Re: RDF* semantics

On 9/2/19 7:01 AM, thomas lörtsch wrote:
>> On 2. Sep 2019, at 11:15, Pierre-Antoine Champin <pierre-antoine.champin@univ-lyon1.fr> wrote:
>>
>> Dear Thomas,
>>
>> I have to strongly disagree with you.
>>
>> On Fri, 30 Aug 2019 at 18:16, thomas lörtsch <tl@rat.io> wrote:
>>> We are on the Semantic Web here which in general is not aiming at such subtleties. The semantics of identification in RDF are vague at best. Does https://paris.com refer to the website or the city?
>> The semantics of identification is *not* vague. It is precisely defined in the RDF Semantics [1] specification. You can also refer to the RDF Concepts [2] spec, section 1.3. It clearly talks about "the resource denoted by an IRI" (note the *singular* use of "resource").
> But you have heard of "social meaning", right?


Hi Thomas,

Comments follow...

>
>> I'm not saying that it is always easy to decide what exactly an IRI identifies. Especially with HTTP IRIs, which intuitively identify web resources (i.e. digital objects) rather than persons or city. There has been a long controversy [3] in the community about that, but it has been settled. For example, http://dbpedia.org/resource/Lyon identifies a city, while http://dbpedia.org/page/Lyon identifies the HTML document describing that city.
> I wasn’t aware of that settlement. 


Using HTTP URIs to identify anything has always been in place. Confusion
arises due to stylistic preferences.

For instance, despite the popularity (an my personal proximity) to
DBpedia, http://dbpedia.org/resource/Lyon (identifies a Place) and
http://dbpedia.org/page/Lyon (identifies a document about Lyon, the
Place) aren't my goto examples.

My goto example would be based on leveraging the indexical prowess of
"#" which is why I tend to share examples inline of the form:

@prefix : <#> .

:this :relatedTo :that .

The identities of what's denoted by the sentence subject, predicate, and
object are implicitly unambiguous due to the nature of HTTP URIs
courtesy of the "#" indexical. I even have a complete tutorial about
this matter that can be run from anywhere [1
<https://medium.com/openlink-software-blog/simple-linked-data-deployment-tutorial-a532e568c82f>].

DBpedia offers a pattern for scenarios where disambiguation is achieved
without a "#" indexical while still using an HTTP URI.


> I don’t think it ends the debate about how to disambiguate denotation from indication on the semantic web in general. 


An HTTP URI combines denotation and connotation in a single identifier,
that's its superpower :)

This issue has indeed been closed, but general understanding and
acceptance has remained stubbornly mercurial.


> Note also that further scrutiny (or zealotry?) could demand to disambiguate the web page from the web site, the historic city center from some modern times administrative area etc etc. Note also that the other examples I gave are still unresolved. So, the semantics of RDF are vague in contrast to the subtle differentiations that natural language can express and they’ll always be.


Natural Language is much vaguer than Logic. RDF is about logic (never
English) and it isn't vague about denotation. Confusion does arise (for
reasons explained above) when RDF is deployed using Linked Data
principles (where HTTP URIs come into play).

[SNIP]

>>> If I wasn’t too lazy I would now check the RDF specifications as I’m sure there’s citable proof that blank nodes in RDF are not a means to speak about the shere existance of certain things
>>>
>> RDF Semantics [1],  section 5.1, first sentence:  "Blank nodes are treated as simply indicating the existence of a thing".
>> You couldn't have chose better words -- except for "not", obviously ;-)


Blank Nodes are Indefinite Pronouns. From my vantage point, Olaf jumped
to conclusions about my initial examples by not seeking clarification
about the nature of  predicates used in my sentence examples. That said,
he might have taken that liberty based on the subject of the thread
itself etc..

Personally, I think stating the nature of a predicate used in a sentence
example is extremely important with regards to clarity regarding  both
intention and intuition (that follows).


> No, because you are missing the most important word in that sentence: "simply" :-) This precisely tries to convey the intuition that one shouldn’t interpret too much into blank nodes. RDF is much to simple, simplistic even, to carry the meatphysical load that you and Olaf want it to carry. Blank nodes are used as existential quantifiers with all the logical consequences that come with it but they are also used as mere anchor points to collect a few properties of something that is not meant to get its own URI. A good example is
>  :Bob :hasAdress [ :street "abc-street 1"; :city "Hometown"; :zip 12345 .]


Yes, but your example is always going to be clearer if you state the
nature of :hasAddress (the predicate in your sentence) too i.e., state
its rdfs:domain and rdfs:range .


