class to which the term to be defined
belongs, and then it names the characteristic that distinguishes that term
from all other members of the same class. The class is termed the _genus_,
and the distinguishing characteristics of the different members of the
class are termed the _differentia_. Notice the following division into
genus and differentia.

TERM TO BE | CLASS | DISTINGUISHING
DEFINED | _(Genus)_ | CHARACTERISTIC
| | _(Differentia)_
| |
A parallelogram | is a quadrilateral | whose opposite sides
| | are parallel
| |
Exposition | is that form of | which seeks to explain
| discourse | the meaning of a term.
| |

Each definition includes three elements: the term to be defined, the
genus, and the differentia; but these are not necessarily arranged in the
order named.

EXERCISE

Select the three elements (the term to be defined, the genus, and the
differentia) in each of the following:--

1. A polygon of three sides is called a triangle.

2. A square is an equilateral rectangle.

3. A rectangle whose sides are equal is a square.

4. Description is that form of discourse which aims to present a picture.

5. The characters composing written words are called letters.

6. The olfactory nerves are the first pair of cranial nerves.

7. Person is that modification of a noun or pronoun which denotes the
speaker, the person spoken to, or the person or things spoken of.

8. The diptera, or true flies, are readily distinguishable from other
insects by their having a single pair of wings instead of two pairs, the
hind wings being transformed into small knob-headed pedicles called
balancers or halters.

+160. Difficulty of Framing Exact Definitions.+--In order to frame a
logical definition, exactness of thought is essential. Even when the
thought is exact, it will be found difficult and often impossible to frame
a satisfactory definition. Usually there is little difficulty in selecting
the genus, still care should be taken to select one that includes the term
to be defined. We might begin the definition of iron by saying, "Iron is a
metal," since all iron is metal, but it would be incorrect to begin the
defini