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Genus–differentia definition
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{{Short description|Type of intensional definition}} {{More citations needed|date=December 2021}} A '''genus–differentia [[definition]]''' is a type of [[Extensional and intensional definitions#Intensional definition|intensional definition]], and it is composed of two parts: # '''a [[Genus (philosophy)|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" # ''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 use of a genus (Greek: ''genos'') and a differentia (Greek: ''diaphora'') in constructing a definition goes back at least as far as [[Aristotle]] (384–322 BCE).<ref> {{cite book | last1 = Parry | first1 = William Thomas | last2 = Hacker | first2 = Edward A. | title = Aristotelian Logic | series = G - Reference, Information and Interdisciplinary Subjects Series | url = https://books.google.com/books?id=rJceFowdGEAC | location = Albany | publisher = State University of New York Press | publication-date = 1991 | page = 86 | isbn = 9780791406892 | access-date = 8 Feb 2019 | quote = Aristotle recognized only one method of real definition, namely, the method of ''genus'' and ''differentia'', applied to defining real things, not words. }} </ref> Furthermore, a genus may fulfill certain characteristics (described below) that qualify it to be referred to as ''a species'', a term derived from the Greek word ''eidos'', which means "[[Platonic form|form]]" in [[Plato]]'s dialogues but should be taken to mean "species" in [[Aristotle]]'s corpus.
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