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Re: OWL Full proposal (sort of) - addressing my Action

From: Jim Hendler <hendler@cs.rpi.edu>
Date: Fri, 8 Feb 2008 16:51:10 -0500
Cc: "Web Ontology Language ((OWL)) Working Group WG" <public-owl-wg@w3.org>
Message-Id: <4EFEE71D-3C74-43CF-A155-AD9143314ACE@cs.rpi.edu>
To: Ivan Herman <ivan@w3.org>


On Feb 8, 2008, at 9:04 AM, Ivan Herman wrote:

> Jim,
>
> thanks. I would like to understand the consequences, so let me ask  
> some questions for clarification...
>
> - If my understanding is correct, in your proposal the RDF  
> compatible semantics of OWL, ie,
>
> http://www.w3.org/TR/2004/REC-owl-semantics-20040210/rdfs.html
>
> would become, sort of, deprecated (I am not looking for the exact  
> 'process' term here). It would certainly not be used and extended by  
> this group when adding new features like the ones proposed by the  
> OWL1.1 submission. Is that correct?

my understanding is that that was being "deprecated" as the new OWL  
1.1 semantics replaces that document - so we as a WG would need to  
decide whether we wanted to do something like that for OWL Full or  
something different.  My idea was that we could do something that  
would be less work, and just as useful to the community.  I would,  
indeed, expect that whatever we produced would need to (sorry, I have  
to say it) comply with our charter - i.e. would try to have maximal  
backward compatibility and would have to be compatible with the RDF  
semantics.


>
>
> I just wonder whether that would have any consequence on deployed  
> application. In practice maybe not, but I just do not know...

as best I can tell, and obviously I cannot prove it for all  
implementations, I think that people in the Web applications community  
(and here I only mean those people in that community who are not  
worrying about  OWL DL) have tended to use the "operational" stuff  
(semantics and test cases) more than that formal model - I think if we  
stay as backwards compatible to the effects as possible  (i.e. we  
might change how we describe the use of "sameAs", but the entailments  
wlll still be the same), we would not hear a lot of complaints - of  
course, that would need to be tested, and would be when we released WDs

>
>
> - What would be the more 'formal' relationship to RDF/S? RDF/S is  
> defined in terms of model theory. If course,
>
> http://www.w3.org/TR/rdf-mt/
>
> includes a bunch of entailment rules which are the type of axiomatic  
> rules that you refer to, but they are only informative  
> (formally...). We may live with that, but I just wanted to be sure...

see above, IMO, we would use the RDF semantics as the base (as we do  
in the current OWL Full) and only have to define the behavior of the  
OWL constructs

>
>
> - I would expect (but, again, maybe I misunderstand you) that the  
> axiomatic logic formulae should be formally described for each term  
> (old an new) somewhere. I guess that is important for  
> interoperability. Or do you envisage to do that only for the various  
> fragments like the OWL Full 'version' of OWL Prime only?


