Logical Definition
An exact definition is supplied by the logical definition. In this there are three parts—the term to be defined, the class (or genus) to which it belongs, and the distinguishing characteristics (differentia) which mark it off from all the other members of that same class. You can represent this graphically by inclosing the word term in a small circle. Around this draw a larger circle in which you write the word class. Now what divides the term from the class in which it belongs?
Indicate the line around the term as distinguishing characteristics, and you will clearly see how accurate a logical definition is. The class should be just larger than the term itself. The main difficulty is in finding exact and satisfying distinguishing characteristics. There are some terms which are so large that no classes can be found for them. Others cannot be marked by acceptable distinguishing characteristics, so it is not possible to make logical definitions for all terms. Consider such words as infinity, electricity, gravity, man.
The words of the definition should be simple, more readily understood than the term to be defined.
Term |
Class |
Distinguishing characteristics |
| A biplane | is an airplane | with two sets of supporting surfaces. |
| A waitress | is a woman | who serves meals. |
| Narration | is that form of discourse | which relates events. |
| A word | is a combination of | suggesting an idea. |
| letters | ||
| A dictionary | is a book | of definitions. |
| A corporal | is an army officer | just higher than a private. |