Definition
How ideas are explained by defining them: the types of definition (nominal, real essential, descriptive) and the four rules every valid definition must satisfy.
Definition explains an idea by analysing its Comprehension and stating the essential notes that constitute it. The principal types are: Nominal definition (states the meaning of the name), Real Essential definition (the classical scholastic form: Proximate Genus plus Specific Difference, e.g., 'Man is a rational animal'), and Descriptive definition (uses Properties or contingent marks when essential definition is not available). Four rules govern every valid definition: it must be adequate (co-extensive with the thing defined), essential (using essential notes, not accidents), clear (intelligible — not circular, not metaphorical, not obscure), and positive (not negative unless the thing is essentially defined by negation). The commonest defects are circular definition, definition by the opposite, and obscure or purely metaphorical definition.
Ideas may be obscure or vague, and explanation is needed to render them clear and distinct. This is required not only for conveying ideas clearly to others, but for clarifying them in our own minds.
Ideas are explained by analysing them and setting forth the results of that analysis. We may analyse the Comprehension of an idea and show the essential notes that make it up; expressing the results of such analysis is an essential Definition. We may also analyse the Extension of an idea and group its inferiors into convenient classes; such grouping is called Logical Division (treated in the next Article).
a) Doctrine of Definition
In the widest sense, definition is the explanation of an idea or term according to its content or use. We distinguish the following types:
1. Nominal Definition
Nominal definition explains a term by telling something about it considered as a name. It may be constructed:
- By etymology: “‘Hippopotamus’ is derived from two Greek words, hippos (horse) and potamos (river).”
- By translation: “‘Hippopotamus’ means ‘river horse.’”
- By substitution: “Theodicy is Natural Theology.”
2. Real Definition
Real definition tells the content of an idea with more or less completeness and accuracy — something about the thing for which the idea and term stand. It is:
i. Essential, when it declares exactly what the essence of a thing is. Essential definition is either:
- Physical: expresses the essential objective elements that make up a reality (its physical parts). “Man is a creature made of body and soul.”
- Metaphysical: expresses the essential notes that make up an idea in the mind. “Man is a rational animal.” In simpler language: a physical definition explains an essence by naming its parts; a metaphysical definition explains an idea by naming its essential notes.
ii. Descriptive (or simply Description), when it tells something about an essence — its properties or accidents — but does not adequately express what that essence is. Description may be:
- Accidental: “Man is a biped without feathers.”
- Attributive (proper): “Man is a walking and talking being.”
- Genetic (showing origins): “Man is formed of the slime of the earth.”
- Causal: “Man is made by God (efficient cause) to know and love God (final cause).”
Dialectics is concerned only with real essential definition.
b) Rules of Definition
No one can think correctly — in intricate and involved matters — without a clear grasp of the elements of thought (ideas), and such clear knowledge depends largely upon adequate essential definition. The test of definition, and the manner of constructing it, is contained in the following four rules.
Rule I: The definition must be exact
The definition must square precisely with the idea defined — neither falling short nor extending beyond its limits. Put another way: Let the definition be neither wider nor narrower than the term defined. The following offend against this rule:
- “Cats are domestic animals” (too wide — not all domestic animals are cats)
- “Charity is that which covers a multitude of sins” (too narrow — this describes only one effect)
- “Man is an animal” (too wide)
- “A circle is a plane figure” (too wide)
Rule II: The definition must be clear
The purpose of definition is to clarify ideas. That purpose is defeated if the definition be as obscure as what it should clarify. This rule is violated by ambiguous, metaphorical, or indefinite terminology, and sometimes by ponderous technical expressions:
- “A window is an orifice in an edifice for the exclusion of elemental disturbances and the admission of illumination through translucent substances.”
- “Evolution is the transit from the homogeneous to the heterogeneous.”
- “A biological species is a class marked by morphological discontinuity and interspecific sterility.”
Rule III: The definition must not contain the term defined, even implicitly
Definition is meant to clarify, but one does not clarify a thing by repeating its name. To define circle as “a circular line” is no definition. The following violate this rule:
- “A metal is a metallic substance.”
- “A body is any substance of bodily character.”
- “Psychology is the scientific study of psychic activities.”
- “The duties of an archdeacon are purely archidiaconal.”
Rule IV: Essential definition consists of proximate genus and specific difference
The Proximate Genus sums up all the essential notes in the Comprehension of an idea save the last one. The Specific Difference gives this ultimate note. Hence:
Proximate Genus + Specific Difference = the sum of essential notes = the entire essence.
Definitions that violate this rule amount to mere description (as exampled in the paragraph on Description above).
A mnemonic for the Four Rules:
Defining, be exact and clear; Don’t let the term defined come near; Essential Definition mocks All but Spec. Diff. and Genus Prox.
Summary of the Article
We have learned what is meant by nominal definition (explanation of the sense of a term) and real definition, subdistinguished as essential definition (physical or metaphysical) and description (accidental, attributive, genetic, or causal). We have studied and exemplified the Four Rules of Definition: (I) be exact, (II) be clear, (III) do not include the term defined, (IV) express proximate genus and specific difference.