ion, is less general than the
word defined. However, to say that "a dog is a canine that has been
domesticated," is a definition in which the genus is more general than
the term defined.

Next, the genus should be a term well understood. "Man is a mammal who
reasons" is all right, in having a genus more general than the term
defined, but the definition fails with many because "mammal" is not
well understood. "Botany is that branch of biology which treats of
plant life" has in it the same error. "Biology" is not so well
understood as "botany," though it is a more general term. In cases of
this sort, the writer should go farther toward the more general until
he finds a term perfectly clear to all. "Man is an animal that
reasons," "botany is the branch of science that treats of plant life,"
would both be easily understood. The genus should be a term better
understood than the term defined; and it should be a term more general
than the term defined.

A definition may be faulty in its _differentia_ also. The differentia
is that part of a definition which names the difference between the
term defined and the general class to which it belongs. "Man is a
reasoning animal." "Animal" names the general class, and "reasoning"
is the differentia which separates "man" from other "animals." On the
selection of this limiting word depends the accuracy of the
definition. "Man is an animal that walks," or "that has hands," or
"that talks," are all faulty; because bears walk, monkeys have hands,
and parrots talk. Supposing the following definitions were given: "A
cat is an animal that catches rats and mice;" "A rose is a flower that
bears thorns;" "Gold is a metal that is heavy;" all would be faulty
because the differentia in each is faulty. Notice, too, the
definitions of "dog" and "canine" already given. Even "man is a
reasoning animal" may fail; since many men declare that other animals
reason. The differentia should include all the members that the term
denotes, and it should exclude all that it does not denote.

Requisites of a good Definition.

The requisites of a good definition are: first, that it shall include
or denote all the members of the class; second, that it shall exclude
everything which does not belong to its class; third, that the words
used in the definition shall be better understood than the word
defined; fourth, that it shall be brief.

A definition may perfectly expound a term; and because of the very
quali