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

1 86 DEFINITION.
classification is made on the basis of one fact only.
of words
Examples are easily found : the arrangement
in a dictionary, the object being to find any word as
easily as possible; the arrangement of books in a
library, according to size, for economy of space, accord
ing to the initial letters of the authors names, or accord
ing to the language in which they are written.
9. We shall now give a formal statement of the
rules of a correct
"
logical division," the process of
splitting up a given class into sub-classes. These rules
are only an expanded statement of the relation of a
genus to the subordinate species which compose it.
() In dividing a genus, the basis of division must be
a quality common to the whole extent of the genus ;
and species must be distinguished according to the
different modifications of it which they possess. Hence
the basis cannot be a proprium, or essential quality of
the genus, for this would be possessed equally by all the
species e.g., we cannot take "life," "reason," &c., as
bases for dividing the genus
"
man."
(b] Each act of division must have one basis only.
Violation of this rule leads to
"
cross division," which
practically means that the species overlap. If there is
one basis only, the species will be mutually exclusive.
(c) The constituent species must be together equal
to the genus. In other words, the division must be ex
haustive. It must not "make a leap" i.e., leave gaps.
The basis on which the Division is made (see rule a
above) is called the principium orfimdamentum divisionis.
These terms should not be used to signify the basis on
which a Classification is made. The relation between
the two ideas is seen by referring to the definition of a
of 8
"natural" class, already given at the beginning
(rule c}. It will be seen that a "natural" class is one
formed by the coincidence of several different principia or