Such a complete
and exact division is called _classification_.
Nearly every term may be divided according to more than one principle. We
may divide the term _books_ into ancient and modern, or into religious and
secular, or in any one of a dozen other ways. Which principle of division
we shall choose will depend upon our purpose. If we wish to discuss
_sponges_ with reference to their shapes, our division will be different
from what it would be if we were to discuss them with reference to their
uses. When a principle of division has once been chosen it is essential
that it be followed throughout. The use of two principles causes an
overlapping of divisions, thus producing what is called cross division.
Using the principle of use, a tailor may sort his bolts of cloth into
cloth for overcoats, cloth for suits, and cloth for trousers; using the
principle of weight, into heavy weight and light weight; or he may sort
them with reference to color or price. In any case but a single principle
is used. It would not do to divide them into cloth for suits, light weight
goods, and brown cloth. Such a division would be neither complete nor
exact; for some of the cloth would belong to none of the classes while
other pieces might properly be placed in all three.
In the exact sciences complete exposition is the aim, and classification
is necessary; but in other writing the purpose in hand is often better
accomplished by omitting minor divisions.
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