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 b