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Re: RDF Semantics Editors Draft?

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Sun, 13 May 2012 08:57:10 +0200
Message-ID: <4FAF5B46.5080809@emse.fr>
To: public-rdf-wg@w3.org
A huge +1 to everything Pat says in this email.


Le 13/05/2012 08:45, Pat Hayes a écrit :
>
> Apologies for not getting involved with this thread earlier, I
> somehow missed it while travelling.
>
> I agree that people who know nothing about reasoning and logics may
> find model theory a bit opaque at first. It may be a way of thinking
> that they are not used to. On the other hand, it can be reasonably
> claimed that without a basic grasp of at least the basic ideas of
> interpretation and satisfaction, they will never be able to
> understand how to implement a modern reasoning engine. And I would
> suggest that without this basic grasp, they will never be competent
> ontology engineers. And also that it is not exactly rocket science.
>
> The problem with making the rules normative is that these rules are
> NOT in fact a good way to implement a reasoner. Any sensible reasoner
> will not use rules like se1 or se2, which (as the text explains) will
> generate all kinds of redundant and pointless conclusions, or lg and
> gl, which are much better handled by allowing literals into the
> subject position. But in any case, the whole approach which is
> suggested by these rules and the entailment lemmas, of checking
> entailment by applying rules to exhaustion, is extremely naive and
> cannot even be used for more expressive languages like OWL.
> Encouraging implementors to use this as a viable approach is leading
> them into a dead end. (I now regret even mentioning rules in the
> semantics document, and especially suggesting that they could be the
> basis of an implementation. I had no idea people would take them this
> seriously. I think they should be in a separate doument entirely,
> perhaps as part of the test cases, and no claims should be made as to
> their completeness, and no long and extremely opaque (and flawed)
> completeness proofs should be included, even in an appendix. Nobody
> gives a damn about completeness in any case.)
>
> I am sympathetic to the idea that RDF should simply not have a
> formally defined semantics at all. This would solve all of these (and
> many other) problems at a stroke, and I could get on with other
> things in my life. However, if it does have a semantics, then I don't
> think anything other than a model theory can work. There are too many
> alternative ways to implement a reasoning engine for any one of them
> to be taken as normative; and in any case, model-theoretic semantics
> has been the accepted norm for semantic specifications in
> linguistics, logic, database theory and the theory of computation now
> for about 40 years, and for very good reasons; it seems crazy and
> close to irresponsible to reject it for the foundational language of
> something called the *semantic* web.
>
> For example, who or what determines that a given rule is valid or
> correct? If we put a typo into an inference rule, or simply forget
> some special case, and a later reader notices this and publicises the
> error (as has indeed happened), then what basis do they have for
> claiming that the rule is wrong? If the rule itself is normative,
> then it CANNOT be wrong, even if it sanctions invalid inferences.
> This very notion of validity is a model-theoretic notion (it is that
> the antecedents can be satisfied by an interpretation which makes the
> conclusion false). Without this, we really do not have a *semantic*
> basis for judging the correctness of entailments.
>
> Further detailed comments in-line below.
>
> On May 4, 2012, at 7:27 AM, David Wood wrote:
>
>> On May 4, 2012, at 07:53, Richard Cyganiak wrote:
>>
>>> Hi Peter,
>>>
>>> On 4 May 2012, at 09:52, Peter F. Patel-Schneider wrote:
>>>> Well I think that it would be a horrible idea to deemphasize
>>>> the semantics.
>>>>
>>>> I'm also completely unsure as to what name could be any better
>>>> than RDF Semantics.
>>>
>>> Let's consider who reads the RDF Semantics document, and for what
>>> reasons. From what I gather, we can identify a number of
>>> different cases. I'm sure there are many more:
>>>
>>> 1) Spec writers read it, in order to understand how to make their
>>> specifications compatible with other specifications in the RDF
>>> stack. For example, neither the RDB2RDF WG nor the SPARQL WG
>>> could have done its work without knowing the things formally
>>> specified in RDF Semantics.
>
> OWL could have, but then OWL would have been entirely disconnected
> from RDF.
>
>>>
>>> 2) Implementers of various specs (incl. RDF, RDF Schema, SPARQL)
>>> read it, in order to understand how their systems should behave,
>>> especially with regard to datatypes and RDFS inference.
>
> If they are seeking a guide to how any system should *behave*, then
> they need to understand that semantics is not about behavior but
> rather about truth, satisfaction and consistency, and these ideas
> relate to inference-making only indirectly. They need to grasp the
> relationship between inference and semantics (eg the notions of
> validity and entailment) in order to relate inference behavior to a
> semantic description. If they do not understand this, then they
> probably shouldnt be trying to implement a reasoner. If semantics is
