5. THE KINDS OF DEFINITIONS.
Generally speaking there are three kinds of definitions, namely, (1) Etymological, (2) Descriptive, (3) Logical.[5]
(1) An etymological definition is one based upon the derivation of the term.
This kind of a definition, which gives merely the meaning of the symbol, is sometimes called a nominal or verbal definition; while a real definition is regarded as one which gives the meaning of the notion for which the symbol stands. The modern logician is inclined to ignore this classification on the argument that to make a distinction between a symbol and the notion it symbolizes is simply to misunderstand the relation which exists between them. If the definition does not agree with the thing then it cannot correctly explain the term which represents the thing. Define correctly the term and one has defined correctly the notion signified by the term.
The attributes of a term may be separated into three classes: differentia, property and accident. It would appear possible, therefore, to define a term by enumerating the accidents only or by enumerating the properties, or, finally, by stating the differentiae. But if the enumeration is confined to accidents the chances are that the statement will be a description, not a definition, as accidents are seldom sufficiently characteristic to determine theboundaries of a term. This leaves open two distinct ways of defining a term: First, by naming the properties or properties and accidents only; second, by stating the differentiæ only. The former kind is the so-called descriptive definition, while the latter is the logical.
(2) A descriptive definition of a term is a description of its nature by means of its properties and accidents.
(3) A logical definition of a term is a description of its nature by means of its differentiæ.
THE THREE KINDS OF DEFINITIONS ILLUSTRATED AND COMPARED.
Etymological Definition of Trigon.
A trigon is a figure of three corners.
Descriptive:
A trigon is a figure which has three sides and three angles, the sum of the latter being equal to two right angles.
Logical:
A trigon is a polygon of three angles.
It is seen that an etymological definition is simply a root-word analysis. In the case of trigon, the prefix comes from the Greek, meaning three, while the root-word comes from the Greek meaning corner.
The descriptive definition of trigon names the properties, “three sides and three angles” (differentiæ) and the accident, “the sum of the angles of which equals two right angles.”
The logical definition of trigon simply states the proximate genus, “polygon,” and the differentia, “three angles.”