bodies,--which constitutes one of the two great departments of physical
investigation,--we found to arise from the imperfection of our senses,
that are not sufficiently acute to discover, of themselves, the
component parts of the masses, which nature everywhere presents to us.
We are thus obliged to form to ourselves an _art of analysis_, merely
that we may perceive what is constantly before our eyes, in the same
manner, as we are obliged to have recourse to the contrivances of the
optician, to perceive stars and planets, that are incessantly shedding
on us their light.
There is, indeed, something truly worthy of our astonishment, in
the sort of knowledge of the qualities of matter, which, with our
very imperfect senses, we are still able to attain. What we conceive
ourselves to know is an aggregate of many bodies, of each of which,
individually, we may be said, in the strictest sense of the term, to
be absolutely ignorant; and yet the aggregate, which we know, has no
real existence, but as that very multitude of bodies, of which we are
ignorant. When water was regarded as a _simple substance_, every one
who looked upon a lake or river, conceived that he knew as well what
the liquid was which flowed in it, as the chemist, who now considers it
as compound; and the chemist, who has learned to regard it as compound,
is perhaps as ignorant of the true nature of the separate bodies that
exist in it, as those who formerly regarded it as simple; since one
additional discovery may prove the very elements, which he now regards
as the ultimate constituents of water, to be truly compounded of other
elements, still more minute, and now altogether unknown to him.
That our only knowledge of matter should be of a multitude of
bodies, of the nature of each of which, individually, we are in
absolute ignorance, may seem, at first sight, to justify many of the
most extravagant doubts of the sceptic: and yet there is really no
ground for such scepticism, since, though the _coexisting_ bodies be
_separately_ unknown, the effect, which they produce when coexisting in
the circumstances observed by us, is not the less certain and definite;
and it is this joint effect of the whole, thus certain and definite,
which is the true object of our knowledge; not the uncertain effect,
which the minuter elements might produce, if they existed alone. The
same aggregates, whatever their elementary nature may be, operate on
our senses, as often as they recur, in the same manner; the unknown
elements which constitute an oak, or a tower, or the ivy that clings
around it, exciting in the mind those particular sensations, to the
external causes of which we continue to give the name of _oak_ or
_tower_ or _ivy_; and exciting these, as precisely and uniformly, as
if we were acquainted with each minute element of the objects without.
Our knowledge of nature must in this way, indeed, be confined to the
_mixed effects_ of the masses which it exhibits; but it is not on that
account less valuable, nor less sure; for to the certainty of this
limited knowledge all which is necessary is uniformity of the mixed
effects, whatever their unknown coexisting causes may be. It is with
_masses_ only, not with _elements_ that we are concerned, in all the
important purposes of life; and the provident wisdom of the Author of
Nature, therefore, has in this as in every other case, adapted our
powers to our necessities,--giving to all mankind the knowledge, that
is requisite for the purposes which all mankind must equally have in
view, and leaving to a few philosophic inquirers, the curiosity of
discovering what the substances around us truly are in their elementary
state, and the means of making continual progress, in this never-ending
analysis.
Such then is the nature of _one_ of the views, in which physical
inquiry may be directed to the discovery of elements, that are existing
together, at the same moment. But is not this species of inquiry, it
may be asked, peculiar to _matter_, or may it also be extended to
_mind_? It is easy to conceive that, if matter always have extension,
and therefore necessarily be composed of parts, an inquiry into its
composition may form an important part of physical investigation;
but this sort of inquiry will seem to you altogether inadmissible in
the _philosophy of mind_, since the mind is not composed of parts
that coexist, but is simple and indivisible. If, indeed, the term
composition, in this application of it, be understood strictly in the
same sense as when applied to matter, it is very evident, that there
can be no inquiry into the composition of thoughts and feelings, since
every thought and feeling is as simple and indivisible as the mind
itself; being, in truth, nothing more than the mind itself existing at
a certain moment in a certain state; and yet, in consequence of some
very wonderful laws, which regulate the successions of our mental
phenomena, the science of mind is, in all its most important respects,
a science of analysis, or at least a science which exhibits to our
contemplation the same results as if it were strictly analytical; and
we inquire into the separate ideas or other feelings, involved in
one complex thought or emotion, very nearly as we inquire into the
corpuscular elements, that coexist in one seemingly continuous mass.
