The classes themselves may be, mentally speaking, perfectly definite,
though it may not always be easy to say to which of them a particular
object belongs. When it is uncertain in which of two classes an object
should be placed, if the classification be properly made, and properly
expressed, the uncertainty can turn only upon a matter of fact. It is
uncertain to which class the object belongs, because it is doubtful
whether it possesses in a greater degree the characteristics of the one
class or those of the other. But the characteristics themselves may be
defined and distinguished with the nicest exactness, and always ought to
be so. Especially ought they in a case like the present, because here it
is only the distinction between the ideas which is of any importance.
That we should be able with ease to portion out all employments between
the two classes, does not happen to be of any particular consequence.

It is frequently said that classification is a mere affair of
convenience. This assertion is true in one sense, but not if its meaning
be, that the most proper classification is that in which it is easiest
to say whether an object belongs to one class or to the other. The use
of classification is, to fix attention upon the distinctions which exist
among things; and that is the best classification, which is founded upon
the most important distinctions, whatever be the facilities which it may
afford of ticketing and arranging the different objects which exist in
nature. In fixing, therefore, the meaning of the words productive and
unproductive, we ought to endeavour to render them significative of the
most important distinctions which, without too glaring a violation of
received usage, they can be made to express.

We ought further, when we are restricted to the employment of old words,
to endeavour as far as possible that it shall not be necessary to
struggle against the old associations with those words. We should, if