ANALOGY.
One of the many meanings of _Analogy_ is, Resemblance of Relations. The
value of an analogical argument in this sense depends on the showing
that, on the common circumstance which is the _fundamentum relationis_,
the rest of the circumstances of the case depend. But, generally, _to
argue from analogy_ signifies to infer from resemblance in some points
(not necessarily in _relations_) resemblance in others. Induction does
the same: but analogy differs from induction in not requiring the
previous proof, by comparison of instances, of the invariable
conjunction between the known and the unknown properties; though it
requires that the latter should not have been ascertained to be
_unconnected_ with the common properties.
If a fair proportion of the properties of the two cases are known, every
resemblance affords ground for expecting an indefinite number of other
resemblances, among which the property in question may perhaps be found.
On the other hand, every dissimilarity will lead us to expect that the
two cases differ in an indefinite number of properties, including,
perhaps, the one in question. These dissimilarities may even be such as
would, in regard to one of the two cases, imply the absence of that
property; and then every resemblance, as showing that the two cases have
a similar nature, is even a reason for presuming against the presence of
that property. Hence, the value of an analogical argument depends on
the extent of ascertained resemblance as compared, first, with the
amount of ascertained difference, and next, with the extent of the
unexplored region of unascertained properties.
The conclusions of analogy are not of direct use, unless when the case
to which we reason is a case _adjacent_, not, as before, in time or
place, but in _circumstances_. Even then a complete induction should be
sought after. But the great value of analogy, even when faint, in
science, is that it may suggest observations and experiments, with a
view to establishing positive scientific truths, for which, however, the
hypotheses based on analogies must never be mistaken.