The nature of this very wonderful application of analysis, or at least
of a process which is virtually the same as analysis, to a substance,
that is necessarily at all times simple and indivisible, will, however,
be better understood by you, after we have turned our attention to
the other general division of physical inquiry, which is still to be
considered by us. I need not I hope, repeat, after the remarks which I
made in my last Lecture, that, in leading your thoughts, for so long
a time, to the subject of general science, I have had constantly in
view its application to the phenomena of our own department of it, and
that we are truly learning to study _mind_ with accuracy, when we are
learning what it is, which is to be studied in the great system of
things. There can be no question at least, that he who has erroneous
notions of the objects of physical investigation in the material
universe, will be very likely also to err, or rather cannot fail to
err, in his notions of the objects of physical investigation, as it
relates to mind.
I proceed, then, to consider, what it is which we truly have in
view, when we direct our inquiry, not to the mere composition of
objects existing continuously in _space_, but to the succession of
changes which they exhibit in _time_,--to their susceptibility of
being affected by other substances, or their power of affecting
other substances. The inquiry, as you must perceive, involves the
consideration of some words about which a peculiar mystery has been
very generally supposed to hang--_causation_, _power_, _connexion_ of
events. But we shall perhaps find that what is supposed so peculiarly
mysterious in them, is not in the very simple notions themselves, but
in the misconceptions of those who have treated of them.
It is not in this case, as in the former department of physical
investigation, the mere imperfection of our senses, that produces the
necessity of inquiry. Matter, as existing in space, is wholly before
us, and all which is necessary for perfect knowledge of it, in this
respect, is greater delicacy of our perceptive organs, that we may
distinguish every element of the seemingly continuous mass. To know the
mere _composition_ of a substance, is to know only what is actually
present at the very moment, which we may imagine senses of the highest
perfection to be capable of instantly perceiving; but to know all the
_susceptibilities_ and _powers_ of a substance, the various modes in
which it may affect or be affected by every other, is to know it, not
merely as it exists before us in the particular circumstances of any
one moment, but as it _might_ have existed, or _may_ exist, in all
possible circumstances of combination,--which our senses, that are
necessarily confined to the circumstances of the present moment, never
could teach us, even though they were able to distinguish every atom of
the minutest mass.
If, indeed, there were any thing, in the mere appearance of a body,
which could enable us to predict the changes that would take place
in it, when brought into every possible variety of situation, with
respect to other bodies, or the changes which it would then produce in
those other bodies, the two views, into which I have divided physical
inquiry, would coincide exactly; so that to know the _continuous
elements_ of any substance, would be to know, at the same time, its
_susceptibilities_ and _powers_. But there is nothing, in the mere
sensible qualities of bodies, considered separately, that can give
us even the slightest intimation of the changes, which, _in new
circumstances of union_, they might reciprocally suffer or produce.
Who could infer, from the _similar_ appearance of a lump of sugar and
a lump of calcareous spar, that the one would be soluble in water, and
the other remain unmelted; or, from the _different_ aspect of gunpowder
and snow, that a spark would be extinguished, if it fell upon the one,
and, if it fell upon the other, would excite an explosion that would
be almost irresistable? But for experience, we should be altogether
incapable of predicting any such effects, from either of the objects
compared; or, if we did know, that the peculiar susceptibility belonged
to one of the two, and not the other, we might as readily suppose, that
calcareous spar would melt in water as sugar, and as readily, that
_snow_ as that _gunpowder_ would detonate, by the contact of a spark.
It is _experience_ alone, which teaches us that these effects ever take
place, and that they take place, not in all substances, but only in
some particular substances.
