Thomas Reid.

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

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H.

t AioqQety.fA.mi ovx o$8atXf/,o7s ■ says Plato, followed
by a host, ot philosophers, comparing the senses to
windows of the mind. — H.

§ *■ 'I he mind fees," says Epicharmus — " the mind
hears, all else is deaf and blind"— a saving alluded to
as proverbial by Arislotle, in a passage to the same
effect, which cannot adequately lie translated:—
Xai£((T0£i<rfl& etifBv.trif %iec.*6i<xf, xafluifrEj avxirOr,rt»
tovov t%tt> Sa-Tie S/'jtfraj to, NS? o;f, * a.} tit
i. x qui i. This Vas escaped (he coninieutatois.— H.
Se«p.«7s.n. f76-78l



ciiAP. ii.] OF IMPRESSIONS ON THE ORGANS, &c.



247



conclude that it is the eye that Eees, or
the ear that hears. The telescope is an
artificial organ of sight, but it sees not.
The eye is a natural organ of sight, by
which we see ; but the natural organ sees
as little as the artificial.

The eye is a machine ixnist admirably
contrived for refracting the rays of light,
and forming a distinct picture of objects
upon the retina; but it sees neither the
object nor the picture. It can form the
picture after it is taken out of the head ;
but no vision ensues. Even when it is in
its proper place, and perfectly sound, it is
well known that an obstruction in the optic
nerve takes away vision, though the eye
has performed all that belongs to it.

If anything more were necessary to be
said on a point so evident, we might ob-
serve that, if the faculty of seeing were in
the eye, that of hearing in the ear, and so
of the other senses, the necessary conse-
quence of this would be, that the thinking
principle, which I call myself, is not one,
but many. But this is contrary to the ir-
resistible conviction of every man. When
I say I see, I hear, I feel, I remember,
this implies that it is one and the same self
that performs all these operations; and, as
it would be absurd to say that my memory,
another man's imagination, and a third
man's reason, may make one individual
intelligent being, it would be equally ab-
surd to say that one piece of matter see-
ing, another hearing, and a third feeling,
may make one and the same percipient
being.

These sentiments are not new ; they have
occurred to thinking men from early ages.
Cicero, in his " Tusculan Questions," Book
I., chap. 20, has expressed them very dis-
tinctly. Those who choose may consult the
passage.* [79]



CHAPTER II.

OFTHE IMPRESSIONS ON THE ORGANS, NERVES,
AND BRAINS.

A second law of our nature regarding
perception is, that u-e perceive no object,
unless some impression is made upon the
organ of sense, either by .the immediate
application of the object, or by some medium
which passes between the object and the
organ.

In two of our senses — to wit, touch and
teste — there must be an immediate applica-
tion of the object to the organ. In the
other three, the object is perceived at a dis-
tance, but still by means of a medium, by



* Cicero saya n-tl-ing on this head tliat had not
been said hcfi re him by tl-e Greek rhii ;,Oj-hcrs — H.
[79, POT



which some impression is made upon the
organ."

The effluvia of bodies drawn into the
nostrils with the breath, are the medium of
smell ; the undulations of the air are the
medium of hearing ; and the rays of light
passing from visible objects to the eye, are
the medium of sight. We see no object
unless rays of light come from it to the eye.
We hear not the sound of any body, unless
the vibrations of some elastic medium, oc-
casioned by the tremulous motion of the
sounding body, reach our ear. We per-
ceive no smell, unless the effluvia of the
smelling body enter into the nostrils. We
perceive no taste, unless the sapid body be
applied to the tongue, or some part of the
organ of taste. Nor do we perceive any
tangible quality of a body, unless it touch
the hands, or some part of our bodies.

These are facts known from experience
to hold universally and invariably, both in
men and brutes. By this law of our na-
ture, our powers of perceiving external ob-
jects, are farther limited and circumscribed.
Nor can we give any other reason for this,
than [80] that it is the will of our Maker, who
knows best what powers, and what degrees
of them, are suited to our state. We were
once in a state, I mean in the womb, wherein
our powers of perception were more limited
than in the present, and, in a future state,
they may be more enlarged.

It is likewise a law of our nature, that,
in order to our perceiving objects, the im-
pressions made upon the organs of sense
must be communicated to the nerves, and
by them to the brain. This is perfectly
known to those who know anything of ana-
tomy.

