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 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 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.