I was not saying we must use an axiomatic model, just using that as an  
example - I think even a well written reference might be enough (with  
some test cases),  For OWL Prime, I know it is doable and not too  
hard, I don't know for all of OWL Full what would be involved - again,  
I think the key thing here would be the clarification of the Full v.  
DL semantic relation being looser than it was - but I don't think that  
will change Full very much, it really means that some of the new  
features we add (I think particularly of the "type"ing vocabulary)  
could be available, but primarily intended for DL, meaning that Full  
users would not get as precise a definition as DL users would (but  
also wouldn't be restricted to use these unless they found value in  
it, where DL users might need to use them if they wanted the  
completeness guarantee(
>
>
>
> Ivan
>
> Jim Hendler wrote:
>> Before I get to something resembling a proposal, I first want to  
>> outline my "ideal vision" (which I know is not the same as some  
>> other people's but at least it makes it clear where I'm coming  
>> from), then "define" what I mean by a couple of the terms with  
>> respect to that vision, and then make the proposal  and provide a  
>> justification-- so if you don't care about anything but the latter,  
>> drop to the bottom.
>> Ideal world:  If I could start all over again, there would be one  
>> language called OWL.  The language would be defined as a set of  
>> vocabulary terms "owl:xxx" and the definition of an OWL document  
>> would be any RDF document (by whatever definition of document one  
>> uses) that includes the use of those terms.  The language would be  
>> aimed at being a good common denominator for the definition of  
>> ontologies, not aimed at being a good KR language for AI use (i.e.  
>> it's main function would be interoperability, more precise and  
>> expressive KR languages would use it for exchanging at least some  
>> of their information where possible). The language would be  
>> primarily aimed at "modeling", not reasoning, and thus the  
>> semantics could be less formal than it is now.
>> Attached to OWL would be a privileged fragment called OWL DL, which  
>> would have a very clean semantics.  OWL DL would be intended for  
>> people whose use of OWL fits one of the important niches OWL can  
>> handle - which is classification reasoning (via consistency  
>> checking).  Folks looking for an expressive common denominator with  
>> good reasoning tools would be drawn towards OWL DL, and they would  
>> find that most of the documents they created in OWL either already  
>> worked well with OWL DL tools, or needed a small amount of work to  
>> get there.  There could also be a number of special extensions to  
>> the OWL vocabulary (lets call it owldl:xxx) that could be used  
>> where they were needed for making reasoners better.  The semantics  
>> of this fragment, which need to be extremely clear and clean, would  
>> be very formal.  Syntaxes for developing documents in this language  
>> would still need to talk to RDF, since they want to provide URIs  
>> that would be of use in the Semantic Web (so regular OWL users  
>> would have benefit of the clean ontologies built in OWL DL)
>> There could also be other communities that come to the W3C, or  
>> which work in other forums, and create other priviledged versions  
>> of OWL for other KR approaches or for other purposes. These  
>> fragments would be defined as vocabulary fragments (i.e. the  
>> document could contain only these terms) or they could have  
>> specific semantics -- OWL DL would be an obvious candidate for the  
>> semantic base of these, since that would greatly enhance reuse.
>> Some (loose) definitions:
>> Operational semantics:  I use Java and C all the time.  I have  
>> never seen any semantics for them other than an "operational  
>> semantics" -- that is one that says what the intended use of the  
>> language constructs is.  I also have books on Prolog and Lisp and  
>> Scheme, which have something closer, but in fact when I implemented  
>> my own prolog once upon a time, the formalism wasn't really what it  
>> is today - I used a document that said what the various features of  
>> the language were and how they interacted.  These semantics, like  
>> the ones in Java and C were written in a language called English.   
>> In fact. most languages whether they be programming languages,  
>> markup languages, or modeling languages are defined against this  
>> kind of semantics.  What is called a "reference" (for example, the  
>> "programming in C" book or the "Moonual" for Lisp) is generally the  
>> user guide to understanding these operational semantics.
>> A fallacy about operational semantics: Some on this group keep  
>> telling me that the difference is that OWL is not a programming  
>> language.  I happen not to agree, but even if you take that view, I  
>> point out most other languages including HTML and XML (and, in fact  
>> SQL) have these semantics.  (SQL also has a mathematical  
>> underpinning, but it includes a number of features that are not  
>> covered by that, and are only explaining operationally - what we  
