Thomas Reid.

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

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not to self-motion only, but to all change of
place. Absolute, or real rest, is opposed to
real motion ; and relative rest—that is, rest
with relation to such a body that is supposed
at rest, is opposed to relative motion with
respect to the same body. But a body may
be relatively at rest, and, at the same time,
really in motion. Thus, a house rests upon
its foundation for ages ; but this rest is
relative with respect to the earth. For it
has gone round the earth's axis every day,
and round the sun every year.

The distinction your Lordship makes be-
tween moving and being moved, belongs not
to physics, but to metaphysics. In physics,
you may use the active or the passive verb
as you like best. The reason is, that in
physics we seek not the efficient causes of
phenomena, but only the rules or laws by
which they are regulated. We know, that
a body once put in motion, continues to
move, or, if you please, to be moved, until
some force is applied to stop or retard it.
But, whether this phenomenon is produced
by some real activity in the body itself, or
by the efficiency of some external cause ;
or whether it requires no efficiency at all to
continue in the state into which it is put, is,
perhaps, difficult to determine; and is a
question that belongs not to physics, but to
metaphysics.

Some divines and philosophers have
maintained, that- the preservation of a
created being in existence, is a, continued
act of creation ; and that annihilation is
nothing but the suspending that exertion of
the Creator by which the being was upheld
in existence.

Analogous to this, I think, is the opinion,
that the continuance of motion in a bodv
requires a continued exertion of that active
force which put it into the state of motion.
I am rather inclined to the contrary of both
these opinions, and disposed to think that
continuance of existence, and continuance
of motion in a body, requires no active
cause ; and that it is only a change of state,
and not a continuance of the present state,
that requires active power. But, I suspect,
both questions are rather beyond the reach
of the human faculties. However, they
belong not to the province of physics, but
to that of metaphysics.

I wish I may be intelligible, and that T
do not oppress your Lordship with the gar-
rulity of old age. I find myself, indeed,
growing old, and have no right to plead ex-
emption from the infirniit'es of that stage of
life. For that reason, I have made choice
of an assistant in my office. Yesterday, the
college, at my desire, made choice of Mr
Archibald Arthur, preacher, to bemvassist-



ant and successor.* I think I have done
good service to the college by this, and pro-
cured some leisure to myself, though with a
reduction of my finances. May your Lord-
ship live long and happy. — Yours,

Tho. Reid.



VI.

ON CONJECTURES AND HYPOTHESES IN PHI-
LOSOPHY. — CAUSE WHAT IN RELATION
TO PHYSICS. — DIFFERENT PROVINCES OP
PHYSICAL AND OF METAPHYSICAL SCIENCE.

\6th December 1780.
My Lord, — 1. I am now to answer the
letter you honoured me with of 7th No-
vember. And, first, I disclaim what you
seem to impute to me — to wit, " the valuing
myself upon my ignorance of the cause of
gravity." To confess ignorance when one
is conscious of it, I take to be a sign, not
of pride, but of humility, and of that can-
dour which becomes a philosopher ; and so
I meant it.

2. Your Lordship thinks, " That never
to trust to hypotheses and conjectures about
the works of God, and being persuaded that
they are more like to be false than true, is
a discouraging doctrine, and damps the
spirit of inquiry," &c. Now, my Lord, I
have, ever since I was acquainted with
Bacon and Newton, thought that this doc-
trine is the very key to natural philosophy,
and the touchstone by which everything
that is legitimate and solid in that science,
is to be distinguished from what is spurious
and hollow ; and I cau hardly think, that
we can differ in so capital a point, if we
understood each other's meaning.

