[1] 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 "form" in Plato's dialogues but should be taken to mean "species" in Aristotle's corpus.
For instance, consider the following: A part of that definition may be singled out (using parentheses here): and with that part, an abstraction may be formed: Then, the definition of a square may be recast with that abstraction as its genus: Similarly, the definition of a square may be rearranged and another portion singled out: leading to the following abstraction: Then, the definition of a square may be recast with that abstraction as its genus: In fact, the definition of a square may be recast in terms of both of the abstractions, where one acts as the genus and the other acts as the differentia: Hence, abstraction is a means of simplifying definitions.
In such a case, it is notationally convenient to consolidate the definitions into one definition that is expressed with multiple genera (and possibly no differentia, as in the following): or completely equivalently: More generally, a collection of
[citation needed] Thus, the following: could be recast as: A genus of a definition provides a means by which to specify an is-a relationship: The non-genus portion of the differentia of a definition provides a means by which to specify a has-a relationship: 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 predecessor is a 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 successor is a most differentiated (or a most derived) definition.
For instance, consider the following definition: In this case: 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 synecdoche.