Relation:Is a

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The relations is a was introduced to describe that an object belongs to some class. However, since Semantic MediaWiki incorporates MediaWiki categories, the use of "is a" is now deprecated. Instead, if you want to say that something is of a particular kind, just write [[Category:Something]] within the according article.

Discussion (somewhat historical)

Effects of having "is a"

  • (+) Simpler Syntax because all Properties are in the Wikipedia Namespace.
  • (-) All Elements of XML Schema, RDF, RDFS, OWL, OWLS, and some others, would need to be Part of MediaWiki. This blows up the Wikipedia Namespace.
  • (-) If people specify directly whether they want to use RDFS or OWL or whatever, then Wikipedia becomes dependent on these particular technologies. It would be preferrable if relations correspond to a fixed intuitive meaning within the wiki, and are later mapped to whatever technical formalism is considered appropriate for applications. Of course, it must be clear what "is a" means. But the according "instance of" from RDFS and OWL are still quite incompatible in many sitiuation (having a totally different formal semantics), and it would be a pity if we end up in a two-worlds scenario of people using OWL and people using RDFS. For "Category:", both RDFS and OWL would be applicable without violating the intuitive semantics, and everybody who uses Wikipedia's data is free to choose which language to use.
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