3. I would discourage no man from con-
jecturing, only I wish him not to take his
conjectures for knowledge, or to expect that
others should do so. Conjecturing may be
a useful step even in natural philosophy.
Thus, attending to such a phenomenon, I
conjecture that it may be owing to such a
cause. This may lead me to make the ex-
periments or observations, proper for dis-
covering whether that is really the cause or
not : and if I can discover, either that it
is or is not, my knowledge is improved;
and my conjecture was a step to that im-



* Mr Arthur, a man of learning, abilities, atid
worth, filled the Chair of Moral Philosophy in the
University of Glasgow for fifteen ye.irs, wilh a repu-
tation which did not disappoint the hopes of his
respectai.le prrdece*sor. A volume of* Discourses
on Theological and l.ilerarv Subjects," which give a
very favourable idea of his Ulents, the justness of
his tasic, and the rectiiude of his moral and religious
principles, has been published, since his death, by
I'lofessor Richardson of the same college — a gentle-
man distinguished in the literarv world, and who hat
done honour to the memory ol his friend, by an inter-
esting sketrh of his life and character, .-unjoined to
these discourses — Loan Wooimonsl UU-.



LETTERS TO LORD KAMES.



57



provement. But, while I rest in my con-
jecture, my judgment remains in suspense,
and all I can say is, it may be so, and it
may be otherwise.

4. A cause that is conjectured ought to
be such, that, if it really does exist, it will
produce the effect. If it have not this
quality, it hardly deserves the name of a
conjecture. Supposing it to have this
quality, the question remains — Whether
does it exist or not ? And this, being a
question of fact, is to be tried by positive
evidence. Thus, Des Cartes conjectured,
that the planets are carried round the sun
in a vortex of subtile matter. The cause
here assigned is si.fficient to produce the
effect. It may, therefore, be entitled to
the name of a conjecture. But where is
the evidence of the existence of such a vor-
tex ? If there be no evidence for it, even
though there were none against it, it is a
conjecture only, and ought to have no
admittance into chaste natural philosophy.

5. All investigation of what we call the
causes of natural phenomena may be reduced
to this syllogism — If such a cause exists, it
will produce such a phenomenon : but that
cause does exist : Therefore, &c. The first
proposition is merely hypothetical. And a
man in his closet, without consulting nature,
may make a thousand such propositions,
and connect them into a system ; but this
is only a system of hypotheses, conjectures,
or theories ; and there cannot be one con-
clusion in natural philosophy drawn from it,
until he consults nature, and discovers
whether the causes he has conjectured
do really exist. As far as he can shew that
they do, he makes a real progress in the
knowledge of nature, and not a step further.
I hope in all this your Lordship will agree
with me. But it remains (o be considered
how the second proposition of the syllogism
is to be proved — to wit, that such a cause
does really exist. Will nothing satisfy
here but demonstration ?

6. I am so far from thinking so, my Lord,
that I am persuaded we never can have
demonstration in this case. All that we
know of til e material world, m ust be grounded
on the testimony of our senses. Our senses
testify particular facts only : from these we
collect, by induction, general facts, which
we call laws of nature, or natural causes.
Thus, ascending by a just and cautious in-
duction, from what is less to what is more
general, we discover, as far as we are able,
natural causes, or laws of nature. This is
the analytical part of natural philosophy.
The synthetical part takes for granted, as
principles, the causes discovered by induc-
tion, and from these explaius or accounts
for the phenomena which result from them.
This analysis and synthesis make up. the
whole theory of natural philosophy. The



practical part consists in applying the laws
of nature to produce effects useful in life.

7. From this view of natural philosophy,
which I have learned from Newton, your
Lordship will perceive that no man who
understands it will pretend to demon-
strate any of its principles. Nay, the most
certain and best established of them may,
for anything we know, admit of exceptions.
For instance, there is no principle in natu-
ral philosophy better established than the
universal gravitation of matter. But, can
this be demonstrated ? By no means.
What is the evidence of it, then ? It is
collected by induction, partly from our
daily experience, and from the experience
of all nations, in all ages, in all places of
earth, sea, and air, which we can reach ;
and partly from the observations and expe-
riments of philosophers, which shew that
even air and smoke, and every body upon
which experiments have been made, gravi-
tate precisely in proportion to the quantity
of matter ; that the sea and earth gravitate
towards the moon, and the moon towards
them ; that the planets and comets gravi-
tate towards the sun, and towards one
another, and the sun towards them. This
is the sum of evidence ; and it is as differ-
ent from demonstration, on the one hand,
as from conjecture on the other. It is the
same kind of evidence which we have, that
fire will burn and water drown, that bread will
nourish and arsenic poison, which, I think,
would not properly be called conjecture.

