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Re: RDF* semantics

From: thomas lörtsch <tl@rat.io>
Date: Fri, 30 Aug 2019 19:03:16 +0200
To: public-rdf-star@w3.org,Olaf Hartig <olaf.hartig@liu.se>,"public-rdf-star@w3.org" <public-rdf-star@w3.org>
CC: Pierre-Antoine Champin <pierre-antoine.champin@univ-lyon1.fr>
Message-ID: <E8E1FCFE-5839-49C2-B13B-1FB131EBB19C@rat.io>


Am 30. August 2019 14:04:32 MESZ schrieb Olaf Hartig <olaf.hartig@liu.se>:
>Thomas,
>
>Just to add to Pierre-Antoine's response, perhaps it is necessary to
>emphasize that you should not get hung up on the fact that the example
>uses blank nodes. That's also secondary to the point I am trying to
>make. So, instead of using blank nodes, we may present the example data
>with two IRIs that denote Alice and Bob, respectively. For instance, if
>we replace _:b1 by IRI http://example.org/Alice and _:b2 by IRI
>http://example.org/Bob, then we have the following snippet of Turtle
>(prefix declarations omitted).
>
><http://example.org/Alice>  rdf:type  foaf:Person .
><http://example.org/Alice>  foaf:name  "Alice" .
><http://example.org/Alice>  :claims  <http://example.org/Bob> .
><http://example.org/Bob>  rdf:type  foaf:Person .
><http://example.org/Bob>  foaf:name  "Bob" .
><http://example.org/Bob>  foaf:age "23"^^xsd:integer .
>
>Perhaps now it becomes more apparent that, according to this data,
>person Alice claims the person Bob (who is of age 23). This is
>different
>than saying that person Alice makes the following claim: the thing
>denoted by IRI is a person named Bob who is of age 23.

No, it's the same. The situation may change when you want to describe Alice's claiming activity. Claiming something as such is no different from liking, being located in etc. You wouldn't insist on putting the objects of such statements in parentheses. Something like

    ex:Alice ex:likes [ a foaf:Person; foaf:name "Bob" ] 

would be perfectly alright, wouldn't it? 


Actually we had a similar discussion already a few mails back and we agreed (I think) that you can model everything with pure triples. You don't *need* meta modelling (I never thought I would write that one day) if you nest your triples deep enough, although abstraction through meta modelling certainly can be very helpful. If the conversation itself is about meta modelling then it can be had with pure basic triples. 


Cheers, 
Thomas 


>Best,
>Olaf
>
>
>On Fri, 2019-08-30 at 13:43 +0200, Pierre-Antoine Champin wrote:
>> Dear Thomas,
>> 
>> 
>> my 2c to that precise question:
>> > what else could a blank node possibly mean then the sum of its
>> attributes? 
>> 
>> 
>> 
>> The sentence "the sum of its attribute" is highly ambiguous... so the
>> answer to your question really depends on what you actually mean by
>> that.
>> 
>> 
>> 
>> First, let me rephrase in a slighly different way what Olaf meant
>when
>> insisting on the word "that". "a person Bob of age 23" does not mean
>> the same thing as "the fact that Bob is a person and has age 23". One
>> of them is a person, the other one is a fact. I hope we agree that
>> facts and persons are two disjoint classes. Which of them is,
>> according to you, "the sum of the attributes"?
>> 
>> 
>> 
>> Second, RDF semantics is defined under the Open World Assumption:
>> whatever you know about a given node, you have to assume that there
>> *may* be other triples about that node that you are not aware of. So
>> by the "sum of its attributes", do you mean "everything that is true
>> about the node, whether you know it or not" (which would be
>consistent
>> with the OWA), or "everything that is stated in a given graph" (which
>> would seem more appropriate for representing a give claim by Alice)?
>> 
>> 
>> 
>>   pa
>> 
>> 
>> On Fri, 30 Aug 2019 at 11:46, thomas lörtsch <tl@rat.io> wrote:
>> 
>>         
>>         
>>         > 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 17:03:48 UTC

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