> Usually nobody would want to get philosophical about that blank node and the sort of existential questions that can arise from it. In other situations they are used exactly for the purpose of discussing existential questions of existance. It depends. I’d say Postels law - "Be liberal in what you accept…" - applies here in slightly modified form: "Be liberal in what you expect a blank node to express". Or, in other words: curb your enthusiasm ;-)
>
> Olaf’s initial critique of Kingsley’s proposal already showed the difficulties that arise when we interpret too much in RDF structures: he interpreted the blank node in the object position different from the blank node in the subject position of Kinglsey’s example.


Yes, but I shouldn't have started with a riddle i.e., I too could have
just stated the rdfs:domain and rdfs:range of the predicate used in my
examples.

Olaf did make a bit of a leap, initially. That said, the topic was about
reification which could be acceptable justification for the leap.


>  It only got worse from there when he drew a conclusion about the range of :claims - the slightly bizarre ownership interpretation - from the syntactic structure of the object alone. This is precisely what I critcize: RDF doesn’t support such subtleties.


RDF actually does handle subtleties, we just need to get in the habit of
making precise examples. That why I go the extra  mile to say:

@prefix rdfs: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix : <#> .

:this :relatedTo :that .

:relatedTo rdfs:domain owl:Thing ;
                 rdfs:range owl:Thing .


Dropping prefixes and the description of an rdf:Property instance that
functions as the predicate in a sentence leads to confusion.


>  It has blank nodes, reification of statement types (but not tokens) and in a hackish way also graphs. How much expressivity can you expect from that on its own? Of course you can model everything with the help of suitable classes and properties but in and on itself RDF is very limited and barely express some structure in the sea of triples. Relying on that structure to transport a lot of meaning leads into dangerous territories of - and that is an empirical fact that you can gather from countless threads like this one on semantic web related mailing lists - undissolvable disputations about the semantics of this, that and "but no I meant *that*". That's the exact opposite of easy interoperability that the Semantic Web is meant to enable.  


The notion of a Semantic Web isn't the problem. Methinks, habit and
tools are the big problem :)

Links:

[1]
https://medium.com/openlink-software-blog/simple-linked-data-deployment-tutorial-a532e568c82f
-- Simple Linked Data Deployment Tutorial


Kingsley

>
> Thomas


>
>>   pa
>>
>> [1] https://www.w3.org/TR/rdf11-mt/
>> [2] https://www.w3.org/TR/rdf11-concepts/
>> [3] https://en.wikipedia.org/wiki/HTTPRange-14
>>  
>> Does a graph name name or label the graph? Is a statement reification referring to a type of statement or a statement token of that type? Cowpath defaults and "social meaning" rule at every level of identification in RDF and in general I think that in its alternativelessness this is a big problem and needs to be solved. However I think it needs to be solved through defaults and on demand disambiguation - late binding if you will - , not through arbitrary descents into semantic rat holes and tedious disambiguation when it isn't needed. 
>> The default mode of the Semantic Web is straightforward exchange of facts like "there's a person named Bob of age 23 and alice, also a person, claims something about him". A blank node is nothing more than a structural helper, the semantic equivalent of a throw away plastic bag (I know we don't do that anymore although we have conferences at the end of the world, 11800 kg of CO2 from northern germany by plane). "There exists..." yadayada. The most intelligent thing you can teach your dumb machine to do is treat these facts at face value. Further subtleties may be encoded in the vocabulary: some specific Alice vocabulary may define alice:claims to have domain slaveholder and range slave as Olaf suggested. 
>> If you want to speak about the fact as it has been stated (provenance etc) you need to reify it. Concise syntax and appropriate semantics would help as I like to keep repeating. To RDF the person and the fact are both just subjects of discourse, resources referenced by URIs. To RDF there is no difference here. Which has the fine property of making endlessly nested meta-meta-meta-... constructs possible - which is probably the least we would need to model this conversation in RDF. 
>> But maybe you are not even suggesting to distinguish indication and denotation but want to speak about something like the factuality of that fact, no matter if or by whom it was stated? I wouldn't be able to follow you there - to dangerous... ;-) 
>>
>>> 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)?
>>
>> That's an orthogonal question. Named Graphs could help if their semantics are properly specified (easy, just define an appropriate vocabulary, and use the RDF extension mechanism to define a suitable semantics). 
>>
>>
>> Thomas
>>
>>
>>>  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
>>>>>
>>>>
>>>>
>
>

-- 
Regards,

Kingsley Idehen       
Founder & CEO 
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