OBSERVATION AND EXPERIMENT.
Since the whole of the present facts are the infallible result of the
whole of the past, so that if the prior state of the entire universe
could recur it would be followed by the present, the process of
ascertaining the relations of cause and effect is an analysis or
resolution of this complex uniformity into the simpler uniformities
which make it up. We must first mentally analyse the facts, not making
this analysis minuter than is needed for our object at the time, but at
the same time not regarding (as did the Greeks their verbal
classifications) a mental decomposition of facts as ultimate. When we
have thus succeeded in looking at any two successive chaotic masses (for
such nature keeps at each instant presenting to us) as so many distinct
antecedents and consequents, we must analyse the facts themselves, and
try, by varying the circumstances, to discover which of the antecedents
and consequents (for many are always present together) are related to
each other.
Experiment and observation are the two instruments for thus varying the
circumstances. When the enquiry is, What are the effects of a given
cause? experiment is far the superior, since it enables us not merely to
produce many more and more opportune variations than nature, which is
not arranged on the plan of facilitating our studies, offers
spontaneously, but, what is a greater advantage, though one less
attended to, also to insulate the phenomenon by placing it among known
circumstances, which can be then infinitely varied by introducing a
succession of well-defined new ones.
Observation cannot ascertain the effects of a given cause, because it
cannot, except in the simplest cases, discover what are the concomitant
circumstances; and therefore sciences in which experiment cannot be
used, either at all, as in astronomy, or commonly, as in mental and
social science, must be mainly deductive, not inductive. When, however,
the object is to discover causes by means of their effects, observation
alone is primarily available, since new effects could be artificially
produced only through their causes, and these are, in the supposed case,
unknown. But even then observation by itself cannot directly discover
causes, as appears from the case of zoology, which yet contains many
recognised uniformities. We have, indeed, ascertained a real uniformity
when we observe some one antecedent to be invariably found along with
the effects presented by nature. But it is only by reversing the
process, and experimentally producing the effects by means of that
antecedent, that we can prove it to be unconditional, i.e. the cause.
CHAPTER VIII. AND NOTE TO CHAPTER IX.[1]
THE FOUR METHODS OF EXPERIMENTAL ENQUIRY.
Five canons may be laid down as the principles of experimental enquiry.
The first is that of the Method of Agreement, viz.: _If two or more
instances of the phenomenon under investigation have only one
circumstance in common, the circumstance in which alone all the
circumstances agree is the cause or the effect of the given phenomenon._
The second canon is that of the Method of Difference, viz.: _If an
instance in which the phenomenon occurs and an instance in which it does
not occur have every circumstance in common, save one, and that one
occurs only in the former, that one circumstance is the effect, or the
cause, or a necessary part of the cause, of the phenomenon._
These two are the simplest modes of singling out from the facts which
precede or follow a phenomenon, those with which it is connected by an
invariable law. Both are methods of elimination, their basis being, for
the method of agreement, that whatever can be eliminated _is not_, and
for that of difference, that whatever cannot be eliminated _is_
connected with the given phenomenon by a law. It is only, however, by
the method of difference, which is a method of artificial experiment
(and by experiment we can introduce into the pre-existing facts a change
perfectly definite), that we can, at least by direct experience, arrive
with certainty at causes. The method of agreement is chiefly useful as
preliminary to and suggestive of applications of the method of
difference, or as an inferior resource in its stead, when, as in the
case of many spontaneous operations of nature, we have no power of
producing the phenomenon.
When we have power to produce the phenomenon, but only by the agency,
not of a single antecedent, but of a combination, the method of
agreement can be improved (though it is even then inferior to the direct
method of difference) by a double process being used, each proof being
independent and corroborative of the other. This may be called the
Indirect Method of Difference, or the Joint Method of Agreement and
Difference, and its canon will be: _If two or more instances in which
the phenomenon occurs have only one circumstance in common, while two or
more instances in which it does not occur have nothing in common save
the absence of that circumstance, the circumstance in which alone the
two sets of instances differ, is the effect, or the cause, or a
necessary part of the cause, of the phenomenon._
The fourth canon is that of the Method of Residues, viz.: _Subduct from
any phenomenon such part as is known by previous inductions to be the
effect of certain antecedents, and the residue of the phenomenon is the
effect of the remaining antecedents._ This method is a modification of
the method of difference, from which it differs in obtaining, of the
two required instances, only the positive instance, by observation or
experiment, but the negative, by deduction. Its certainty, therefore, in
any given case, is conditional on the previous inductions having been
obtained by the method of difference, and on there being in reality no
remaining antecedents _besides_ those given as such.
The fifth canon is that of the Method of Concomitant Variations, viz.:
_Whatever phenomenon varies in any manner whenever another phenomenon
varies in some particular manner, is either a cause or an effect of that
phenomenon, or_ (since they may be effects of a common cause) _is
connected with it through some fact of causation._ Through this method
alone can we find the laws of the permanent causes. For, though those of
the permanent causes whose influence is local may be escaped from by
changing the scene of the observation or experiment, many can neither be
excluded nor even kept isolated from each other; and, therefore, in such
cases, the method of difference, which requires a negative instance, and
that of agreement, which requires the different instances to agree only
in one circumstance, in order to prove causation, are (together with the
methods which are merely forms of these) equally inapplicable. But,
though many permanent antecedents insist on being always present, and
never present alone, yet we have the resource of making or finding
instances in which (the accompanying antecedents remaining unchanged)
their influence is _varied_ and _modified_. This method can be used most
effectually when the variations of the cause are variations of
quantity; and then, if we know the absolute quantities of the cause and
the effect, we may affirm generally that, at least within our limits of
observation, the variations of the cause will be attended by similar
variations of the effect; it being a corollary from the principle of the
composition of causes, that more of the cause is followed by more of the
effect. This method is employed usually when the method of difference is
impossible; but it is also of use to determine according to what law the
quantity or different relations of an effect ascertained by the method
of difference follow those of the cause.
These four methods are the only possible modes of experimental enquiry.
Dr. Whewell attacks them, first, on the ground (and the canon of
ratiocination was attacked on the same) that they assume the reduction
of an argument to formulae, which (with the procuring the evidence) is
itself the chief difficulty. And this is in truth the case: but, to
reduce an argument to a particular form, we must first know what the
form is; and in showing us this, Inductive Logic does a service the
value of which is tested by the number of faulty inductions in vogue.
Dr. Whewell next implies a complaint that no discoveries have ever been
made by these four methods. But, as the analogous argument against the
syllogism was invalidated by applying equally as against all reasoning,
which must be reducible to syllogism, so this also falls by its own
generality, since, if true against these methods, it must be true
against all observation and experiment, since these must ever proceed by
one of the four. And, moreover, even if the four methods were not
methods of discovery, as they are, they would yet be subjects for logic,
as being, at all events, the sole methods of Proof, which (unless Dr.
Whewell be correct in his view that inductions are simply conceptions
consistent with the facts they colligate) is the principal topic of
logic.
FOOTNOTE:
[1] Chap. IX. consists of 'Miscellaneous Examples of the Four Methods,'
which cannot be well represented in an abridged form.