- From: Ivan Herman <ivan@w3.org>
- Date: Fri, 08 Feb 2008 15:04:17 +0100
- To: Jim Hendler <hendler@cs.rpi.edu>
- CC: "Web Ontology Language ((OWL)) Working Group WG" <public-owl-wg@w3.org>
- Message-ID: <47AC6161.5090508@w3.org>
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? I just wonder whether that would have any consequence on deployed application. In practice maybe not, but I just do not know... - 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... - 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? 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
Received on Friday, 8 February 2008 14:04:17 UTC