Genus–differentia definition
A genus–differentia definition is a type of intensional definition, and it is composed of two parts:
a genus (or family): An existing definition that serves as a portion of the new definition; all definitions with the same genus are considered members of that genus.
the differentia: The portion of the definition that is not provided by the genus.
For example, consider these two definitions:
a triangle: A plane figure that has 3 straight bounding sides.
a quadrilateral: A plane figure that has 4 straight bounding sides.
Those definitions can be expressed as one genus and two differentiae:
one genus:
- the genus for both a triangle and a quadrilateral: "A plane figure"
the genus for both a triangle and a quadrilateral: "A plane figure"
two differentiae:
- the differentia for a triangle: "that has 3 straight bounding sides."
- the differentia for a quadrilateral: "that has 4 straight bounding sides."
the differentia for a triangle: "that has 3 straight bounding sides."