen before now,
I should not perceive either the distance or
tangible figure of the tree ; and it would
have required the practice of seeing for
many months, to change that original per-
ception which nature gave me by my eyes,
into that which I now have by custom.

The objects which we see naturally and
originally, as hath been before observed,
have length and breadth, but no thickness
nor distance from the eye. Custom, by a
kind of legerdemain, withdraws gradually
these original and proper objects of sight,
and substitutes in their place objects of
touch, which have length, breadth, and
thickness, and a determinate distance from
the eye. By what means this change is
brought about, and what principles of the

human mind concur in it, we are next to

Section XX,


Sensation, and the perception-}- of exter-
nal objects by the senses, though very dif-
ferent in their nature, have commonly been
considered as one and the same thing.£
The purposes of common life do not make
it necessary to distinguish them 5 and the
received opinions of philosophers tend ra-
ther to confound them ; but, without at-
tending carefully to this distinction, it is
impossible to have any just conception of
the operations of our senses. The most
simple operations of the mind, admit not of
a logical definition : all we can do is to de-
scribe them, so as to lead those who are
conscious of them in themselves, to attend
to them, and reflect upon them ; and it is
often very difficult to describe them so as to
answer this intention.

The same mode of expression is used to
denote sensation and perception ; and, there-
fore, we are apt to look upon them as things
of the same nature. Thus, I feel a pain ;
I see a tree : the first denoteth a sensation,
the last a perception. The grammatical
analysis of both expressions is the same :

• Nothing in the compass of inductive reasoning
appears more satisfactory than Berkeley's demon-
stration of the necessity and manner of our learn,
ing, by a slow processof observation and comparison
alone, the connection between the perceptions of
vision and touch, and, in general, all that relates to
the distance and real magnitude of external things.
But, although the same necessity seems in theory
equally incumbent on thelower animals as on man,
yet this theory is provokingly— and that by the most
manifest experience— found totally at fault with re-
gard to them ; for we find that ,all the animals who
possess at birth the power of regulated motion (and
these are those only through whom the truth of the
theory canbe brought to the test of a decisive ex-
periment) possess also from birth the whole appre-
hension of distance, &c , which they are ever known
to exhibit. The solution of this difference, by a
resort to instinct, is unsatisfactory ; for instinct is,
in fact, an occult principle— a kind of natural revel-
ation— and the hypothesis of instinct, therefore, only
a confession of our ignorance ; and, at the same time,
if instinct he allowed in the lower animals, how
can we determine whether and how far instinct
may not in like manner operate to the same result
in man ?— I have discovered, and, by a wide indue
tion, estaLlished, that the power ot regulated mo-
tion at birth is, in all animals, governed by the de-
velopement, at that period, of the cerebellum, in pro.
por tion to the brain proper. Is this law to be extended
to the faculty of determiningdistances, &c, by sight ?

f On the distinction of Sensation proper^ from
Perception proper, see « Essays on the Intellectual
Powers," Essay II., chap. 16, and Note I).* Reid
himself, especially in this work, has not been always
rigid in observing their discrimination.— H.

J Not only are they different, but— what has escaped
our philoaophen-the law ot their manifestation
is, that, while both are co-existent, each is always in
the inverse ratio of the other. Perception is theobjec.
tive, bensation the subjective, element. This by the



for both consist of an active verb and an
object. But, if we attend to the things sig-
nified by these expressions, we shall find
that, in the first, the distinction between the
act and the object is not real but gramma-
tical ; in the second, the distinction is not
only grammatical but real.

The form of the expression, I feel pain,
might seem to imply that the feeling is
something distinct from the pain felt ; yet,
in reality, there is no distinction. As
thinking a thought is an expression which
could signify no more than thinking, so
feeling a pain signifies no more than being
pained. What we have said of pain is ap-
licable to every other mere sensation. It
is difficult to give instances, very few of
our sensations having names ; and, where
they have, the name being common to the
sensation, and to something else which is
associated with it. But, when we attend
to the sensation by itself, and separate it
from other things which are conjoined
with it in the imagination, it appears to
be something which can have no existence
but in a sentient mind, no distinction
from the act of the mind by which it is

Perception, as we here understand it,
hath always an object distinct from the act
by which it is perceived; an object which
may exist whether it be perceived or not.
I perceive a tree that grows before my win-
dow ; there is here an object which is per-
ceived, and an act of the mind by which it
is perceived ; and these two are not only
distinguishable, but they are extremely un-
like in their natures. The object is made
up of a trunk, branches, and leaves ; but
the act of the mind by which it is per-
ceived hath neither trunk, branches, nor
leaves. I am conscious of this act of my
mind, and I can reflect upon it ; but it is
too simple to admit of an analysis, and I
cannot find proper words to describe it. I
find nothing that resembles it so much as
the remembrance of the tree, or the ima-
gination of it. Yet both these differ essen-
tially from perception ; they differ likewise
one from another. It is in vain that »
philosopher assures me, that the imagina-
tion of the tree, the remembrance of it, and
the perception of it, are all one, and differ
only in degree of vivacity. I know the
contrary ; for I am as well acquainted with
all the three as I am with the apartments
of my own house. I know this also, that
the perception of an object implies both a
conception of its form, and a belief of its
present existence.* I know, moreover, that

* It is to be observed that Reid himself does not
discriminate perception and imagination by any
essential difference. According to him, perception
is only the conception (imagination) of an object, ac-
companied with a belief of its present existence; and
even this last distinction, a mere " faith without

this belief is not the effect of argumentation
and reasoning ; it is the immediate effect of
my constitution.

