m of
nature has been constructed upon a line intermediate between the
extremes of sameness and diversity. If the measure of difference between
classes and individuals had been much greater or much smaller than it
is, the accumulation of knowledge would have been extremely difficult,
or altogether impossible. It is by the combination of similarity and
dissimilarity among sensible objects that science from its lowest to its
highest measures becomes possible. If all animals, or all plants had
been in their sensible qualities precisely the same, there would have
been of animals or vegetables only one class: we could have had no
knowledge regarding them, except as individuals: our knowledge would at
this day have been less than that of savages. Again, if all animals or
all plants had been in their sensible qualities wholly dissimilar--all
from each, and each from all, it would have been impossible to frame
classes; our knowledge, as on the opposite supposition, would have been
limited to our observation of individuals. In either case Zoology or
Botany would have been impossible. Man, endowed with intelligence, could
not, in such a world, have found exercise for his faculties. It would
have been like a seeing eye without a shining light. The power would
have lain dormant for want of a suitable object. Ask the Botanist, the
Naturalist, the Chemist--ask the votary of any science, what makes
accumulated knowledge possible; he will tell you, it is the similarity
which enables him to classify, accompanied by the diversity which
enables him to distinguish. Wanting these two qualities in balanced
union there could be no analogy; and wanting analogy, man could not be
capable of occupying the place which has been assigned to him in
creation.[1]

[1] But in order to employ analogy with effect more is needful than
to make sure that the two objects or acts compared are similar
without being identical: the design for which a comparison is made
enters as an essential element, and decisively determines its value.
Between two given objects an analogy may exist, good for one purpose
but worthless for another. Given two balls, spherical in form and
equal in size, the one of wood and the other of iron; and let the
question be, Do these two objects bear any analogy to each other,
real in itself and capable of being usefully employed? The question
cannot yet be answered: we must first ascertain for what purpose the
comparis