8. It is proper here to explain what is
meant by the cause of a phenomenon, when
that word is used in natural philosophy.
The word cause is so ambiguous, that I fear
many mistake its meaning, and take it to
mean the efficient cause, which I think it
never does in this science.

9. By the cause of a phenomenon, nothing
is meant but the law of nature, of which
that phenomenon is an instance, or a neces-
sary consequence- The cause of a body's
falling to the ground is its gravity. But
gravity is not an efficient cause, but a gene-
ral law, that obtains in nature, of which
law the fall of this body is a particular in-
stance. The cause why a body projected
moves in a parabola, is, that this motion is
the necessary consequence of the projectile
force and gravity united. But these are
not efficient causes ; they are only laws of
nature. In natural philosophy, therefore,
we seek only the general laws, according to
which nature works, and these we call the
causes of what is done according to them.
But such laws cannot be the efficient cause
of anything. They are only the rule accord-
ing to which the efficient cause operates.

10. A natural philosopher may search
after the cause of a law of nature ; but
this means r.o more than searching for a



58



CORRESPONDENCE OF DR REID.



more general law, which includes that par-
ticular law, and perhaps many others under
it. This was all that Newton aimed at by
his ether. He thought it possible, that, if
there was such an ether, the gravitation of
bodies, the reflection and refraction of the
rays of light, and many other laws of nature,
might be the necessary consequences of the
elasticity and repelling force of the ether.
But, supposing this ether to exist, its elas-
ticity and repelling force must be considered
as a law of nature ; and the efficient cause
of this elasticity would still have been latent
11. Efficient causes, properly so called,
are not within the sphere of natural philo-
sophy. Its business is, from particular
facts in the material world, to collect, by
just induction, the laws that are general,
and from these the more general, as far as
we can go. And when this is done, natural
philosophy has no more to do. It exhibits
to our view the grand machine of the mate-
rial world, analysed, as it were, and taken
to pieces, with the connexions and depend-
encies of its several parts, and the laws of
its several movements. It belongs to
another branch of philosophy to consider
whether this machine is the work of chance
or of design, and whether of good or of bad
design ; whether there is not an intelligent
first Mover who contrived the whole, and
gives motion to the whole, according to the
laws which the natural philosopher has dis-
covered, or, perhaps, according to laws
still more general, of which we can only
discover some branches; and whether he
does these things by his own hand, so to
speak, or employs subordinate efficient
causes to execute his purposes. These are
very noble and important inquiries, but they
do not belong to natural philosophy ; nor
can we proceed in them in the way of ex-
periment and induction, the only instru-
ments the natural philosopher uses in his
researches.

12. "Whether you call this branch of
philosophy Natural Theology or Meta-
physics, I care not ; but I think it ought
not to be confounded with Natural Philo-
sophy; and neither of them with Mathe-
matics. Let the mathematician demon-
strate the relation of abstract quantity ; the
natural philosopher investigate the laws of
the material system by induction ; and the
metaphysician, the final causes, and the
efficient causes of what we see and what
natural philosophy discovers in the world
we live in.

13. As to final causes, they stare us in
the face wherever we cast our eyes. I can
no more doubt whether the eye was made
for the purpose of seeing, and the ear of
hearing, than I can doubt of a mathema-
tical axiom ; yet the evidence is neither
mathematical demonstration, nor is it in-



duction. In a word, final causes, good final
causes, are seen plainly everywhere : in the
heavens and in the earth ; in the constitu-
tion of every animal, and in our own consti-
tution of body and of mind ; and they are
most worthy of observation, and have a
charm in them that delights the soul.

14. As to Efficient Causes, I am afraid
our faculties carry us but a very little way,
and almost only to general conclusions. I
hold it to be self-evident, that every pro-
duction, and every change in nature, must
have an efficient cause that has powei
to produce the effect ; and that an effect
which has the most manifest marks of in-
telligence, wisdom, and goodness, must have
an intelligent, wise, and good efficient cause.
From these, and some such self-evident
truths, we may discover the principles of
natural theology, and that the Deity is the
first efficient cause of all nature. But how
far he operates in nature immediately, or
how far by the ministry of subordinate effi-
cient causes, to which he has given power
adequate to the task committed to them, I
am afraid our reason is not able to discover,
and we can do little else than conjecture.
We are led by nature to believe ourselves
to be the efficient causes of our own volun-
tary actions ; and, from analogy, we judge
the same of other intelligent beings. But
with regard to the works of nature, I can-
not recollect a single instance wherein I can
say, with any degree of assurance, that such
a thing is the efficient cause of such a phe-
nomenon of nature.

