PLURALITY OF CAUSES, AND INTERMIXTURE OF EFFECTS.
The difficulty in tracing the laws of nature arises chiefly from the
Intermixture of Effects, and from the Plurality of Causes. The
possibility of the latter in any given case--that is, the possibility
that the same effect may have been produced by different causes--makes
the Method of Agreement (when applied to positive instances)
inconclusive, if the instances are few; for that Method involves a tacit
supposition, that the same effect in different instances, which have
_one_ common antecedent, must follow in all from the same cause, viz.
from their common antecedent. When the instances are varied and very
many (how many, it is for the Theory of Probability to consider), the
supposition, that the presence in all of the common antecedent may be
simply a coincidence, is rebutted; and this is the sole reason why mere
_number_ of instances, differing only in immaterial points, is of any
value. As applied, indeed, to negative instances, i.e. to those
resembling each other in the absence of a certain circumstance, the
Method of Agreement is not vitiated by Plurality of Causes. But the
negative premiss cannot generally be worked unless an affirmative be
joined with it: and then the Method is the Joint Method of Agreement and
Difference. Thus, to find the cause of Transparency, we do not enquire
in what circumstance the numberless _non_-transparent objects agree; but
we enquire, first, in what the few transparent ones agree; and then,
whether all the opaque do not agree in the _absence_ of this
circumstance.
Not only may there be Plurality of Causes, the whole of the effect being
produced now by one, now by another antecedent; but there may also be
Intermixture of Effects, through the interference of different causes
with each other, so that part of the total effect is due to one, and
part to another cause. This latter contingency, which, more than all
else, complicates, the study of nature, does not affect the enquiry into
those (the exceptional) cases, where, as in chemistry, the total effect
is something quite different to the separate effects, and governed by
different laws. There the great problem is to discover, not the
properties, but the cause of the new phenomenon, i.e. the particular
conjunction of agents whence it results; which could indeed never be
ascertained by specific enquiry, were it not for the peculiarity, not of
all these cases (e.g. not of mental phenomena), but of many, viz. that
the heterogeneous effects of combined causes often reproduce, i.e. are
_transformed into_ their causes (as, e.g. water into its components,
hydrogen and oxygen). The great difficulty is _not_ there to discover
the properties of the new phenomenon itself, for these can be found by
experiment like the _simple_ effects of any other cause; since, in this
class of cases the effects of the separate causes give place to a new
effect, and thereby cease to need consideration as separate effects. But
in the far larger class of cases, viz. when the total effect is the
exact sum of the separate effects of all the causes (the case of the
Composition of Causes), at no point may it be overlooked that the effect
is not simple but complex, the result of various separate causes, all of
which are always tending to produce the whole of their several natural
effects; having, it may be, their _effects_ modified, disturbed, or even
prevented by each other, but always preserving their _action_, since
laws of causation cannot have exceptions.
These complex effects must be investigated by _deducing_ the law of the
effect from the laws of the separate causes on the combination of which
it depends. No inductive method is conclusive in such cases (e.g. in
physiology, or _a fortiori_, in politics and history), whether it be the
method of simple observation, which compares instances, whether positive
or negative, to see if they agree in the presence or the absence of one
common antecedent, or the empirical method, which proceeds by directly
trying different combinations (either made or found) of causes, and
watching what is the effect. Both are inconclusive; the former, because
an effect may be due to the concurrence of many causes, and the latter,
because we can rarely know what all the coexisting causes are; and still
more rarely whether a certain portion (if not all) of the total effect
is not due to these other causes, and not to the combination of causes
which we are observing.