The nerves are fine cords, which pass
from the brain, or from the spinal marrow,
which is a production of the brain, to all
parts of the body, dividing into smaller
branches as they proceed, until at last they
escape our eyesight : and it is found by
experience, that all the voluntary and in-
voluntary motions of the body are performed
by their means. When the nerves that
serve any limb, are cut, or tied hard, we
have then no more power to move that limb
than if it was no part of the body.

As there are nerves that serve the mus-
cular motions, so there are others that serve
the several senses ; and as without the for-
mer we cannot move a limb, so without the
latter we can have no perception.

• This distinction of a mediate and immediate ob.
Ject. or of an object and a medium, in perception, is
inaccurate, and a eource of sad confusion. We per-
ceive, and can perceive, nothing but what is in rela.
tion to the organ, and nothing is in relation to the
organ that is not present to it. Ail tbesenses are, in
tact, modifications of touch, as Dcmticritus of old
taught. Wc reach the distant reality, not tty senses
not by perception, tint by inference. Keicl, how.
ever, in this only follows his predecessor* — H,



243



ON THE INTELLECTUAL POWERS.



[ESSAY II.



This train of machinery the wisdom of
God has made necessary to our perceiving
objects. Various parts of the body concur
to it, and each has its own function. First,
The object, either immediately, or by some
medium, must make an impression on the
organ. The organ serves only as a medium
by which an impression is made on the
nerve ; and the nerve serves as a medium
to make an impression upon the brain.
Here the material part ends; at least we
can trace it no farther ; the rest is all in-
tellectual. *

The proof of these impressions upon the
nerves and brain in [81] perception is this,
that, from many observations and experi-
ments, it is found that, when the organ of
any sense is perfectly sound, and has the
impression made upon it by the object ever
so strongly, yet, if the nerve which serves
that organ be cut or tied hard, there is no
perception ; and it is well known that dis-
orders in the brain deprive us of the power
of perception when both the organ and its
nerve are sound.

There is, therefore, sufficient reason to
conclude that, in perception, the object pro-
duces some change in the organ ; that the
organ produces some change upon the
nerve; and that the nerve produces some
change in the brain. And we give the
name of an impression to those changes,
because we have not a name more proper to
express, in a general manner, any change
produced in a body, by an external cause,
without specifying the nature of that
change. Whether it be pressure, or at-
traction, or repulsion, or vibration, or some-
thing unknown, for which we have no
name, still it may be called an impression.
But, with regard to the particular kind of
this change or impression, philosophers
have never been able to discover anything
at all.

But, whatever be the nature of those im-
pressions upon the organs, nerves, and
brain, we perceive nothing without them.
Experience informs that it is so ; but we
cannot give a reason why it is so. In the
constitution of man, perception, by fixed
laws of nature, is connected with those im-
pressions ; but we can discover no neces-
sary connection. The Supreme Being has
seen fit to limit our power of perception ; so
that we perceive not without such impres-
sions; and this is all we know of the
matter.

This, however, we have reason to con-



* There can be no doubt that the whole organism
ofthescnse, from periphery to centre, must co-operate
simultaneously in perception ; but there is no rea-
son to place the mind at the central extremity nlotic,
Hhtl to hold that not only a certain series of organic
changes, but a scn-ation, must precede the mental
cognition This is mere hypothesis, and opyoacd 10
the testimony of consciousness, — K.



elude in general — that, as the impressions on
the organs, nerves, and brain, correspond
exactly to the nature and conditions of the
objects by which they are made, so our
perceptions and sensations correspond to
those impressions, and vary in kind, and in
degree, as they vary. [ 82 ] Without this exact
correspondence, the information we receive
by our senses would not only be imperfect,
as it undoubtedly is, but would be fallacious,
which we have no reason to think it is.



CHAPTER III.

HYPOTHESES CONCERNING THE NERVES AND
BRAIN.

We are informed by anatomists, that, al-
though the two coats which inclose a nerve,
and which it derives from the coats of the
brain, are tough and elastic, yet the nerve
itself has a very small degree of consistence,
being almost like marrow. It has, how-
ever, a fibrous texture, and may be divided
and subdivided, till its fibres escape our
senses ; and, as we know so very little about
the texture of the nerves, there is great
room left for those who choose to indulge
themselves in conjecture.