I am aware that this belief which I have
in perception stands exposed to the strongest
batteries of scepticism. But they make no
great impression upon it. The sceptic asks
me, Why do you believe the existence of
the external object which you perceive ?
This belief, sir, is none of my manufacture ;
it came from the mint of Nature 5 it bears
her image and superscription ; and, if it is
not right, the fault is not mine : I even took
it upon trust, and without suspicion. Kea-
son, says the sceptic, is the only judge 01
truth, and you ought to throw off every opi-
nion and every belief that is not grounded
on reason. Why, sir, should I believe the
faculty of reason more than that of percep-
tion ? — they came both out of the same shop,
and were made by the same artist ; and if
he puts one piece of false ware into my
hands, what should hinder him from put-
ting another ?*

Perhaps the sceptic will agree to distrust
reason, rather than give any credit to per-
ception. For, says he, since, by your own
concession, the object which you perceive,
and that act of your mind by which you
perceive it, are quite different things, the
one may exist without the other ; and, as
the object may exist without being per-
ceived, so the perception may exist without
an object. There is nothing so shameful
in a philosopher as to be deceived and de-
luded ; and, therefore, you ought to resolve
firmly to withhold assent, and to throw off
this belief of external objects, which may be
all delusion. For my part, I will never
attempt to throw it off ; and, although the
sober part of mankind will not be very
anxious to know my reasons, yet, if they
can be of use to any sceptic, they are
these : —

First, because it is not in my power : why,
then, should I make a vain attempt ? It
would be agreeable to fly to the moon, and
to make a visit to Jupiter and Saturn ; but,
when I know that Nature has bound me
down by the law of gravitation to this planet
which I inhabit, I rest contented, aud quietly

knowledge," is surrendered by Mr Stewart. Now,
as conception (imagination) is only inriediately cog-
nisant of the ego, so must perception or. tl is doctrine
be a knowledge purely subjective. Perception must
be wholly different in kind from Conception, if we are
to possess a faculty informing us of the existent e and
qualities of an external world ; and, unless we are
possessed of such a faculty, we shall never be compe-
tent to vindicate more than an ideal reality to the
objects of our cognitions. — H.

* This argument would be good in favour of our
belief, that we are really percipient of a non-rgo :
it is not good in favour of our belief that a ?>crn-<'go
really exists, our perception of its re. 1 exi tence
being abandoned. Mankind have the latter belief
only as they have the former ; and, if we are dei eivcti
by our Nature touching the one, it is absurd to ap
peal to her veracity in proof of Hie other. — H.



suffer myself to be carried along in its orbit.
My belief is carried along by perception, as
irresistibly as my body by the earth. And
the greatest sceptic will find himself to be
in the same condition. He may struggle
hard to disbelieve the informations of his
senses, as a man does to swim against a tor-
rent ; but, ah I it is in vain. It is in rain
that he strains every nerve, and wrestles
with nature, and with every object that
strikes upon his senses. For, after all,
when his strength is spent in the fruitless
attempt, he will be carried down the tor-
rent with the common herd of believers.

Secondly, I think it would not be pru-
dent to throw off this belief, if it were in
my power. If Nature intended to deceive
me, and impose upon me by false appear-
ances, and I, by my great cunning and pro-
found logic, have discovered the imposture,
prudence would dictate to me, in this case,
even to put up [with] this indignity done
me, as quietly as I could, and not to call
her an impostor to her face, lest she should
be even with me in another way. For
what do I gain by resenting this injury ?
You ought at least not to believe what she
says. This indeed seems reasonable, if
she intends to impose upon me. But what
is the consequence ? I resolve not to be-
lieve my senses. I break my nose against
a post that comes in my way ; I step into
a dirty kennel ; and, after twenty such
wise and rational actions, I am taken up
and clapped into a mad-house. Now, I con-
fess I would rather make one of the credu-
lous fools whom Nature imposes upon, than
of those wise and rational philosophers
who resolve to withhold assent at all this
expense. If a man pretends to be a scep-
tic with regard to the informations of
sense, and yet prudently keeps out of harm's
way as other men do, he must excuse my
suspicion, that he either acts the hypocrite,
or imposes upon himself. For, if the scale
of his belief were so evenly poised as to
lean no more to one side than to the con-
trary, it is impossible that his actions could be
directed by any rules of common prudence. *

Thirdly, Although the two reasons al-
ready mentioned are perhaps two more than
enough, I shall offer a third. I gave im-
plicit belief to the informations of Nature
by my senses, for a considerable part of my
life, before I had learned so much logic as
to be able to start a doubt concerning them.
And now, when I reflect upon what is past,
I do not find that I have been imposed upon
by this belief. I find that without it I must
have perished by a thousand accidents. I
find that without it I should have been no
wiser now than when I was born. I should

* This is not a fair consequence of Idealism ; there-

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 42 of 114)