I agree with Peter here. Daniel's statement of "Anything that is a subclass of X, and at the same an instance of Y, where Y is not "class", is problematic." is simply too strong. The classical example is Harry the eagle, and eagle being a species.

The following paper has a much more measured and subtle approach to this question:

http://snap.stanford.edu/wikiworkshop2016/papers/Wiki_Workshop__WWW_2016_paper_11.pdf 

I still think it is potentially and partially too strong, but certainly much better than Daniel's strict statement.



On Mon, Jan 9, 2017 at 7:58 AM Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
On 01/09/2017 07:20 AM, Daniel Kinzler wrote:
> Am 09.01.2017 um 04:36 schrieb Markus Kroetzsch:
>> Only the "current king of Iberia" is a single person, but Wikidata is about all
>> of history, so there are many such kings. The office of "King of Iberia" is
>> still singular (it is a singular class) and it can have its own properties etc.
>> I would therefore say (without having checked the page):
>>
>> King of Iberia    instance of  office
>> King of Iberia    subclass of  king
>
> To be semantically strict, you would need to have two separate items, one for
> the office, and one for the class. Because the individual kinds have not been
> instances of the office - they have been holders of the office. And they have
> been instances of the class, but not holders of the class.
>
> On wikidata, we often conflate these things for sake of simplicity. But when you
> try to write queries, this does not make things simpler, it makes it harder.
>
> Anything that is a subclass of X, and at the same an instance of Y, where Y is
> not "class", is problematic. I think this is the root of the confusion Gerards
> speaks of.

There is no a priori reason that an office cannot be a class.  Some formalisms
don't allow this, but there are others that do.  Some sets of rules for
ontology construction don't allow this, but there are others that do.  There
is certainly no universal semantic consideration, even in any strict notion of
semantics, that would require that there be two separate items here.

As far as I can tell, the Wikidata formalism is not one that would disallow
offices being classes.  As far as I can tell, the rules for constructing the
Wikidata ontology don't disallow it either.

Peter F. Patel-Schneider
Nuance Communications

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