The ancients conjectured that the ner-
vous fibres are fine tubes, filled with a very
subtile spirit, or vapour, which they called
animal spirits ; that the brain is a gland,
by which the animal spirits are secreted
from the finer part of the blood, and their
continual waste repaired ; and that it is by
these animal spirits that the nerves perform
their functions. Des Cartes has shewn
how, by these animal spirits, going and re-
turning in the nerves, muscular motion,
perception, memory, and imagination, are
effected. All this he has described as dis-
tinctly as if he had been an eye-witness of
all those operations. But it happens that
the tubular structure of the nerves was
never perceived by the human eye, nor
shewn by the nicest injections ; and all that
has been said about animal spirits, through
more than fifteen centuries, is mere con-
jecture.

Dr Briggs, who was Sir Isaac Newton's
master in anatomy, was the first, as far as
I know, who advanced a new system
concerning [83] the nerves. - Heconceived
them to be solid filaments of prodigious



- Briggs was not the first The Jesuit, Hon •-
ratus r-ai.ry, had before him denial the old hypothe-
sis of spirits ; and the new hypothesis of cerebral
fibres, and fibril, hj which he explains the phseno-
mer-a (ifscinc, imagination ami memory, is not on v
the first, but perhaps the most ingenious of the class
that has been proposed. Yet ihe very name of Fabry
it. wholly uiiiAiticed by those historians of philosophy
who do not dcpmit ,sui crflurus to dwell on the tire
«inne reveries of Briggs, Hartley, ;md Bonnet. — H.



[81-83]



chap, in.] HYPOTHESES CONCERNING THE NERVES, &c. 24J



tenuity ; and this opinion, as it accords bet-
ter with observation, seems to have been
more generally received since his time. As
to the manner of performing their office,
Dr Briggs thought that, like musical cords,
they have vibrations differing according to
their length and tension. They seem, how-
ever, very unfit for this purpose, on account
of their want of tsuacity, their moisture,
and being through their whole length in
contact with moist substances ; so that, al-
though !>r Briggs wrote a book upon this
system, called Nova Visionis Theoria, it
seems not to have been much followed.

Sir Isaac Newtonjjn all his philosophical
writingsjtook great care to distinguish his
doctrines, which he pretended to prove by
just induction, from his conjectures, which
were to stand or fall according as future
experiments and observations should esta-
blish or refute them. His conjectures he
has put in the form of queries, that they
might not be received as truths, but be
inquired into, and determined according to
the evidence to be found for or against
them. Those who mistake his queries for
a part of his doctrine, do him great injus-
tice, and degrade him to the rank of the
common herd of philosophers, who have in
all ages adulterated philosophy, by mixing
conjecture with truth, and their own fancies
with the oracles of Nature. 1 Among other
queries, this truly great philosopher pro-
posed this, Whether there may not be an
elastic medium, or eether, immensely more
rare than air, which pervades all bodies,
and which is the cause of gravitation ; of
the refraction and reflection of the rays of
light ; of the transmission of heat, through
spaces void of air ; and of many other phse-
nomena ? In the 23d query subjoined to his
" Optics," he puts this question with regard
to the impressions made on the nerves and
brain in perception, Whether vision is
effected chiefly by the vibrations of this
medium, excited in the bottom of the eye
by the rays of light, and propagated along
the solid, pellucid, and uniform capillaments
of the optic nerve ? And whether hearing
is effected [84] by the vibrations of this or
some other medium, excited by the tremor
of the air in the auditory nerves, and pro-
pagated along the solid, pellucid, and uni-
form capillaments of those nerves ? And
so with regard to the other senses.

What Newton only proposed as a matter
to be inquired into, Dr Hartley conceived
to have such evidence, that, in his " Ob-
servations on Man," he has deduced, in a
mathematical form, a very ample system
concerning the faculties of the mind, from
the doctrine of vibrations, joined with that
of association.

His notion of the vibrations excited in
the nerves, is expressed in Propositions 4
[84., 85]



and 5 of the first part of his " Observa-
tions on Man." " Prop. 4. External objects
impressed on the senses occasion, first in
the nerves on which they are impressed,
and then in the brain, vibrations of the
small, and, as one may say, infinitesimal
medullary particles. Prop. 5. The vibra-
tions mentioned in the last proposition are
excited, propagated, and kept up, partly by
the sether — that is, by a very subtile elastic
fluid ; partly by the uniformity, continuity,
softness, and active powers of the medullary
substance of the brain, spinal manrow, and
nerves."

