OF OBSERVATION AND EXPERIMENT.
§ 1. It results from the preceding exposition, that the process of
ascertaining what consequents, in nature, are invariably connected with
what antecedents, or in other words what phenomena are related to each
other as causes and effects, is in some sort a process of analysis. That
every fact which begins to exist has a cause, and that this cause must
be found somewhere among the facts which immediately preceded the
occurrence, may be taken for certain. The whole of the present facts are
the infallible result of all past facts, and more immediately of all the
facts which existed at the moment previous. Here, then, is a great
sequence, which we know to be uniform. If the whole prior state of the
entire universe could again recur, it would again be followed by the
present state. The question is, how to resolve this complex uniformity
into the simpler uniformities which compose it, and assign to each
portion of the vast antecedent the portion of the consequent which is
attendant on it.
This operation, which we have called analytical, inasmuch as it is the
resolution of a complex whole into the component elements, is more than
a merely mental analysis. No mere contemplation of the phenomena, and
partition of them by the intellect alone, will of itself accomplish the
end we have now in view. Nevertheless, such a mental partition is an
indispensable first step. The order of nature, as perceived at a first
glance, presents at every instant a chaos followed by another chaos. We
must decompose each chaos into single facts. We must learn to see in the
chaotic antecedent a multitude of distinct antecedents, in the chaotic
consequent a multitude of distinct consequents. This, supposing it done,
will not of itself tell us on which of the antecedents each consequent
is invariably attendant. To determine that point, we must endeavour to
effect a separation of the facts from one another, not in our minds
only, but in nature. The mental analysis, however, must take place
first. And every one knows that in the mode of performing it, one
intellect differs immensely from another. It is the essence of the act
of observing; for the observer is not he who merely sees the thing which
is before his eyes, but he who sees what parts that thing is composed
of. To do this well is a rare talent. One person, from inattention, or
attending only in the wrong place, overlooks half of what he sees:
another sets down much more than he sees, confounding it with what he
imagines, or with what he infers; another takes note of the _kind_ of
all the circumstances, but being inexpert in estimating their degree,
leaves the quantity of each vague and uncertain; another sees indeed the
whole, but makes such an awkward division of it into parts, throwing
things into one mass which require to be separated, and separating
others which might more conveniently be considered as one, that the
result is much the same, sometimes even worse, than if no analysis had
been attempted at all. It would be possible to point out what qualities
of mind, and modes of mental culture, fit a person for being a good
observer: that, however, is a question not of Logic, but of the Theory
of Education, in the most enlarged sense of the term. There is not
properly an Art of Observing. There may be rules for observing. But
these, like rules for inventing, are properly instructions for the
preparation of one's own mind; for putting it into the state in which it
will be most fitted to observe, or most likely to invent. They are,
therefore, essentially rules of self-education, which is a different
thing from Logic. They do not teach how to do the thing, but how to make
ourselves capable of doing it. They are an art of strengthening the
limbs, not an art of using them.
The extent and minuteness of observation which may be requisite, and the
degree of decomposition to which it may be necessary to carry the mental
analysis, depend on the particular purpose in view. To ascertain the
state of the whole universe at any particular moment is impossible, but
would also be useless. In making chemical experiments, we do not think
it necessary to note the position of the planets; because experience has
shown, as a very superficial experience is sufficient to show, that in
such cases that circumstance is not material to the result: and,
accordingly, in the ages when men believed in the occult influences of
the heavenly bodies, it might have been unphilosophical to omit
ascertaining the precise condition of those bodies at the moment of the
experiment. As to the degree of minuteness of the mental subdivision; if
we were obliged to break down what we observe into its very simplest
elements, that is, literally into single facts, it would be difficult to
say where we should find them: we can hardly ever affirm that our
divisions of any kind have reached the ultimate unit. But this too is
fortunately unnecessary. The only object of the mental separation is to
suggest the requisite physical separation, so that we may either
accomplish it ourselves, or seek for it in nature; and we have done
enough when we have carried the subdivision as far as the point at which
we are able to see what observations or experiments we require. It is
only essential, at whatever point our mental decomposition of facts may
for the present have stopped, that we should hold ourselves ready and
able to carry it farther as occasion requires, and should not allow the
freedom of our discriminating faculty to be imprisoned by the swathes
and bands of ordinary classification; as was the case with all early
speculative inquirers, not excepting the Greeks, to whom it seldom
occurred that what was called by one abstract name might, in reality, be
several phenomena, or that there was a possibility of decomposing the
facts of the universe into any elements but those which ordinary
language already recognised.