> too opaque, the many cases of entailment and non-entailment in the
> test cases document give a pretty good idea of most of what they
> need.
>
>>>
>>> 3) Authors of RDF Schema vocabularies read it, in order to
>>> understand the consequences of declaring domains, ranges,
>>> subproperties and so on.
>>>
>>> 4) RDF newbies read it, because this is where they happened to
>>> end up after some googling or link-clicking and they don't
>>> understand the big scheme of things yet.
>
> The second paragraph of the document starts "This is one document in
> a set of six (Primer, Concepts, Syntax, Semantics, Vocabulary, and
> Test Cases) ..." and they might then hazard a wild guess that the one
> called "Primer" might possibly be a better place to start.
>
> If they habitually read documents without checking the first page,
> then I really don't think we have any responsibility for what kind of
> a mess they get into, frankly.
>
>>>
>>> How well does the current document serve these needs?
>>>
>>> For 1), I think it works well.
>>>
>>> For 2) and 3), I think the document as it stands does not serve
>>> these readers well, and it could do a *much* better job. We have
>>> reports that such readers tend to find the informative entailment
>>> rules in Section 7 extremely useful, and much of the rest of the
>>> document rather impenetrable. It has thus been suggested that the
>>> entailment rules should be given more prominence. One minimally
>>> invasive way of achieving this would be to just move the words
>>> “normative” and “informative” around a bit. I suppose this is
>>> what Ivan refers to when he says “reorganize to make the rules
>>> normative and deemphasize the model-theoretic semantics”.
>>
>> *Personally*, I agree with this as an implementer of specs and
>> author of vocabularies.
>
> What would it mean to make those rules normative? Would an efficient
> tableax-based reasoner be then illegal? What about an modified
> resolution-based FO reasoner adapted to RDF? What about a rule engine
> which used a different suite of rules (there are many other rules
> that could be used and would give similar results.) What about an
> implementation based on Prolog? It is crazy to try to make one
> (flawed and inefficient) implementation technique be the normative
> definition of a semantic standard.
>
>>> For 4), the document as it stands doesn't work at all, and it
>>> can't, because that's not its purpose. In fact, when newbies try
>>> reading this document, it's pretty much guaranteed to end in
>>> disaster.
>
> That depends entirely on the background that the newbie happens to
> have.
>
>>> I think we all agree that this is *not* the document you should
>>> be looking at in your first encounter with RDF.
>
> If you know nothing about logical methods, inference engines, machine
> inference? Yes, it might not be a good starting point if you are this
> ignorant, indeed. I did try to write some introductory, almost
> tutorial, material in the first sections, but no doubt these are not
> often read.
>
>>> There isn't *too* much we can do about this, but I think that a
>>> title that is a bit scarier to newbies could help. Can we put
>>> something like “model theory” or “formal representation logic”
>>> into the title?
>>
>> This problem could be solved easily with the insertion of some
>> language in the introduction pointing to the Primer in the first
>> paragraph (instead of just the Vocab and Concepts, as it does
>> now).
>
> See above. It refers to the primer in the second paragraph of the
> document.
>
>>
>> Regards, Dave
>>
>>
>>>
>>>> Of course, if the WG wants to make RDF no longer be a formal
>>>> representational logic, then ....
>>>
>>> I thought RDF is a data model?
>
> It is both. Data models ARE formal representational logics (ref.
> Codd.)
>
> Pat
>
>>>
>>> Best, Richard
>>>
>>>
>>>
>>>>
>>>> peter
>>>>
>>>>
>>>> On 04/25/2012 11:27 AM, David Wood wrote:
>>>>> Hi Peter and Pat,
>>>>>
>>>>> The RDF WG briefly discussed the need for an RDF Semantics
>>>>> editors draft at today's telecon.  I am aware that there are
>>>>> a lot of open issues and therefore hard to produce a draft,
>>>>> but perhaps it makes sense to have a single document that
>>>>> lists the issues in one place.
>>>>>
>>>>> In any event, we would like to discuss this at next week's
>>>>> telecon if you can make it.  Thanks.
>>>>>
>>>>> Relevant comments from IRC (no log published yet since the
>>>>> meeting isn't over): [[ ivan: one thing that came up early
>>>>> was discussion to change title of RDF Semantics document,
>>>>> reorganize to make the rules normative and deemphasize the
>>>>> model-theoretic semantics AlexHall @ 11:20 ... think it's a
>>>>> good thing to do but huge amount of editorial work AlexHall @
>>>>> 11:20 cygri: is there an editors draft of RDF Semantics yet?
>>>>> 11:21 [no] 11:21 cygri: given that there are larger changes
>>>>> to the doc, would feel better if there were an editors draft
>>>>> by now. 11:21 guus: suggest we should put it on the agenda
>>>>> for next week ]]
>>>>>
>>>>> Regards, Dave
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>>
>>
>
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-- 
Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
42023 Saint-Étienne Cedex 2
France
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Received on Sunday, 13 May 2012 06:57:54 GMT

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