's arms are
homologous with a horse's fore legs, but they are not analogous
inasmuch as they are not used for progression. Apart from these
technical employments, the word is loosely used in common speech for
any kind of resemblance. Thus De Quincey speaks of the "analogical"
power in memory, meaning thereby the power of recalling things by
their inherent likeness as distinguished from their casual connexions
or their order in a series. But even in common speech, there is a
trace of the original meaning: generally when we speak of analogy we
have in our minds more than one pair of things, and what we call
the analogy is some resemblance between the different pairs. This
is probably what Whately had in view when he defined analogy as
"resemblance of relations".

In a strict logical sense, however, as defined by Mill, sanctioned
by the previous usage of Butler and Kant, analogy means more than
a resemblance of relations. It means a preponderating resemblance
between two things such as to warrant us in inferring that the
resemblance extends further. This is a species of argument distinct
from the extension of an empirical law. In the extension of an
empirical law, the ground of inference is a coincidence frequently
repeated within our experience, and the inference is that it has
occurred or will occur beyond that experience: in the argument from
analogy, the ground of inference is the resemblance between two
individual objects or kinds of objects in a certain number of points,
and the inference is that they resemble one another in some other
point, known to belong to the one, but not known to belong to the
other. "Two things go together in many cases, therefore in all,
including this one," is the argument in extending a generalisation:
"Two things agree in many respects, therefore in this other," is the
argument from analogy.

The example given by Reid in his _Intellectual Powers_ has become the
standard illustration of the peculiar argument from analogy.

We may observe a very great similitude between this earth
which we inhabit, and the other planets, Saturn, Jupiter,
Mars, Venus and Mercury. They all revolve round the sun,
as the earth does, although at different distances and in
different periods. They borrow all their light from the sun,
as the earth does. Several of them are known to revolve
round their axis like the earth, and by that means have like
succession of day and night.