6. WHEN THE THREE KINDS OF DEFINITIONS ARE SERVICEABLE.
The etymological definition is helpful in furnishing a cue for remembering the descriptive and logical definitions. It also leads to precision of expression—the right word in the right place. Here is where the knowledge of a foreign language, particularly Latin, is helpful.
The descriptive definition is best adapted to the child-mind. Children think in the large; are not given to hair-splitting discriminations, and, therefore, many characteristic marks must be mentioned in order to insure a mastery of the content. With children the logical definition is often too brief to be clear. For example, it is easy to see which of the following definitions would be better adapted to the child-mind. Logical: A square is an equilateral rectangle. Descriptive: A square is a figure of four equal sides and four right angles.
The logical definition may be introduced to the student of the secondary school.
Few exercises are better adapted to the development of powers of discrimination and precision than practice in defining logically the common terms of every-day life. For example: “A book is a pack of paper-sheets bound together.” “A chair is a piece of furniture with back and seat, designed for the seating of one person.” “A lead pencil is a cylindrical writing implement with lead through the center.” “A door is an obstacle designed to swing in and out to open and close an entrance.” “An eraser is an implement made to rub out written or printed characters.”
These definitions, coming from training school students, are not above criticism, yet they illustrate the point in hand.
7. THE RULES OF LOGICAL DEFINITION.
Five rules summarize the requirements to which a logical definition must conform.
FIRST RULE.
A logical definition should state the essential attributes of the species defined.