We sort items sharing similar traits into categories
to understand and analyze them. The
set of things to be classified—movies, books, college courses, sports
cars—becomes the genus; items within the genus must then be subdivided
into distinct species.
How items are classified depends upon the purpose
of the classification. Books, for example, can be sorted according
to subject,
size, or even color. Generally,
however, items are grouped according to significant traits. Classes must be:
Sorted according to a
single, consistent principle.
Mutually
exclusive.
Contain all
significant items in the genus.