The chief difficulty of framing logical definitions arises in the selection of differentia.  In many cases it is not easy to decide just what characteristics distinguish one member of a class from all other members of that class.  We all know that iron is a metal, but most of us would find it difficult to add to the definition just those things which distinguish iron from other metals.  We may say, “A flute is a musical instrument”; so much of the definition is easily given.  The difficulty lies in distinguishing it from all other musical instruments.

EXERCISES

A. Select proper differentia for the following:—­

|
TERM TO BE DEFINED |     CLASS (Genus)         |   DISTINGUISHING
|                           |   CHARACTERISTIC
|                           |  (Differentia)
|                           |
1.  Narration       | is that form of discourse |         ?
|                           |
2.  A circle        | is a portion of a plane   |         ?
|                           |
3.  A dog           | is an animal              |         ?
|                           |
4.  A hawk          | is a bird                 |         ?
|                           |
5.  Physiography    | is the science            |         ?
|                           |
6.  A sneak         | is a person               |         ?
|                           |
7.  A quadrilateral | is a plane figure         |         ?
|                           |
8.  A barn          | is a building             |         ?
|                           |
9.  A bicycle       | is a machine              |         ?
|                           |
10.  A lady         | is a woman                |         ?

B. Give logical definitions for at least four words in the list below.

1.  Telephone.

2.  Square.

3.  Hammer.

4.  Novel

5.  Curiosity.

6.  Door.

7.  Camera.

8.  Brick.

9.  Microscope.

+161.  Inexact Definitions.+—­If the distinguishing characteristics are not properly selected, the definition though logical in form may be inexact, because the differentia do not exclude all but the term to be defined.  If we say, “Exposition is that form of discourse which gives information,” the definition is inexact because there are other forms of discourse that give information.  Many definitions given in text-books are inexact.  Care should be taken to distinguish them from those which are logically exact.

EXERCISE

Which of the following are exact?

1.  A sheep is a gregarious animal that produces wool.

2.  A squash is a garden plant much liked by striped bugs.

3.  A pronoun is a word used for a noun.

4.  The diaphragm is a sheet of muscle and tendon, convex on its upper side, and attached by bands of striped muscle to the lower ribs at the side, to the sternum, and to the cartilage of the ribs which join it in front, and at the back by very strong bands to the lumbar vertebrae.