Genus and differentia
from 350 BCE Topics and Posterior Analytics
Name the family a thing belongs to, then the single feature that separates it from everything else in that family — and stop.
- Active
- from 350 BCE
- Where
- Athens
- Also
- genos kai diaphora
- Status
- In use
- Tradition
- Classical procedures
What it is
A definition of this kind has exactly two parts. The genus places the thing among its nearest relatives; the differentia says what distinguishes it from all of them. Together they should pick out everything that is the thing and nothing that is not.
Aristotle's requirement is that the differentia be a single feature, not a list. A definition that needs five clauses to fence the thing in is a symptom that the genus was chosen badly.
How it works
Find the nearest kind that contains the target and is not much wider than it. Ask what separates the target from the rest of that kind. Check the definition both ways: does it admit anything it should exclude, and does it exclude anything it should admit?
The two-way check is the discipline, and it is what the elenchus applies when it goes after a definition.
The moves, in order
- Identify the nearest containing kind
- Identify the one feature separating the target from the rest of that kind
- State the definition as kind plus feature
- Test for admitted cases that should be excluded
- Test for excluded cases that should be admitted
The procedure
Proposition A granny flat is a dwelling for rating purposes.
- The nearest containing kind is not building but dwelling: a structure in which a household lives independently.
- Within dwellings, the feature separating a granny flat from a house is that it sits on a lot already carrying a principal dwelling and is subordinate to it.
- So: a granny flat is a dwelling subordinate to a principal dwelling on the same lot.
- Test one, for false admissions: does this admit a garden shed? No, because a shed is not a dwelling and fails the genus.
- Test two, for false exclusions: does this exclude a converted garage occupied as a separate household? No, and it should not — it is a dwelling and it is subordinate, so it is correctly caught.
Therefore The granny flat is a dwelling, and the definition also settles the converted garage without a second rule being written.
The two tests at the end are the whole method. Anyone can produce a form of words; only the tests show whether the words do the work.
What it is good for
Legislation, standards, contracts, and any situation where a term must be applied consistently by people who did not write it. It is the single most useful method on this list in ordinary working life.
Where it fails
It assumes things have essences — that there is a fact about what a thing really is, waiting to be captured. Where that assumption fails the method produces arbitrary results dressed as discoveries, and Wittgenstein's family-resemblance argument holds that it fails for most interesting concepts, including *game* and probably including *definition*.
It also cannot define the widest kinds. If every definition needs a genus above it, the highest categories have nothing above them and cannot be defined this way at all — a limitation Aristotle acknowledged and did not solve.
Common misconceptions
That the genus should be as broad as possible. The opposite: it should be the nearest kind, because a distant genus forces the differentia to do too much work and the definition becomes a list.
Where it came from
Aristotle's Topics and Posterior Analytics, refined by Porphyry into the tree of predicables that dominated logic teaching for a thousand years.
What became of it
Absorbed into legal drafting, scientific nomenclature and lexicography, where it is still the default shape of a definition.
Lasting influence
Every statutory definitions section and every dictionary entry follows it. So does every class hierarchy in object-oriented programming.
Why it still matters
The most practically applied ancient method here.
Put another way
An address: the street, then the number.
Texts
Aristotle, Topics
Book VI is on definition and is essentially a manual for constructing and demolishing them.
Where to start
Take any definitions section from a piece of legislation and find the genus and the differentia in each entry. They are almost always both there.
The surprising bit
Wittgenstein's attack on it is now more famous than the method, and drafters of legislation have carried on using it regardless, because a family-resemblance concept cannot be applied by a magistrate.
In their own words
A definition is a phrase signifying a thing's essence
Who held it
the medieval logicians
Connections
- Relies on
- Diairesis
- Set against
- Ordinary language
- Influenced
- Conceptual analysis, Explication
- Parallel to
- Necessary and sufficient
The verdict
Indispensable where terms must be applied by strangers, and philosophically much shakier than its usefulness suggests.