The modesty and diffidence with which
Dr Hartley offers his system to the world —
by desiring his reader " to expect nothing
but hints and conjectures in difficult and
obscure matters, and a short detail of the
principal reasons and evidences in those
that are clear ; by acknowledging, that he
shall not be able to execute, with any ac-
curacy, the proper method of philosophising,
recommended and followed by Sir Isaac
Newton ; and that he will attempt a sketch
only for the benefit of future enquirers" —
seem to forbid any criticism upon it. One
cannot, without reluctance, criticise what is
proposed in such a ' manner, and with so
good intention ; yet, as the tendency of this
system of vibrations is to make all the oper-
ations of the mind mere mechanism, depend-
ent [85] on the laws of matter and motion,
and, as it has been held forth by its vota-
ries, as in a manner demonstrated, I shall
make some remarks on that part of the sys-
tem which relates to the impressions made
on the nerves and brain in perception.

It may be observed, in general, that Dr
Hartley's work consists of a chain of pro-
positions, with their proofs and corollarieo,
digested in good order, and in a scientific
form. A great part of them, however, are,
as he candidly acknowledges, conjectures
and hints only ; yet these are mixed with
the propositions legitimately proved, with-
out any distinction. Corollaries are drawn
from them, and other propositions grounded
upon them, which, all taken together, make
up a system. A system of this kind re-
sembles a chain, of which some links are
abundantly strong, others very weak. The
strength of the chain is determined by that
of the weakest links ; for, if they give way,
the whole falls to pieces, and the weight
supported by it falls to the ground.

Philosophy has been, in all ages, adul-
terated by hypotheses ; that is, by systems
built partly on facts, and much upon con-
jecture. It is pity that a man of Dr Hart-
ley's knowledge and candour should have
followed the multitude in this fallacious
tract, after expressing his approbation of
the proper method of philosophising, pointed
out by Bacon and Newton. The last con-



250



ON THE INTELLECTUAL POWERS.



£essat II.



sidered it as a reproach when his system
was called his hypothesis ; and says, with
disdain of such imputation, Hypotheses non
Jingo. And it is very strange that Dr
Hartley should not only follow such a me-
thod of philosophising himself, but that he
should direct others in their inquiries to
follow it. So he does in Proposition 87,
Part I., where he deduces rules for the
ascertainment of truth, from the rule of
false, in arithmetic, and from the art of
decyphering ; and in other places.

As to the vibrations and vibratiuncles,
whether of an elastic aether, or of the in-
finitesimal particles of the brain and nerves,
there [86] may be such things for what we
know ; and men may rationally inquire
whether they can find any evidence of their
existence ; but, while we have no proof of
their existence, to apply them to the solu-
tion of pliEenomena, and to build a system
upon them, is what I conceive we call build-
ing a castle in the air.

When men pretend to account for any
of the operations of Nature, the causes
assigned by them ought, as Sir Isaac New-
ton has taught us, to have two conditions,
otherwise they are good for nothing. First,
They ought to be true, to have a real exist-
ence, and not to be barely conjectured to
exist, without proof. Secondly, They ought
to be sufficient to produce the effect.

As to the existence of vibratory motions
in the medullary substance of the nerves
and brain, the evidence produced is this :
First, It is observed that the sensations of
seeing and hearing, and some sensations of
touch, have some short duration and con-
tinuance. Secondly, Though there be no
direct evidence that the sensations of taste
and smell, and the greater part of these of
touch, have the like continuance, yet, says
the author, analogy would incline one to
believe that they must resemble the sensa-
tions of sight and hearing in this particular.
Thirdly, The continuance of all our sensa-
tions being thus established it follows, that
external objects impress vibratory motions
on the medullary substance of the nerves
and brain ; because no motion, besides a
vibratory one, can reside in any part for a
moment of time.

This is the chain of proof, in which the
first link is strong, being confirmed by ex-
perience ; the second is very weak ; and the
third still weaker. For other kinds of mo-
tion, besides that of vibration, may have
some continuance — such as rotation, bending
or unbending of a spring, and perhaps others
v\ Inch we are unacquainted with ; nor do
we know whether it is motion that is pro-
duced in the nerves — it may be pressure,
attraction, repulsion, or something we do
not know. This, indeed, is the common
refuge of all hypotheses, [87] that wc know



no other way in which the phenomena may
be produced, and, therefore, they must be
produced in this way. There is, therefore,
no proof of vibrations in the infinitesimal
particles of the brain and nerves.