15. Malebranche, and many of the Car-
tesians, ascribed all to the immediate oper-
ation of the Deity, except the determinations
of the will of free agents. Leibnitz, and all
his followers, maintain, that God finished
his work at the creation, having endowed
every creature and every individual particle
of matter, with such internal powers as
necessarily produce all its actions, motions,
and changes, to the end of time- Others
have held, that various intelligent beings,
appointed by the Deity to their several
departments, are the efficient causes of the
various operations of nature. Others, that
there are beings endowed with power with-
out intelligence, which are the efficient
causes in nature's operations ; and they
have given them the name of Plastic
Fowers, or Plastic Natures. A late author
of your Lordship's acquaintance,* has given
it as ancient metaphysics, That every body
in the universe is compounded of two sub-
stances united— to wit, an immaterial mind
or soul, which, in the inanimate creation,
has the power of motion without thought ;
and of inert matter as the other part. The
celebrated Dr Priestley maintains, that



• Lord Monboddo H.



LETTERS TO LORD KAMES.



59



matter, properly organized, has not only the
power of motion, but of thought and intel-
ligence ; and that a man is only a piece of
matter properly organized.

16. Of all these systems about the effi-
cient causes of the phenomena of nature,
there is not one that, in my opinion, can be
either proved or refuted from the principles
of natural philosophy. They belong to
metaphysics, and affect not natural philo-
sophy, whether they be true or false. Some
of them, I think, may be refuted upon meta-
physical principles ; but, as to the others, I
can neither see such evidence for them or
against them as determines my belief.
They seem to me to be conjectures only
about matters where we have not evidence ;
and, therefore, I must confess my ignor-
ance.

17. As to the point which gave occasion
to this long detail, Whether there is reason
to think that matter gravitates by an in-
herent power, and is the efficient cause of
its own gravitation, I say, first, This is a
metaphysical question, which concerns not
natural philosophy, and can neither be
proved nor refuted by auy principle in that
science. Natural philosophy informs us,
that matter gravitates according to a certain
law ; and it says no more. Whether mat-
ter be active or passive in gravitation, can-
not be determined by any experiment I can
think of. If it should be said that we ought
to conclude it to Tse active, because we per-
ceive no external cause of its gravitation,
this argument, I fear, will go too far. Be-
sides it is very weak, amounting only to
this : I do not perceive such a thing, there-
f ire it does not exist.

18. I never could see good reason to
believe that matter has any active power at
all. And, indeed, if it were evident that it
has one,! think there could be no good reason
assigned for not allowing it others. Your
Lordship speaks of the power of resisting
motion, and some others, as acknowledged
active powers inherent in matter. As to
the resistance to motion, and the continu-
ance in motion, I never could satisfy my-
self whether these are not the necessary
consequences of matter being inactive. If
they imply activity, that may lie in some
other cause.

1!). I am not able to form any distinct
conception of active power but such as I find
in myself. I can only exert my active
power by will, which supposes thought. It
seems to me, that, if I was not conscious of
activity in myself, I could never, from things
I see about me, have had the conception or
idea of active power. I see a succession of
changes, but I fee not the power, that is,
the efficient cause of them ; but, having got
the notion of active power, from the con-
sciousness of my own activity, and finding



it a first principle, that every production
requires active power, I can reason about
an active power of that kind I am acquainted
with — that is, such as supposes thought and
choice, and is exerted by will. But, if there
is anything in an unthinking inanimate being
that can be called active power, I know not
what it is, and cannot reason about it.

20. If you conceive that the activity of
matter is directed . by thought and will in
matter, every particle of matter must know
the situation and distance of every other
particle within the planetary system ; but
this, I am apt to think, is not your Lord-
ship's opinion.

