An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

DEFINITION. 183
the greatest number of other characters, we observe that
such a classification satisfies the following conditions :
(a) It shall enable the greatest number of general
assertions to be made about the class.
(b) It shall enable us to infer of any other member
a great part of what we know about any one.
(c)
Its members shall have the greatest number of
points of mutual resemblance, and the fewest
points
of resemblance to members of other
groups.
Such a class is said to be natural. This term, as
used of classes, takes us back to the ancient view, that
in Nature there are fixed, permanent kinds or classes of
things which can never pass into one another. This
idea is now abandoned, although we may retain the term
"natural" as applied to methods of classification.
The impossibility
of drawing any clear dividing line is a
fact of universal experience. "To admit of degrees is the
character of all natural facts ; there are no hard lines in
nature. Between the animal and the vegetable kingdoms,
for example, where is the line to he drawn? ... I reply
that I do not believe that there is any absolute distinction
whatever. External objects and events shade off into one
another by imperceptible differences ; and, consequently,
definitions whose aim it is to classify such objects and
events must of necessity be founded on circumstances par
taking of this character. ... It is, therefore, no valid
objection to a classification, nor, consequently, to the defini
be found which
tion founded upon it, that instances may
fall, or seem to fall, on our lines of demarcation. This is
inevitable in the nature of things. But, this notwithstand
ing, the classification, and therefore the definition, is a good
one if in those instances which do not fall on the line, the
distinctions marked by
the definition are such as it is
important to mark, such that the recognition of them will
help the inquirer forward towards the desiderated goal"
(Cairnes, Logical Method of Political Economy, p. 139).