Definition
A definition is a statement specifying the necessary and sufficient conditions for a term to apply to an object. Good definitions are the foundation of clear argument: when parties to a debate mean different things by their key terms, they are not really arguing about the same thing. Definitions embed knowledge — they stand in relation to each other in a web of meaning.
How It Appears Per Course
PHIL 252
Central to Unit 4. Definitions enable arguments from definition (Unit 4) and prevent equivocation (Unit 6). The unit provides a taxonomy of definition types and six rules for evaluating them.
Necessary vs. Sufficient Conditions
- Necessary condition: must be present for category membership. Being a mammal is necessary to be a dog (all dogs are mammals).
- Sufficient condition: alone guarantees membership. Being a poodle is sufficient to be a dog (if it’s a poodle, it’s a dog).
- A good definition specifies conditions that are jointly necessary AND sufficient: all and only members of the category satisfy them.
Genus-Species Structure
Place the term in its broader class (genus), then add the distinguishing features (differentia) that mark the specific type (species).
- “A bungalow is a house [genus] with only one storey [differentia].”
- “A dog is a domesticated canine mammal.”
Types of Definition
| Type | Description | Example |
|---|---|---|
| Lexical | Standard dictionary/common meaning | ”Bachelor: unmarried man” |
| Stipulative | Assigned for a specific purpose or context | ”In this paper, ‘species’ means X” |
| Precising | Clarifies a vague or broad term | Legal definition of “reasonable person” |
| Persuasive | Seeks to influence attitudes, not just clarify | ”Abortion is murder” as a definition |
The Six Rules for a Good Definition
| Rule | Problem If Violated | Example of Bad Definition |
|---|---|---|
| Not too broad | Admits things that shouldn’t qualify | ”Horse: an animal” — includes all animals |
| Not too narrow | Excludes things that should qualify | ”Canadian citizen: born in Canada” — misses naturalized citizens |
| Not vague or obscure | Unclear what is being defined | Using jargon the audience doesn’t understand |
| Not circular | Uses the term itself in the definition | ”Detoxification: removing toxins from the body” (just repeats the root) |
| Not negative only | Tells you what something isn’t, not what it is | ”Vitamin: not a mineral” |
| Not slanted or biased | Contains evaluative/emotional content | ”Politicians: professional liars” |
Arguments from Definition
When the conclusion of an argument follows necessarily by virtue of the meaning of the terms involved. If definitions are clear and mutually accepted, such arguments are deductively valid.
- “Bruce is a parent [premise]. Therefore, Bruce has a child [conclusion].”
- The validity comes entirely from what “parent” means.
Cross-Course Connections
ClassificationSystems — definitions are classification rules for individual terms
Enthymeme — many enthymeme implicit premises are definitional claims
FallaciesOfAmbiguity — equivocation occurs when definitions shift within an argument
Validity — arguments from definition are valid when definitions are precise and shared
Key Points for Exam/Study
- A good definition = jointly necessary AND sufficient conditions
- Know all six rules and be able to identify violations (exam will likely test this)
- Necessary ≠ sufficient: being a mammal is necessary but not sufficient to be a dog (cats are mammals)
- “Talking past each other” happens when parties use the same term with different definitions
- Persuasive definitions masquerade as neutral definitions while actually pushing an agenda
Open Questions
- What do we do when a term (like “species” in biology) has multiple legitimate technical definitions for different purposes?