21. I must therefore conclude, that this
active power is guided in all its operations
by some intelligent Being, who knows both
the law of gravitation, and the distance and
situation of every particle of matter with
regard to every other particle, in all the
changes that happen in the material world.
I can only conceive two ways in which this
particle .pf matter can be guided, in all the
exertions of its active power, by an intelli-
gent Being. Either it was formed, in its
creation, upon a foreknowledge of all the
situations it shall ever be in with respect to
other particles, and had such an internal
structure given it, as necessarily produces,
in succession, all the motions, and tend-
encies to motion, it shall ever exert. This
would make every particle of matter a ma-
chine or automaton, and every particle of a
different structure from every other particle
in the universe. This is indeed the opinion
of Leibnitz ; but I am not prejudiced against
it upon that account ; I only wished to know
whether your Lordship adopted it or not.
Another way, and the only other way, in
which I can conceive the active power of a
particle of matter, guided by an intelligent
Being, is by a continual influence exerted
according to its situation and the situation
of other particles. In this case, the particle
would be guided as a horse is by his rider ;
and I think it would be improper to ascribe
to it the power of gravitation. It has only
the power of obeying its guide. Whether
your Lordship chooses the first or the last
in this alternative, I should be glad to
know ; or whether you can think of a third
way better than either.

22. I will not add to the length of so
immoderately long a letter by criticising
upon the passages you quote from Newton.
I have a great regard for his j udgment ; but
where he differs from me, I think him
wrong.

The idea of natural philosophy I have
given in this letter, I think I had from him.
If in scholia and queries he gives a range to
his thoughts, and sometimes enters the
regions of natural theology and metaphysics,
this I think is very allowab'c, and is not to



60



CORRESPONDENCE OF DR REID.



be considered a part of his physics, which
are contained in his propositions and corol-
laries. Even his queries and conjectures
are valuable ; but I think he never intended
that they should be taken for granted, but
made the subject of inquiry.

Tho. Reid.



VII.

LAWS OF MOTION PRESSURE OF FLUIDS.

January 25, 1781.

My Lord, — To what cause is it owing
that I differ so much from your Lordship
in Fhysics, when we differ so little in Meta-
physics ? I am at a loss to account for this
phenomenon. Whether is it owing to our
having different conceptions to the same
words ? — or, as I rather think it is, to
your being dissatisfied with the three gene-
ral laws of motion ? Without them I know
not indeed how to reason in physics. Ar-
chimedes reasoned from them both in me-
chanics and hydrostatics. Galileo, Huy-
gens, Wren, Wallis, Mariotte, and many
others, reasoned from them, without ob-
serving that they did so.

I have not indeed any scruples about the
principles of hydrostatics. They seem to
me to be the necessary consequences of the
definition of a fluid, the three laws of motion,
and the law of gravitation ; and, therefore,
I cannot assent to your Lordship's reason-
ing, either about the pressure of fluids, or
about the suspension of the mercury in the
barometer.

As to the first, the experiments which shew
that fluids do, in fact, press undequaque,
are so numerous, and so well known to your
Lordship, that I apprehend it is not the fact
you question, but the cause. You think
that gravity is not the cause. Why ? Be-
cause gravity gives to every part of the fluid
a tendency downwards only ; and what is
true of every part, is true of the whole :
therefore, the whole has no other tendency
but downward. This argument is specious,
but there is a fallacy in it. If the parts did
not act upon one another, and counteract
one another, the argument would be good ;
but the parts are so connected, that one
cannot go down but auother must go up,
and, therefore, that very gravity which
presses down one part presses up another :
so that every part is pressed down by its
own gravity, and pressed up, at the same
time, by the gravity of other parts ; and
the contrary pressures being equal, it re-
mains at rest.

This may be illustrated by a balance
equilibrating by equal weights in both scales.
1 say each arm of the balance is equally
pressed upwards and downwards at the same



time, and from that cause is at rest ; although
the tendency of the weights, in each of the
scales, is downwards only. I prove it a pos-
teriori ; because the arm of a balance being
moveable by the least force, if it was pressed
in oue direction only, it would move in that
direction : but it does not move. I prove



Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 15 of 114)