On 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 in some fact or concourse of 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 endeavor 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 further 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 recognized.
§ 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 (_a 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 recognized: 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 every thing 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
connection 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 practiced at a
disadvantage in most cases equivalent to impracticability; from which it
follows that the methods of those sciences, in order to accomplish any
thing 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 recognized
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 can not 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 can not, 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 favorable 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.(135) 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
co-existences, but can not 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 co-existence, 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 can not 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.