+Theme Lx.x.xVII.+--_Following the outline, write an exposition on one of the subjects chosen._

(Notice the transition from one paragraph to another. See Section 87.)

+158. Exposition of Terms--Definition.+--Explanation of the meaning of general terms is one form of exposition (Section 63). The first step in the exposition of a term is the giving of a definition. This may be accomplished by the use of a synonym (Section 64). We make a term intelligible to the reader by the use of a synonym with which he is familiar; and though such a definition is inexact, it gives a rough idea of the meaning of the term in question, and so serves a useful purpose.

If, however, we wish exactness, we shall need to make use of the logical definition.

+159. The Logical Definition.+--The logical definition sets exact limits to the meaning of a term. An exact definition must include all the members of a cla.s.s indicated by the term defined, and it must exclude everything that does not belong to that cla.s.s. A logical definition is composed of two parts. It first names the cla.s.s 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 cla.s.s. The cla.s.s is termed the _genus_, and the distinguishing characteristics of the different members of the cla.s.s are termed the _differentia_. Notice the following division into genus and differentia.

TERM TO BE | CLa.s.s | 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 p.r.o.noun 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 k.n.o.b-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 definition of rodent by saying, "A rodent is a beaver," because the term beaver does not include all rodents. We must also take care to choose for the genus some term familiar to the reader, because the object of the definition is to make the meaning clear to him.

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 cla.s.s from all other members of that cla.s.s. 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 | CLa.s.s (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 p.r.o.noun is a word used for a noun.