§ 2. The different antecedents and consequents, being, then, supposed to
be, so far as the case requires, ascertained and discriminated from one
another; we are to inquire which is connected with which. In every
instance which comes under our observation, there are many antecedents
and many consequents. If those antecedents could not be severed from
one another except in thought, or if those consequents never were found
apart, it would be impossible for us to distinguish (_à posteriori_ at
least) the real laws, or to assign to any cause its effect, or to any
effect its cause. To do so, we must be able to meet with some of the
antecedents apart from the rest, and observe what follows from them; or
some of the consequents, and observe by what they are preceded. We must,
in short, follow the Baconian rule of _varying the circumstances_. This
is, indeed, only the first rule of physical inquiry, and not, as some
have thought, the sole rule; but it is the foundation of all the rest.
For the purpose of varying the circumstances, we may have recourse
(according to a distinction commonly made) either to observation or to
experiment; we may either _find_ an instance in nature, suited to our
purposes, or, by an artificial arrangement of circumstances, _make_ one.
The value of the instance depends on what it is in itself, not on the
mode in which it is obtained: its employment for the purposes of
induction depends on the same principles in the one case and in the
other; as the uses of money are the same whether it is inherited or
acquired. There is, in short, no difference in kind, no real logical
distinction, between the two processes of investigation. There are,
however, practical distinctions to which it is of considerable
importance to advert.
§ 3. The first and most obvious distinction between Observation and
Experiment is, that the latter is an immense extension of the former. It
not only enables us to produce a much greater number of variations in
the circumstances than nature spontaneously offers, but also, in
thousands of cases, to produce the precise _sort_ of variation which we
are in want of for discovering the law of the phenomenon; a service
which nature, being constructed on a quite different scheme from that of
facilitating our studies, is seldom so friendly as to bestow upon us.
For example, in order to ascertain what principle in the atmosphere
enables it to sustain life, the variation we require is that a living
animal should be immersed in each component element of the atmosphere
separately. But nature does not supply either oxygen or azote in a
separate state. We are indebted to artificial experiment for our
knowledge that it is the former, and not the latter, which supports
respiration; and for our knowledge of the very existence of the two
ingredients.
Thus far the advantage of experimentation over simple observation is
universally recognised: all are aware that it enables us to obtain
innumerable combinations of circumstances which are not to be found in
nature, and so add to nature's experiments a multitude of experiments of
our own. But there is another superiority (or, as Bacon would have
expressed it, another prerogative) of instances artificially obtained
over spontaneous instances,--of our own experiments over even the same
experiments when made by nature,--which is not of less importance, and
which is far from being felt and acknowledged in the same degree.
When we can produce a phenomenon artificially, we can take it, as it
were, home with us, and observe it in the midst of circumstances with
which in all other respects we are accurately acquainted. If we desire
to know what are the effects of the cause A, and are able to produce A
by means at our disposal, we can generally determine at our own
discretion, so far as is compatible with the nature of the phenomenon A,
the whole of the circumstances which shall be present along with it: and
thus, knowing exactly the simultaneous state of everything else which is
within the reach of A's influence, we have only to observe what
alteration is made in that state by the presence of A.
For example, by the electric machine we can produce in the midst of
known circumstances, the phenomena which nature exhibits on a grander
scale in the form of lightning and thunder. Now let any one consider
what amount of knowledge of the effects and laws of electric agency
mankind could have obtained from the mere observation of thunder-storms,
and compare it with that which they have gained, and may expect to gain,
from electrical and galvanic experiments. This example is the more
striking, now that we have reason to believe that electric action is of
all natural phenomena (except heat) the most pervading and universal,
which, therefore, it might antecedently have been supposed could stand
least in need of artificial means of production to enable it to be
studied; while the fact is so much the contrary, that without the
electric machine, the Leyden jar, and the voltaic battery, we probably
should never have suspected the existence of electricity as one of the
great agents in nature; the few electric phenomena we should have known
of would have continued to be regarded either as supernatural, or as a
sort of anomalies and eccentricities in the order of the universe.
