Of the kinds of definitions, the classical way to write a lexical one goes back to Aristotle: place the thing in a bigger group, then say what sets it apart.
Key idea
A definition by genus and difference names the genus (the larger kind the thing belongs to) and the difference (the feature that sets it apart from the other members of that kind).
Why it works
The genus tells you roughly where to look; the difference picks out exactly the right members. "A triangle is a polygon (genus) with three sides (difference)": every triangle is a polygon with three sides, and nothing else is.
| Term | Genus | Difference |
|---|---|---|
| triangle | polygon | with three sides |
| puppy | dog | that is young |
| island | piece of land | surrounded by water |
| square | rectangle | whose four sides are equal |
Worked example: Find the parts
"A peninsula is a piece of land that is almost surrounded by water but joined to the mainland."
- The bigger kind it belongs to: a piece of land. That is the genus.
- What sets it apart from other pieces of land: almost surrounded by water but joined to the mainland. That is the difference.
Answer: Genus: piece of land. Difference: almost surrounded by water but joined to the mainland.
Worked example: Not a definition
Which defines "island" by genus and difference? (a) "An island is a place like Hawaii." (b) "An island is an isle." (c) "An island is a piece of land surrounded by water."
- (a) gives an example, which tells you one island but not what makes something an island.
- (b) gives a synonym, which helps only if you already know the other word.
- (c) names a genus and a difference.
Answer: (c).
Common mistake
Giving an example or a synonym instead of a definition, or naming a difference that other members of the genus also have ("A dog is an animal with four legs": so is a cat). Check: does the difference apply to all and only the things being defined?