On 31.12.2014 16:18, Thomas Douillard wrote:
Not sure either it's writeable as punning imply to treat the class/individual as different things ...
TL;DR: This subtlety is important for powerful ontology modelling languages such as OWL, but we don't need to worry about this in Wikidata. Even if classes and individuals are kept distinct for technical purposes, a modeller may of course consider an entity as a single conceptual thing that has a "class nature" and an "individual nature" in different contexts.
It is true that the class and the individual are "different things" from a technical perspective when it comes to defining the formal semantics in the W3C OWL ontology language (under the common Direct Semantics). However, this has very little or no effect in Wikidata. The reason is that the things you say about the individual normally have no bearing on what you know about the class, and vice versa. In other words: whether we consider the class to be different from the individual or not will not have any effect on system behaviour.
The reason why we need to care about these subtleties in OWL is that OWL has features that we do not have in Wikidata. The main feature that matters here is equality of individuals. In OWL, you can say that two identifiers refer to the same semantic individual (owl:sameAs). If the individual and the class are treated as one, then "A sameAs B" would imply that A and B are also the same classes (with the same instances, the same subclasses, etc.). It is possible to define a semantics in this way, but for a powerful ontology language such as OWL, it makes reasoning (query answering, consistency checking, etc.) undecidable. In other words, there is no algorithm that can return exactly the correct answers to all fact queries under this semantics.
The reason why this is such a problem is that OWL has more involved ways of saying that two individuals are the same. OWL has /number restrictions/ on relations, for example, to say that "a person has at most one biological mother". In contrast to the constraints we have in Wikidata, these number restrictions will not be "violated" if you specify more individuals (like a person with two mothers). Instead, an OWL system would infer that the two individuals you specify must be the same. In this way, you get additional sameAs relationships, sometimes only after a rather complicated reasoning process.
If you want to know more background information and a comparison of two possible decidable semantics (one of which is what we now call "punning"), you should have a look at Motik's paper: "On the Properties of Metamodeling in OWL." Journal of Logic and Computation, 17(4):617–637, 2007, http://www.cs.ox.ac.uk/boris.motik/pubs/motik07metamodeling-journal.pdf
For a discussion on why an unrestricted metamodelling is generally problematic when supporting more expressive languages, a classic read would be Patel-Schneider's paper: "Building the Semantic Web Tower from RDF Straw." Nineteenth International Joint Conference on Artificial Intelligence. Edinburgh, Scotland, August 2005, http://ect.bell-labs.com/who/pfps/publications/fol.pdf Indeed, it is very hard to combine such meta-modelling with OWL, and the OWL RDF-Based Semantics ("OWL Full") had to be revised substantially in OWL 2 to ensure that the specification is at least consistent (= not contradictory in itself). You will have similar issues in other powerful languages, including SWRL, if you allow for certain metamodelling.
Nevertheless, the official policy of the W3C is still that every URI refers to "one thing". Even if this thing has a "class nature" and an "individual nature" that are kept separate by reasoning systems, we can still view these different natures as different aspects of the "same thing" (conceptually speaking). This could also be a sound approach for Wikidata if we ever get to a level of expressivity where we need to keep individuals and classes technically distinct like in OWL.
Cheers, Markus
tried to dig if it is possible in SWRL (see http://dior.ics.muni.cz/~makub/owl/ for example), seems not so easy either, found this topic on semanticweb.com <http://semanticweb.com> (google cache <http://webcache.googleusercontent.com/search?q=cache:dAcXbpivQJoJ:answers.semanticweb.com/questions/26553/property-chains-or-swrl-rules-with-subclassof-and-type+&cd=1&hl=fr&ct=clnk&gl=fr&client=iceweasel-a>, the site seems down ATM, original URL : swrl rules with subclass of and type <http://answers.semanticweb.com/questions/26553/property-chains-or-swrl-rules-with-subclassof-and-type>) which is related to the problems I faced and seem to imply it needs to be done in SPARQL. Happy Q11269 either ;) 2014-12-31 14:59 GMT+01:00 Emw <[email protected] <mailto:[email protected]>>: Automobile (Q1420) had the claims [1]: /subclass of/ motor road vehicle /instance of/ motor road vehicle That was incorrect. An instance of motor road vehicle is something like the Peekskill Meteorite Car (Q7756463) [2]. It is generally incorrect when an item has /instance of/ and /subclass of/ claims with the same value. I am not aware of a Wikidata constraint template which can encode that rule. (Off hand I'm not sure how it would be encoded in OWL, either. Ontology experts: how would we do that?) If we wanted use both /instance of/ and /subclass of/ in automobile, then we would need to do something like: /subclass of/ motor road vehicle /instance of/ motor road vehicle class In my opinion, /instance of/ claims like that are not very useful, because they simply restate what is directly implied in the /subclass of/ claim. Punning that is not a mere rephrasing can be useful, e.g. Chevrolet Malibu (Q287723) [3] /"subclass of/ mid-size car, /instance of/ car model". See also Markus's comment from September about using /subclass of/ and /instance of/ in the same item, which conveniently also discusses automobiles [4]. Happy Q11269! Eric https://www.wikidata.org/wiki/User:Emw 1. https://www.wikidata.org/w/index.php?title=Q1420&oldid=184512429#P279 2. https://www.wikidata.org/wiki/Q7756463 3. https://www.wikidata.org/wiki/Q287723 4. https://lists.wikimedia.org/pipermail/wikidata-l/2014-September/004649.html _______________________________________________ Wikidata-l mailing list [email protected] <mailto:[email protected]> https://lists.wikimedia.org/mailman/listinfo/wikidata-l _______________________________________________ Wikidata-l mailing list [email protected] https://lists.wikimedia.org/mailman/listinfo/wikidata-l
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