A definition by genus and difference can still go wrong. Logic books list a few rules that catch the common faults.
Key idea
A good definition is not too broad (it lets in nothing that doesn't belong), not too narrow (it leaves out nothing that does), not circular, not negative when it can be positive, and not figurative or obscure.
Why it works
A definition should pick out exactly the right things and help someone who does not know the word. Too broad or too narrow picks out the wrong things. A circular definition uses the word itself, so it helps no one. A negative or figurative one leaves you not knowing what the thing is.
| Definition | Fault |
|---|---|
| A dog is an animal with four legs. | Too broad: cats have four legs |
| A bird is an animal with feathers that can fly. | Too narrow: penguins are birds |
| A teacher is a person who teaches. | Circular |
| A circle is a shape that is not a square. | Negative |
| Music is the language of the heart. | Figurative |
Worked example: Broad or narrow?
"A chair is a wooden seat with four legs."
- Look for chairs it leaves out: metal chairs, plastic chairs, three-legged chairs.
- It shuts out real chairs, so it is too narrow.
- Does it let in non-chairs? A wooden stool with four legs, so it is also a little broad. The main fault is narrowness.
Answer: Too narrow (mainly).
Worked example: Circular
"A friend is someone who is friendly with you."
- "Friendly" is built from "friend".
- Anyone who does not know what a friend is will not know what friendly means either.
Answer: Circular.
Common mistake
Mixing up too broad and too narrow. Too broad lets in things that do not belong (look for an outsider it includes). Too narrow shuts out things that do belong (look for a member it leaves out).