>> would call non-semantic features).  The fallacy has been that if  
>> one says "OWL" and means "OWL DL" then they are right that the  
>> language is not like a programming or markup langauge, it is a KR  
>> language and that does indeed need a formal semantics.  So what I  
>> call OWL in the above is the main language, and if it has any  
>> semantics at all, it should be operational (and maybe we have to  
>> abide by the fact that RDF has a semantics - but that was a mistake  
>> in my opinion).
>> An axiomatic semantics - the term has been used in a number of  
>> different ways.  I think Boris and I came close to agreeing that  
>> many things in OWL and OWL DL could be defined in a fairly straight  
>> forward way against traditional rules of logic.  This is using  
>> logic as the "spec" for the operational semantics -- that is, we  
>> could say  (making up a simple example)
>> A owl:sameInstanceAs B <=> forall(x) P(A,x) -> P(B,x)
>> there was one of these done in KIF for DAML, so it is pretty clear  
>> that a great amount of what is in the current OWL Full could be  
>> defined this way.  Note that this does not mean one necessarily  
>> should, or could, implement the language by simply inputting these  
>> to a logic engine (although that is probably possible for at least  
>> the great bulk of the language) but rather that the axioms are  
>> being used to provide a more precise description than NL does - but  
>> I would still claim this is primarily an operational semantic  
>> approach.
>> OK - my proposal.
>> I believe we can actually reach the ideal world above with one  
>> exception - the language called "OWL" is going to have to be called  
>> OWL Full for historical reasons.
>> Argument
>> We would need to update the OWL reference, which pretty much  
>> contains the operational semantics for OWL Full to include those  
>> new constructs we think are of use, and we can remove the part  
>> about the restrictions for OWL DL (because there is no longer the  
>> need for the 1-1 mapping) - so the OWL 1.0 reference would be  
>> extended into the OWL Full document.
>> We would define OWL DL precisely as we have already and, in my  
>> opinion, other than cleanup and resolving the OWL DL only issues,  
>> much of the complexity would disappear - because we could have  
>> features (and syntax) that are particular to the needs of reasoners  
>> that extend the above.  The semantics would be aimed at reasoners  
>> so and would be based on the current model theory.
>> Interesting observation
>> Taking this approach has some nice features:
>>  1 - we are almost done (we would need to decide which OWL DL  
>> features need to be added to the OWL Full reference and write them  
>> up)- I guess we would still need to write a UCR document, but that  
>> we have in our charter whichever way we go (in fact, we could edit  
>> the old UCR document into the "OWL Full UCR" document, and just add  
>> use cases for the OWL DL specific features - so that would also be  
>> easier)
>>  2 - we would be compliant with our charter -we would be defining  
>> both Full and DL, the feature mapping would be clear where needed.   
>> Features and syntax of DL that seem not to have Full realization  
>> would with (a) be added or (b) be declared as "extensions" that are  
>> not needed in Full.
>> 3 - We could have fragments like OWL Prime that are defined for OWL  
>> Full easily.  If we want corresponding fragments in OWL DL, we need  
>> to do the semantic work, but that is relatively straightforward.   
>> Fragments of DL, like those in the current fragments document,  
>> would also be easily derived as they wouldn't necessarily impact  
>> the Full world.  (Folks who want mappings from OWL to provably  
>> polynomial fragments would either use the OWL DL semantics, or  
>> could invent their own - we don't have to do that work for them)
>> hmm, looks like my ideal world might be closer to realizable than I  
>> thought...
>> I look forward to being torn to shreds since I know this was  
>> written in a language very foreign from that of many in the WG.  On  
>> the other hand, when we invented Web ontologies at Maryland (SHOE  
>> claims earliest use of the term), this is the vision I had in mind,  
>> so I guess I'm at least consistent....
>> cheers
>> JH
>> "If we knew what we were doing, it wouldn't be called research,  
>> would it?." - Albert Einstein
>> Prof James Hendler                http://www.cs.rpi.edu/~hendler
>> Tetherless World Constellation Chair
>> Computer Science Dept
>> Rensselaer Polytechnic Institute, Troy NY 12180
>
> -- 
>
> Ivan Herman, W3C Semantic Web Activity Lead
> Home: http://www.w3.org/People/Ivan/
> PGP Key: http://www.ivan-herman.net/pgpkey.html
> FOAF: http://www.ivan-herman.net/foaf.rdf

"If we knew what we were doing, it wouldn't be called research, would  
it?." - Albert Einstein

Prof James Hendler				http://www.cs.rpi.edu/~hendler
Tetherless World Constellation Chair
Computer Science Dept
Rensselaer Polytechnic Institute, Troy NY 12180
Received on Friday, 8 February 2008 21:51:35 GMT

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