When we have succeeded in insulating the phenomenon which is the subject
of inquiry, by placing it among known circumstances, we may produce
further variations of circumstances to any extent, and of such kinds as
we think best calculated to bring the laws of the phenomenon into a
clear light. By introducing one well-defined circumstance after another
into the experiment, we obtain assurance of the manner in which the
phenomenon behaves under an indefinite variety of possible
circumstances. Thus, chemists, after having obtained some
newly-discovered substance in a pure state, (that is, having made sure
that there is nothing present which can interfere with and modify its
agency,) introduce various other substances, one by one, to ascertain
whether it will combine with them, or decompose them, and with what
result; and also apply heat, or electricity, or pressure, to discover
what will happen to the substance under each of these circumstances.
But if, on the other hand, it is out of our power to produce the
phenomenon, and we have to seek for instances in which nature produces
it, the task before us is very different. Instead of being able to
choose what the concomitant circumstances shall be, we now have to
discover what they are; which, when we go beyond the simplest and most
accessible cases, it is next to impossible to do, with any precision and
completeness. Let us take, as an exemplification of a phenomenon which
we have no means of fabricating artificially, a human mind. Nature
produces many; but the consequence of our not being able to produce
them by art is, that in every instance in which we see a human mind
developing itself, or acting upon other things, we see it surrounded and
obscured by an indefinite multitude of unascertainable circumstances,
rendering the use of the common experimental methods almost delusive. We
may conceive to what extent this is true, if we consider, among other
things, that whenever nature produces a human mind, she produces, in
close connexion with it, a body; that is, a vast complication of
physical facts, in no two cases perhaps exactly similar, and most of
which (except the mere structure, which we can examine in a sort of
coarse way after it has ceased to act), are radically out of the reach
of our means of exploration. If, instead of a human mind, we suppose the
subject of investigation to be a human society or State, all the same
difficulties recur in a greatly augmented degree.
We have thus already come within sight of a conclusion, which the
progress of the inquiry will, I think, bring before us with the clearest
evidence: namely, that in the sciences which deal with phenomena in
which artificial experiments are impossible (as in the case of
astronomy), or in which they have a very limited range (as in mental
philosophy, social science, and even physiology), induction from direct
experience is practised at a disadvantage in most cases equivalent to
impracticability: from which it follows that the methods of those
sciences, in order to accomplish anything worthy of attainment, must be
to a great extent, if not principally, deductive. This is already known
to be the case with the first of the sciences we have mentioned,
astronomy; that it is not generally recognised as true of the others, is
probably one of the reasons why they are not in a more advanced state.
§ 4. If what is called pure observation is at so great a disadvantage,
compared with artificial experimentation, in one department of the
direct exploration of phenomena, there is another branch in which the
advantage is all on the side of the former.
Inductive inquiry having for its object to ascertain what causes are
connected with what effects, we may begin this search at either end of
the road which leads from the one point to the other: we may either
inquire into the effects of a given cause, or into the causes of a given
effect. The fact that light blackens chloride of silver might have been
discovered either by experiments on light, trying what effect it would
produce on various substances, or by observing that portions of the
chloride had repeatedly become black, and inquiring into the
circumstances. The effect of the urali poison might have become known
either by administering it to animals, or by examining how it happened
that the wounds which the Indians of Guiana inflict with their arrows
prove so uniformly mortal. Now it is manifest from the mere statement of
the examples, without any theoretical discussion, that artificial
experimentation is applicable only to the former of these modes of
investigation. We can take a cause, and try what it will produce: but we
cannot take an effect, and try what it will be produced by. We can only
watch till we see it produced, or are enabled to produce it by accident.
This would be of little importance, if it always depended on our choice
from which of the two ends of the sequence we would undertake our
inquiries. But we have seldom any option. As we can only travel from the
known to the unknown, we are obliged to commence at whichever end we are
best acquainted with. If the agent is more familiar to us than its
effects, we watch for, or contrive, instances of the agent, under such
varieties of circumstances as are open to us, and observe the result.
