Hi everybody,

while I understand that there are cases, where it is not the best idea, at least at the data entry point, to decide if something is a class or an individual, like in the case of car brands, models, etc. I don't understand why the obvious individuals, such as (instances) of people, countries or cities, like Bill Gates, United States of America or New York, should not be marked as such. Obviously they cannot have instances and they cannot be subclasses of "classes" (whatever that means) and IMHO automatically verifying these constraints could be beneficial, the same way verifying predicate constraints makes sense in Wikidata.

Please explain if there are any plans to encode that information in Wikidata or provide a rational for not doing so.

Kind regards,
Aleksander Smywinski-Pohl

---- Wł. So, 03 sty 2015 15:22:27 +0100 Markus Krötzsch<markus@semantic-mediawiki.org> napisał(a) ----
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,

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,
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.



> 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 <emw.wiki@gmail.com
> >:
> 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
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