skos:definition
| - A specialization of SetOrCollection (q.v.). This is the collection of all collections of things. Each Collection is a kind or type of thing whose instances share a certain property, attribute, or feature. For example, Cat is the collection of all and only cats, and BinaryFunction is the collection of all and only functions taking two arguments. The notion of collection is fundamental to the Cyc ontology, and is thus difficult to define in a precise and substantive way. But it is instructive to compare it to the mathematical notion of a set (see Set_Mathematical). Like a set, a collection is an AbstractThing (i.e. aspatial and atemporal) that is not an Individual (cf.), but instead has elements (see elementOf). Unlike a set, however, the elements or "instances" (see isa) of a given collection can vary from context to context, and it is possible for distinct collections to have exactly the same elements (i.e. to have the same extent) with respect to a given context. For example, the (non-reified) collections "Chordate" and "Renate" are co-extensional in the context of the present-day actual world, but have differring extents with respect to hypothetical contexts in which there exist creatures having hearts but not kidneys. This difference is sometimes summarized by saying that sets are "extensional" things and collections are "intensional" things. Many, though by no means all, of the collections reified in the Cyc ontology correspond to natural kinds or classes. It is particulary useful to reify collections that are difficult to define precisely, but about which there are many common-sense rules and other things to state. For example, it would not be worthwile to introduce a constant 'WhiteCat', both because it's easy to define the collection of white cats in terms of more general, already-reified notions (e.g. '(CollectionIntersection2Fn Cat WhiteColor)'), and because there's not much to say about that particular collection. WhiteCollarWorker, on the other hand, is a good collection to have reified, as it is hard to define precisely, yet is a collection about which there are many things to say. Some of the many reified specializations of Collection are FirstOrderCollection, TimeDependentCollection, and ObjectType. (en)
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