If, on the contrary, the conditions on which a phenomenon depends are
obscure, but the phenomenon itself familiar, we must commence our
inquiry from the effect. If we are struck with the fact that chloride of
silver has been blackened, and have no suspicion of the cause, we have
no resource but to compare instances in which the fact has chanced to
occur, until by that comparison we discover that in all those instances
the substances had been exposed to light. If we knew nothing of the
Indian arrows but their fatal effect, accident alone could turn our
attention to experiments on the urali; in the regular course of
investigation, we could only inquire, or try to observe, what had been
done to the arrows in particular instances.
Wherever, having nothing to guide us to the cause, we are obliged to set
out from the effect, and to apply the rule of varying the circumstances
to the consequents, not the antecedents, we are necessarily destitute of
the resource of artificial experimentation. We cannot, at our choice,
obtain consequents, as we can antecedents, under any set of
circumstances compatible with their nature. There are no means of
producing effects but through their causes, and by the supposition the
causes of the effect in question are not known to us. We have therefore
no expedient but to study it where it offers itself spontaneously. If
nature happens to present us with instances sufficiently varied in their
circumstances, and if we are able to discover, either among the
proximate antecedents or among some other order of antecedents,
something which is always found when the effect is found, however
various the circumstances, and never found when it is not; we may
discover, by mere observation without experiment, a real uniformity in
nature.
But though this is certainly the most favourable case for sciences of
pure observation, as contrasted with those in which artificial
experiments are possible, there is in reality no case which more
strikingly illustrates the inherent imperfection of direct induction
when not founded on experimentation. Suppose that, by a comparison of
cases of the effect, we have found an antecedent which appears to be,
and perhaps is, invariably connected with it: we have not yet proved
that antecedent to be the cause, until we have reversed the process, and
produced the effect by means of that antecedent. If we can produce the
antecedent artificially, and if, when we do so, the effect follows, the
induction is complete; that antecedent is the cause of that
consequent.[32] But we have then added the evidence of experiment to
that of simple observation. Until we had done so, we had only proved
_invariable_ antecedence within the limits of experience, but not
_unconditional_ antecedence, or causation. Until it had been shown by
the actual production of the antecedent under known circumstances, and
the occurrence thereupon of the consequent, that the antecedent was
really the condition on which it depended; the uniformity of succession
which was proved to exist between them might, for aught we knew, be
(like the succession of day and night) not a case of causation at all;
both antecedent and consequent might be successive stages of the effect
of an ulterior cause. Observation, in short, without experiment
(supposing no aid from deduction) can ascertain sequences and
coexistences, but cannot prove causation.
In order to see these remarks verified by the actual state of the
sciences, we have only to think of the condition of natural history. In
zoology, for example, there is an immense number of uniformities
ascertained, some of coexistence, others of succession, to many of
which, notwithstanding considerable variations of the attendant
circumstances, we know not any exception: but the antecedents, for the
most part, are such as we cannot artificially produce; or if we can, it
is only by setting in motion the exact process by which nature produces
them; and this being to us a mysterious process, of which the main
circumstances are not only unknown but unobservable, we do not succeed
in obtaining the antecedents under known circumstances. What is the
result? That on this vast subject, which affords so much and such varied
scope for observation, we have made most scanty progress in ascertaining
any laws of causation. We know not with certainty, in the case of most
of the phenomena that we find conjoined, which is the condition of the
other; which is cause, and which effect, or whether either of them is
so, or they are not rather conjunct effects of causes yet to be
discovered, complex results of laws hitherto unknown.
Although some of the foregoing observations may be, in technical
strictness of arrangement, premature in this place, it seemed that a few
general remarks on the difference between sciences of mere observation
and sciences of experimentation, and the extreme disadvantage under
which directly inductive inquiry is necessarily carried on in the
former, were the best preparation for discussing the methods of direct
induction; a preparation rendering superfluous much that must otherwise
have been introduced, with some inconvenience, into the heart of that
discussion. To the consideration of these methods we now proceed.