Traditional Definition: Genus and Differentia

It distinguishes the thing. Naming the differentia tells you what marks it off, and therefore how to tell it from its neighbours. A decree is distinguished from an order by conclusive determination of rights, and that single phrase is what a hundred applications turn on.

And it yields a test. A definition of this form gives a two-part checklist: is this within the genus, and does it have the differentia? Applying a statutory definition is exactly that operation.

The limits of the form

Some terms have no genus. The most general terms of all, being, existence, thing, cannot be defined this way, because there is no wider class to put them in. This is why definitions of the very general words in law, "law", "justice", "right", are so unsatisfactory: they have nothing above them.

Some terms have no differentia that can be stated. A colour cannot be defined by genus and differentia to somebody who has never seen it. Traditional logic accepted that such terms are defined by pointing, which is the ostensive method at sequence 580.

Individuals cannot be defined at all on this scheme. A proper name has no connotation, as sequence 180 said, so there are no attributes to state. What can be given for an individual is a description, and a description is not a definition.

And a practical limit that matters most in law: a genus may be chosen for effect. To define a lease as a transfer of an interest in property places it among conveyances; to define it as a contract for occupation places it among agreements. Both can be defended and they carry different consequences, and the choice of genus is very often the real decision hiding inside a definition.

A worked example

A statute is to define "public place".

Choose the genus. Is a public place a kind of place, a kind of premises, or a kind of space to which the public has access? The choice already decides a great deal. If the genus is "premises", a road is probably outside. If the genus is "place", a road is inside.

Choose the differentia. Take the genus as "place". What marks off a public one? Candidates: ownership by the State; a right in the public to be there; actual access by the public whether as of right or by permission; use by the public in fact.

See what each differentia does. Ownership by the State excludes a cinema hall. A right in the public excludes a shop, since a shopkeeper may refuse entry. Access by permission includes a cinema and a shop, and probably a members' club during a public event. Use in fact includes a private field that people cross.

And now the two-part test appears. Whatever pair is chosen, applying the definition to a disputed case is a matter of asking two questions in order, is it within the genus and does it have the differentia. A student who can set out those two questions from a statutory definition can apply any statutory definition, which is the practical skill this chapter teaches.

A note on why litigation clusters where it does. Almost every reported case on a definition clause is about the differentia and almost none about the genus. That is because the genus is usually obvious and the differentia is where the drafter had to make a choice, and where a choice was made, a party will one day want it made differently.

Distinctions that carry marks

What this does not mean

Genus and species are not fixed classes. They are relative: a class is a species of what is above it and a genus of what is below.

A property is not part of the definition. It follows from the definition and is true of everything defined, which is exactly why it must not be used as the differentia.

Not every term can be defined this way. The most general terms have no genus, some qualities have no statable differentia, and individuals have no connotation to state at all.

Quick revision

Form: definiendum equals genus plus differentia.

Genus: the wider class. Differentia: what marks the species off within it.

Property: follows from the definition, not part of it. Accident: some have it, some do not.

Genus and species are relative terms, and the ladder between the summum genus and the infima species is the tree of Porphyry.

Its strength: it places the thing and distinguishes it, and yields a two-part test.

Its limits: the most general terms have no genus; some qualities have no statable differentia; individuals cannot be defined; and the choice of genus is often the real decision.

In law: litigation clusters on the differentia, because that is where the drafter made a choice.

Test yourself

1. State the traditional form of definition and identify its parts in an example.

Definition per genus et differentiam: the definiendum is defined by naming the genus, the wider class it falls under, and the differentia, the attribute that marks it off from the other species of that genus. In "a contract is an agreement enforceable by law", the genus is "agreement" and the differentia is "enforceable by law", so contracts are marked off within agreements by enforceability.

2. Distinguish genus, differentia, property and accident.

The genus is the wider class containing the species defined. The differentia is the attribute distinguishing the species within that genus. A property belongs to every member of the species and is not part of the definition but follows from it, as the enforceability of the obligations follows from a contract being enforceable at law. An accident is an attribute some members happen to have and others do not, such as being in writing.

3. Why are genus and species called relative terms?

Because the same class is a species of the class above it and a genus of the classes below it. "Contract" is a species of "agreement" and a genus of "contract of sale". Nothing is a genus or a species absolutely, except the summum genus, which has nothing above it, and the infima species, below which there are only individuals.

4. Why was this form thought to be the only real kind of definition?

Because it does two things at once. Naming the genus places the thing, telling you what family of rules and questions applies to it. Naming the differentia distinguishes it, telling you how to separate it from its neighbours. Together they yield a two-part test that can be applied to a disputed case: is it within the genus, and does it have the differentia.

5. Give three limits of the form.

The most general terms, such as being or thing, have no genus above them and so cannot be defined this way, which is why definitions of "law" and "justice" are always unsatisfactory. Some qualities, such as a colour, have no differentia that can be stated to someone who has not experienced them, and are defined by pointing instead. And individuals cannot be defined at all, since a proper name has no connotation; what can be given for an individual is a description.

6. Why does litigation about a definition clause almost always concern the differentia?

Because the genus is usually obvious and uncontested, while the differentia is where the drafter had to choose a boundary. A choice about where a line falls is a choice somebody will one day want made differently, and the words that draw the line are therefore what is argued about. The choice of genus is rarer in dispute but more consequential when it is, since it settles which family of rules applies at all.