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Re: RDF* semantics

From: thomas lörtsch <tl@rat.io>
Date: Fri, 30 Aug 2019 11:45:10 +0200
Cc: "public-rdf-star@w3.org" <public-rdf-star@w3.org>
Message-Id: <65D0BAE9-BF40-4D88-8BAA-7D0BD7CBDD0D@rat.io>
To: Olaf Hartig <olaf.hartig@liu.se>


> On 30. Aug 2019, at 10:29, Olaf Hartig <olaf.hartig@liu.se> wrote:
> 
> Hi Thomas,
> 
> On torsdag 29 augusti 2019 kl. 10:18:48 CEST thomas lörtsch wrote:
>> [...]
>> Ah, you are really taking all those little ’that’ words very serious ;-)
> 
> I better do; we are talking about semantics here ;-)
> 
>> [...] your translation, "a person Bob who is of age 23", captures the sense
>> of factualness even better.
> 
> Good.
> 
>>> Therefore, all the triples together seem to say that a person named
>>> Alice claims a person named Bob who is of age 23. My initial example
>>> said something else, namely: person Alice claims *that* person Bob is of
>>> age 23.
>> 
>> Hmm, that *that* again ;-) So you mean the difference between Alice claiming
>> that there exists a "Bob, person, aged 23" and Alice claiming that some
>> already introduced and agreed upon person Bob is "aged 23"?
> 
> While the fact that the person Bob has already been introduced and agreed upon 
> is necessary to make single-statement claims about this person, this is 
> secondary to the main point I keep on trying to make. Again, in my opinion, 
> Kingsley's data cannot be interpreted as you do in your sentence above (person 
> Alice claims "that there exists" a person Bob of age 23). In contrast, since 
> bnode _:b2 represents 'a person Bob of age 23', the :claims triple with _:b2 
> in the object position is to be interpreted as: person Alice claims the person 
> Bob (rather than claiming the existence of such a person). Hence, the verb 
> "claim" here is used with its meaning of demanding ownership instead of its 
> meaning of stating (potentially false) facts. See:
> 
> https://en.wiktionary.org/wiki/claim#Verb

Well, so "claims" has more meanings then that usually assumed in the context of semantic web discussions about reification, provenance etc. However: either Alice claims the existence or ownership of "a person Bob who is of age 23". I don’t see what difference this makes with respect to our discussion about RDF* etc. Anyway I wouldn't want the semantics of some property to have such wide ranging consequences on the meaning of basic structural constructs like a blank node (… amybe a bit too bold a statement - I hope there aren’t any non-marginal counter examples proofing me wrong).

> If you would only want to capture that person Alice claims "that there exists" 
> a person Bob of age 23, then the object of the :claims triple cannot be the 
> bnode _:b2, but instead the object needs to be a graph that contains the three 
> triples that have bnode _:b2 in their subject position. 

That’s quite strong as a requirement. As I said before: what else could a blank node possibly mean then the sum of its attributes? Can you give a convincing answer to that question? And with convincing I mean "obvious", "intuitive", "in wide use". One might want to talk about the blank node *itself* but that is really a corner case and there are much wider gaps in the identification semantics of the Semantic Web that would need a fix first.
I think the other way round - you have to be specific if you want to address the triple, otherwise you address all that’s said about the blank node - is practicable and unsurprising. We have to find idioms that are easy to use and have intuitive defaults. There is never an end to even more precision but that doesn’t scale. 
What I would endorse however is rather an 80/20 style approach like a specific property to talk about the blank node itself - sensible defaults, specific instruments where required. Disambiguating identification is also a case by case problem: identifiers play different roles in different situations. Concise statement attribution could make it feasible to disambiguate those roles when necessary. That would be great.


Thomas



>> Technically that is the difference between talking about a set
>> of triples with the same subject (lines 4-6 in the above example) and a
>> single triple (line 6), right?
> 
> Almost. See above.
> 
> Best,
> Olaf
> 
> 
>>>> [...]
>>>> However I would also like to stress that such modelling is not
>>>> meta-modelling and it is not equivalent to a layer of abstraction
>>>> (vulgo taking one step back) like reification or named graphs.
>>> 
>>> Exactly! That's the point I am trying to make with this example. To
>>> capture the statement that "Alice claims *that* Bob is of age 23," we
>>> need a form of meta-modeling.
>> 
>> And I just wanted to express my endorsement of your position in that
>> respect.
>>>> [...]
>>>> Well, as I’m on it, a shameless plug: I recently posted an unhaelthily
>>>> long mail to this list . That mail started with [...] I wonder if anybody
>>>> bothered to read that sermon.
>>> 
>>> I did ;-)
>> 
>> Great! :-)
>> 
>>> ...and I was planning to respond to it. However, since I am on this list
>>> here in my spare time, I couldn't get to it right away.
>> 
>> No pressure! ;-)
>> 
>> Thomas
>> 
>>> Olaf
> 
> 
Received on Friday, 30 August 2019 09:45:43 UTC

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