Editing Genus–differentia definition (section)

== Structure ==

A genus of a definition provides a means by which to specify an ''[[is-a|is-a relationship]]'':
* A square is a rectangle, which is a quadrilateral, which is a plane figure, which is a...
* A square is a rhombus, which is a quadrilateral, which is a plane figure, which is a...
* A square is a quadrilateral, which is a plane figure, which is a...
* A square is a plane figure, which is a...
* A square is a...
The non-genus portion of the differentia of a definition provides a means by which to specify a ''[[has-a|has-a relationship]]'':
* A square has an interior angle that is a right angle.
* A square has a straight bounding side.
* A square has a...

When a system of definitions is constructed with genera and differentiae, the definitions can be thought of as nodes forming a [[hierarchy]] or—more generally—a [[directed acyclic graph]]; a node that has no [[Directed graph|predecessor]] is ''a&nbsp;most general definition''; each node along a directed path is ''more '''differentiated''''' (or ''more '''derived''''') than any one of its predecessors, and a node with no [[Directed graph|successor]] is ''a&nbsp;most differentiated'' (or ''a&nbsp;most derived'') definition.

When a definition, ''S'', is the [[Directed graph|tail]] of each of its successors (that is, ''S'' has at least one successor and each [[Directed graph|direct successor]] of ''S'' is a most differentiated definition), then ''S'' is often called <span style="white-space: nowrap">''the '''[[species]]'''''</span> of each of its successors, and each direct successor of ''S'' is often called <span style="white-space: nowrap">''an '''[[individual]]'''''</span> (or <span style="white-space: nowrap">''an '''[[wikt:entity|entity]]'''''</span>) of the species ''S''; that is, the genus of an individual is synonymously called ''the&nbsp;species'' of that individual. Furthermore, the differentia of an individual is synonymously called <span style="white-space: nowrap">''the [[Identity (philosophy)|identity]]''</span> of that individual. For instance, consider the following definition:
* ''[the] John Smith'': a human that has the name 'John&nbsp;Smith'.
In this case:
* The whole definition is ''an&nbsp;individual''; that is, ''[the] John Smith'' is an individual.
* The genus of ''[the] John Smith'' (which is "a&nbsp;human") may be called synonymously ''the&nbsp;species'' of ''[the] John Smith''; that is, ''[the] John Smith'' is an individual of the species ''[a] human''.
* The differentia of ''[the] John Smith'' (which is "that has the name 'John&nbsp;Smith'") may be called synonymously ''the&nbsp;identity'' of ''[the] John Smith''; that is, ''[the] John Smith'' is identified among other individuals of the same species by the fact that ''[the] John Smith'' is the one "that has the name 'John Smith'".

As in that example, the identity itself (or some part of it) is often used to refer to the entire individual, a phenomenon that is known in [[linguistics]] as a ''[[pars pro toto|pars&nbsp;pro&nbsp;toto]] [[synecdoche]]''.