It has, indeed, been supposed by many ingenious philosophers, that,
if we were acquainted with what they term the _intimate structure_
of bodies, we should then see, not merely _what_ corpuscular changes
take place in them, but _why_ these changes take place in them; and
should thus be able to predict, before experience, the effects which
they would reciprocally produce. “I doubt not,” says Locke, “but
if we could discover the figure, size, texture, and motion of the
minute constituent parts of any two bodies, we should know without
trial several of their operations one upon another, as we do now the
properties of a square or a triangle. Did we know the mechanical
affections of the particles of rhubarb, hemlock, opium, and a man; as a
watch-maker does those of a watch, whereby it performs its operations,
and of a file, which by rubbing on them will alter the figure of any
of the wheels; we should be able to tell before-hand, that rhubarb
will purge, hemlock kill, and opium make a man sleep; as well as a
watch-maker can, that a little piece of paper laid on the balance will
keep the watch from going, till it be removed; or that, some small part
of it being rubbed by a file, the machine would quite lose its motion,
and the watch go no more. The dissolving of silver in aquafortis, and
gold in aqua regia, and not _vice versa_, would be then perhaps no more
difficult to know, than it is to a smith to understand why the turning
of one key will open a lock, and not the turning of another. But
while we are destitute of senses acute enough to discover the minute
particles of bodies, and to give us ideas of the mechanical affections,
we must be content to be ignorant of their properties and ways of
operation; nor can we be assured about them any farther, than some few
trials we make are able to reach. But whether they will succeed again
another time, we cannot be certain. This hinders our certain knowledge
of universal truths concerning natural bodies: and our reason carries
us herein very little beyond particular matter of fact.
“And therefore I am apt to doubt, that how far soever human industry
may advance useful and experimental philosophy in physical things,
scientifical will still be out of our reach; because we want perfect
and adequate ideas of those very bodies which are nearest to us, and
most under our command. Those which we have ranked into classes under
names, and we think ourselves best acquainted with, we have but very
imperfect and incomplete ideas of. Distinct ideas of the several sorts
of bodies that fall under the examination of our senses perhaps we
may have; but adequate ideas, I suspect, we have not of any one among
them. And though the former of these will serve us for common use
and discourse, yet while we want the latter, we are not capable of
scientifical knowledge; nor shall ever be able to discover general,
instructive, unquestionable truths concerning them. Certainty and
demonstration are things we must not, in these matters, pretend to. By
the colour, figure, taste, and smell, and other sensible qualities, we
have as clear and distinct ideas of sage and hemlock, as we have of a
circle and a triangle; but having no ideas of the particular primary
qualities of the minute parts of either of these plants, nor of other
bodies which we would apply them to, we cannot tell what effects they
will produce; nor when we see those effects, can we so much as guess,
much less know, their manner of production. Thus having no ideas of the
particular mechanical affections of the minute parts of bodies that
are within our view and reach, we are ignorant of their constitutions,
powers, and operations: and of bodies more remote we are yet more
ignorant, not knowing so much as their very outward shapes, or the
sensible and grosser parts of their constitutions.”[25]
The fallacy of the reasoning of this very eminent philosopher consists
partly, in the present case, in a sort of _petitio principii_, or,
at least, a false assumption that is involved in the very phrase
_mechanical affections_, and in all the mechanical illustrations
adduced. If rhubarb purge, and hemlock kill, by qualities that can be
said to be mechanical, and if these qualities be PERMANENT, there can
be no question, that to know accurately the _mechanical qualities_ of
these substances, in relation to the human body, would be to know,
that rhubarb must purge, and hemlock kill, as much as to know the
mechanism of a watch would be to know, that the watch must stop, if
a small part of it were rubbed by a file. But the inquiry is still
left, whether it be thus, by the mere principles of mechanical action,
that rhubarb and hemlock produce their peculiar effects on the animal
system, and that silver is dissolved in aqua fortis, and gold in aqua
regia; and, if there be no reason whatever to suppose this, we must
then surely admit that the prophecy would still be beyond our power,
though we were acquainted with “the figure, size, texture, and motion,
of the minute constituent parts” of the different bodies. In the same
manner, as, in the mechanical division of a substance, we must still
come to other substances capable of further division, so, though we
could reduce all the changes that appear to be wrought in the great
masses around us, to the changes wrought in their minute parts, we must
still come to certain ultimate changes as inexplicable as those which
we see at present. It is as difficult to predict, without experience,
the motion of one atom to or from another atom, as the motion of one
mass of atoms to or from another mass of atoms. That the globe of the
earth should tend towards the sun, which is at so great a distance from
it, and should thus be every moment arrested within that orbit, from
which, if there were no such deflecting force, it would every moment
have a tendency to escape by flying off in a straight line, is, indeed,
most wonderful. But precisely the same laws which operate on the whole
globe of the earth, operate on every particle of which the earth is
composed,--since the earth itself is only these separate particles
under another name; and if it be wonderful that all of these should
have a tendency to approach the sun, it must be equally wonderful,
that each minute constituent particle should tend individually,
though, to use Mr Locke's words, we were accurately acquainted with
the “figure, size, texture, and motion of each.” The same original
mystery of gravitation, then, would remain, though our senses enabled
us to discover every gravitating particle in the intimate structure of
the gravitating mass. By knowing the intimate structure of bodies, we
should indeed, know _what_ were their elements mutually affected, but
not _why_ these elements were mutually affected, or were affected in
_one_ way rather than in _another_.
