§ 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 hardly ever 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 voltaic battery, and
the Leyden jar, 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 it 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, also 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 physiology, mental philosophy, and the
social science,) induction from direct experience is practised at a
disadvantage generally 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 still in their infancy.
§ 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 substance 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.(76) 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, 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) no 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, the name of experimentation would here be
completely misapplied. Such are the facts: and what is the result? That on
this vast subject, which affords so much and such varied scope for
observation, we have not, properly speaking, ascertained a single cause, a
single unconditional uniformity. We know not, 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.