The art of thinking · Concepts, categories and definition

Genus and differentia

Also known as: Genus–differentia definition, Per genus et differentiam, Definition by genus and difference, Classical definition

German: Genus proximum et differentia specifica

In logic and terminology work, a definition by genus and differentia states what something is by naming the nearest broader class it belongs to (the genus) and the characteristic that sets it apart from the other members of that class (the differentia). The pattern goes back to Aristotle, for whom a correct definition of X gives the genus — what kind of thing X is — and the difference that identifies X uniquely within that genus; the scholastic short form of the rule is "definitio fit per genus proximum et differentiam specificam". Terminology standards use the same structure under a different name: an intensional definition states the immediate superordinate concept plus the delimiting characteristics. The classic textbook example defines a human being as a rational animal, with "animal" as the genus and "rational" as the differentia.

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In one sentence

Classical pattern of definition: name the nearest broader class (genus) and the feature that sets the thing apart within it (differentia).

Example

In a product glossary, the entry "A changeover is a setup operation that converts a machine from one product variant to another" names the genus (setup operation) and the differentia (conversion between product variants).

How it applies

Where it comes from: Aristotle discusses definition, genus and difference in the Topics and Posterior Analytics and ties the pattern to the Platonic method of division; later commentators, following Porphyry's Isagoge, listed genus, species, differentia, property (proprium) and accident as the five predicables. The Latin formula and the pairing genus proximum / differentia specifica are scholastic, not Aristotle's own wording.
Take the nearest genus: The genus should be the class immediately above the concept, not a catch-all such as "device" or "thing". A nearest genus places the term in an existing hierarchy; a distant genus forces the differentia to carry work that the hierarchy should do.
Make the differentia discriminating: A differentia that is also true of the sibling concepts explains nothing. The test is simple: read the definition back and ask which other members of the genus it would also fit.
Terminology standards use the same shape: Under the name intensional definition, ISO 704 (terminology work — principles and methods) and the vocabulary standard ISO 1087 build definitions from the immediate superordinate concept plus delimiting characteristics, which yields the readable template "X is a Y such that Z". Following that template is a quality practice for terminology work; it does not by itself make a document set compliant with any regulation and never replaces a risk assessment or the safety information a product requires.
Traditional rules of thumb: Classical logic textbooks add that a definition should be neither too broad nor too narrow, should not be circular, should avoid defining by a negation where a positive characteristic is available, and should not smuggle in the term being defined.
In documentation work: The genus–differentia shape fits the first sentence of a glossary entry, the glossdef of a DITA glossentry, and the definition field of a term base. Because the genus names a broader concept, consistent definitions and a consistent concept hierarchy constrain each other — in SKOS terms, the genus of a definition should match its broader relation, and siblings should be separated by differentiae of the same kind.
Where the pattern runs out: A highest genus has no class above it, individuals are identified rather than defined, and some concepts are held together by overlapping similarities rather than one shared essence. If no plausible genus can be named, the label may be an editorial bundle rather than a concept — a signal worth acting on before the term enters a taxonomy or metadata scheme.

Genus and differentia vs. extensional definition

A genus–differentia definition is intensional: it states the characteristics an object must have to fall under the concept. An extensional definition instead lists what falls under the term — its species or its individual members, as in "protective device: fence, interlocking guard, light curtain, pressure-sensitive mat". Lists are often easier to grasp and are useful as a complement, but they go stale as soon as a new member appears and they leave the criterion for membership unstated. Classification and ontology work therefore usually pairs the two: an intensional definition for the concept, examples or enumerated instances for orientation. The distinction between the characteristics of a concept and the set of its instances is treated in intension and extension terms in the companion entry Intension and extension.

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