The chief error of Mr Locke, in this respect, evidently consisted,
as I have said, in his assumption of the very thing to be proved, by
taking for granted, that all the changes of bodies are the effects
of their immediate contact and impulse, and of a kind, therefore,
which may be termed strictly mechanical,--an assumption, indeed,
which harmonized with the mathematical chemistry and medicine of the
age in which he lived, but of the justness of which there is not the
slightest evidence in the general phenomena, chemical and nervous, of
which he speaks. If, instead of confining his attention to the action
of bodies in apparent contact, he had turned his thought to the great
distant agencies of nature in the motions of the planetary world, it is
scarcely possible to conceive that he should not have discovered his
mistake. In another of his works, his _Elements of Natural Philosophy_,
he has stated very justly, as a consequence of the law of gravitation,
that if the earth were the sole body in the universe, and at rest, and
the moon were suddenly created at the same distance from the earth
as at present, the earth and the moon would instantly begin to move
towards one another in a straight line. What knowledge of the “figure,
size, and texture,” of the particles of the earth could have enabled
its human inhabitants to predict this instant change? and if the
particles of gold and aqua regia, and of hemlock, rhubarb, and opium,
which, together with all the other particles of our globe, would in
the case supposed, instantly begin to move towards the moon,--can thus
attract and be attracted, in gravitation, with tendencies that are
independent of every mechanical affection, what authority can there
be for supposing, that the chemical and vital agencies of the same
particles must be mechanical, or that the one set of changes could have
been predicted _a priori_, if the other was confessedly beyond the
power of philosophic divination?
But even with regard to the mechanical affections of matter themselves,
though all the changes which take place in nature were truly reducible
to them, we should still have ultimately the same difficulty in
attempting to predict, without experience, the changes that would ensue
from them. The mechanical properties are indeed the most familiar to
our thought, because they are those which we are constantly witnessing
in the great displays of human power that are most striking to our
senses. The house, the bridge, the carriage, the vessel, every
implement which we use, and the whole wide surface of the cultivated
earth, present to us, as it were, one universal trophy of the victories
of the great mechanist, man. We cannot look back to the time when we
were ignorant of the mechanical properties of matter; but still there
was a time when they first became known to us, and became known by
experience of the motions that resulted from them. What can be simpler
than the phenomena of impulse? That a ball in motion, when it meets
another at rest, should force this to quit its place, appears now to be
something which it required no skill or experience to predict; and yet,
though our faculties were, in every respect, as vigorous as now,--if we
could imagine this most common of all phenomena to be wholly unknown
to us,--what reason should we be able to discover in the circumstances
that immediately precede the shock, for inferring the effect that
truly results, rather than any other effect whatever? Were the laws of
motion previously unknown, it would be in itself as presumable, that
the moving ball should simply stop when it reached the other, or that
it should merely rebound from it, as that the quiescent ball should
be forced by it to quit its state of rest, and move forward in the
same direction. We know, indeed, that the effect is different, but it
is because we have witnessed it that we know it; not because the laws
of motion, or any of the mechanical affections of matter whatever are
qualities that might be inferred independently of observation.
Experience, then, is necessary in every case, for discovering the
mutual tendencies of the _elements_ of bodies, as much as for
determining the reciprocal affections of the _masses_. But experience
teaches us the past only, not the future, and the object of physical
inquiry is, not the mere solitary fact of a change which has taken
place, but the similar changes which will continually take place as
often as the objects are again in the same circumstances,--not the
_phenomena_ only, but the _powers_ by which the phenomena are produced.
Why is it, then, we believe that continual similarity of the future
to the past, which constitutes, or at least is implied, in our notion
of power? A stone tends to the earth,--a stone will always tend to
the earth,--are not the same proposition; nor can the _first_ be said
to involve the _second_. It is not to experience, then, alone that we
must have recourse for the origin of the belief, but to some other
principle, which converts the simple facts of experience into a general
expectation, or confidence, that is afterwards to be physically the
guide of all our plans and actions.