What's an instance?
Each of the entities in a class is said to be an instance of the class. An entity can be an instance of multiple classes, which are called its types.
The class membership relation (called instance-of) that holds between an instance and a class is a binary relation that maps entities to classes. A class is considered to be a unary relation that is true for each instance of the class. That is,
(<=> (holds ?C ?I) (instance-of ?I ?C))The relation type-of is defined as the inverse of relation instance-of. That is,
(<=> (type-of ?C ?I) (instance-of ?I ?C))
Entities that are not classes are referred to as individuals. Thus, the domain of discourse consists of individuals and classes. The unary relation class is true if and only if its argument is a class and the unary relation individual is true if and only if its argument is an individual. The following axiom holds:
(<=> (class ?X) (not (individual ?X)))
See What's a class?
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