Rules for Definitions

Logicians have identified six rules for constructing a type of definition that is suitable for general purposes.

A definition should:

1. Include a genus and a differentia.
2. Be neither too broad or too narrow.
3. State the essential attributes of the concept's referents.
4. Avoid circularity.
5. Avoid negative terms.
6. Avoid vague, obscure, and metaphorical language.


Let us examine each of these in turn.

A Genus and a Differentia

A definition should include a genus and a differentia.

Example:

Humans are rational animals.

The term "animal" names the wider class to which humans belong; it classifies us as a species of the genus ANIMAL.

The term "rational" specifies an attribute that distinguishes us from other species of the same genus. This part of the definition is called the differentia--it differentiates humans from other animals.

Neither Too Broad or Too Narrow

The point of a definition is to identify the referents of a concept. A definition that does not pick out the right referents- -one that includes too much or too little--is not doing its job.

A definition is too broad if it includes things that are not referents of the concept.

Example:

Humans are two-legged animals.

This definition is too broad because the defining phrase "two-legged animal" includes birds as well as humans.

A definition is too narrow if it fails to include things that are referents of the concept.

Example:

Humans are religious animals

This definition is too narrow because, no matter how widespread religious belief may be, some people are atheists.

Essential Attributes

The term "essential" means fundamental: An essential attribute causes or explains the existence of other attributes.

Example:

The heart is an organ that goes "lub-dub, lub-dub".

The "lub-dub" sound is a superficial trait; it is merely a by-product of the heart's essential function, which is to circulate the blood.

Avoid Circularity

A circular definition tells us how a concept relates to itself, but not how it relates to other concepts or to reality. Such a definition doesn't go anywhere; it just moves in a circle.

Example:

Suppose we define "ownership" as the legal relation between people and the things they own. Because this definition uses the word "own," it defines the concept OWNERSHIP in terms of itself.

Avoid Negative Terms

A definition should not use negative terms unnecessarily.

Negative definitions should be avoided because knowing what a thing is not doesn't tell us much about what it is.

Example:

At the turn of the century, the automobile was described as a "horseless carriage." The differentia "horseless" tells us about one source of power that automobiles do not use. However, there are many sources of power automobiles do not use; what we want to know is the source they do use.

Some concepts, however, are inherently negative, and thus require negative terms in their definitions.

Example:

A "bachelor" is a man who is not married.

This has to be defined negatively, since it is an inherently negative concept.

Avoid Vagueness

This is the clarity rule. The purpose of a definition is to clarify our understanding of a concept. At the very least, therefore, the language we use in a definition should not be less clear than the concept being defined.

A vague definition is unclear because it does not give any precise criterion for membership in the concept. A definition shouldn't have borders that are even fuzzier than those of the concept being defined.

Example:

Suppose we define "maturity" as the stage of psychological development in which a person becomes well-adjusted.

Who belongs in the class of well-adjusted people and who doesn't is unclear; the class has fuzzy boundaries that are even fuzzier than those of the concept being defined.

A Definition Should Avoid Obscurity

An obscure definition is unclear because it uses abstract or technical language that is more difficult to understand than the concept itself.

Example:

Suppose we define "death" as the cessation of one's participation in finitude.

The problem here may not necessarily be one of vagueness. Within a specialized context, this definition might have a perfectly clear and definite meaning. The problem is that if technical definitions are used outside of these specialized contexts they are not clear to the layperson.

A Definition Should Avoid Metaphorical Language

A metaphorical definition is unclear because it doesn't convey the literal meaning of the concept, but only an analogy that we have to interpret.

Example:

Suppose we consider the definition: "Life is a cabaret."

Like any good metaphor, this one uses a simple image to convey a complex thought that would take many paragraphs to explain in literal terms. Metaphorical definitions leave too many questions unanswered, which is why we need literal definitions.