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.
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.
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.
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.
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