It may be thought that the existence of
an elastic vibrating aether stands on a firmer
foundation, having the authority of Sir
Isaac Newton. But it ought to be observed
that, although this great man had formed
conjectures about this aether near fifty
years before he died, and had it in his eye
during that long space as a subject of in-
quiry, yet it does not appear that he ever
found any convincing proof of its existence,
but considered it to the last as a question
whether there be such an aether or not.
In the premonition to the reader, prefixed
to the second edition of his " Optics,"
anno 1717, he expresses himself thus with
regard to it : — " Lest any one should think
that I place gravity among the essential
properties of bodies, I have subjoined one
question concerning its cause ; a question,
1 say, for I do not hold it as a thing estab-
lished." If, therefore, we regard the
authority of Sir Isaac Newton, we ought
to hold the existence of such an tether as a
matter not established by proof, but to be
examined into by experiments ; and I have
never heard that, since his time, any new
evidence has been found of its existence.

" But," says Dr Hartley, " supposing
the existence of the sether, and of its pro-
perties, to be destitute of all direct evidence,
still, if it serves to account for a great
variety of phaenomena, it will have an in-
direct evidence in its favour by this means."
There never was an hypothesis invented by
an ingenious man which has not this evi-
dence in its favour. The vortices of Des
Cartes, the sylphs and gnomes of Mr Pope,
serve to account for a great variety of
phaenomena.

When a man has, with labour and in-
genuity, wrought up an hypothesis into a
system, he contracts a fondness for it,
which is apt [88] to warp the best judgment.
This, I humbly think, appears remarkably
in Dr Hartley. In his preface, he declares
his approbation of the method of philoso-
phising recommended and followed by Sir
Isaac Newton ; but, having first deviated
from this method in his practice, he is
brought at last to justify this deviation in
theory, and to bring arguments in defence
of a method diametrically opposite to it.
" Wo admit," says he, " the key of a cypher
to be a true one when it explains the cypher
completely." I answer, To find the key
requires an understanding equal or supe-
rior to that which made the cypher. This
instance, therefore, will then be in point,
when he who attempts to decypher the
works of Nature by an hypothesis, has an

[8G-88"]



ohap. m.] HYPOTHESES CONCERNING THE NERVES, &c. 251



understanding equal or superior to that
which made them. The votaries of hypo-
theses have often been challenged to shew
one useful discovery in the works of Nature
that was ever made in that way. If in-
stances of this kind could be produced, we
ought to conclude that Lord Bacon and
Sir Isaac Newton have done great disser-
vice to philosophy by what they have said
against hypotheses. But, if no such in-
stance can be produced, we must conclude,
with those great men, that every system
which pretends to account for the phseno-
mena of Nature by hypotheses or conjecture,
is spurious and illegitimate, and serves only
to flatter the pride of man with a vain con-
ceit of knowledge which he has not attained.

The author tells us, "that any hypo-
thesis that has so much plausibility as to
explain a considerable number of facts, helps
us to digest these facts in proper order, to
bring new ones to light, and to make e.r-
perimenta crucis for the sake of future
inquirers."

Let hypotheses be put to any of these
uses as far as they can serve. Let them
suggest experiments, ordirect our inquiries ;
but let just induction alone govern our
belief.

" The rule of false affords an obvious and
strong instance of the possibilityof being led,
with precision and certainty, to a [89] true
conclusion from a false position. And it is
of the very essence of algebra to proceed in
the way of supposition."

This is true ; but, when brought to jus-
tify the accounting for natural phaanomena
by hypotheses, is foreign to the purpose.
When an unknown number, or any un-
known quantity, is sought, which must have
certain conditions, it may be found in a
scientific manner by the rule of false, or
by an algebraical analysis ; and, when
found, may be synthetically demonstrated
to be the number or the quantity sought,
by its answering all the conditions required.
But it is one thing to find a quantity which
shall have certain conditions ; it is a very
different thing to find out the laws by which
it pleases God to govern the world and
produce the pliEenomena which fall under



Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 56 of 114)