- From: Ian Horrocks <horrocks@cs.man.ac.uk>
- Date: Thu, 28 Feb 2002 14:56:18 +0000
- To: Jeff Heflin <heflin@cse.lehigh.edu>
- Cc: www-webont-wg@w3.org
On February 27, Jeff Heflin writes: > Ian, > > Please see my responses below... > > (Note, apparently these comments concern the older draft of the > document. This may be my fault, because my message about the new version > along with the new URL only made it out about an hour before this > message was sent. Note that the current document is at: > http://km.aifb.uni-karlsruhe.de/owl/index.html) > > Ian Horrocks wrote: > > > > Here are some comments on the revised text. > > > > 0. The term "definition" is often used to refer to statements in an > > ontology, e.g., in 3.1 "to provide additional definitions", in > > 5.Commitment to ontologies "which set of definitions", 5. Class > > definition primitives, etc. Are we suggesting that all statements in > > an ontology are "definitions". What about statements of the form > > sameClassAs C1 C2, where neither C1 nor C2 is a class name? I think > > that each use of "definition" should be examined and, in most cases, > > changed to something more neutral such as "axiom" or "statement". > > I can understand your objection to word "definition." I think > "statement" would be too vague. "Axiom" is a possibility, but I fear > that there might be a number of readers who don't have the background in > logic to understand what we mean here. Can someone suggest a better > term? As I said in my reply to Guus, "asserted fact" might also be a possibility. > > > 1. The second bullet point at the end of section 2.2 says "The search > > should be able to utilize part structure of objects to return better > > search results". I don't understand what this means. Does "part > > structure of" refer to partonomies (i.e., part-whole relations used to > > specify the physical structure of objects)? Or does it mean "part of > > the structure of". Assuming the latter, I would suggest changing the > > wording to: > > > > [[The search should be able to utilize (part of) the structure of > > objects to return better search results]] > > I believe, that "part-whole" relations is what is intended. The new > version of the document says "utilize the part structure." We will > consider clarifying this. It certainly needs to be clarified. I'm still not sure I believe your interpretation - why should search only (or particularly) utilise part-whole structures in preference to any other information in the ontology? > > > 2. In 3.2, justification, it says "Both compatible and incompatible > > revisions should be allowed, but it should be possible to distinguish > > between the two.". It isn't clear if compatibility is to be computed > > (e.g., by comparing ontologies) or asserted (e.g., by the author of > > the revision). I don't know if it is possible/worthwhile to clarify > > this point. > > I think in most real-world cases, semantic backward compatibility cannot > be computed, because it depends on the intended interpretations of the > terms. The last sentences "... the ontology author needs to be able to > indicate such changes explicitly." argues for the "asserted" case. OK - new text is better. > > > 3.In 3.5, justification, it says "There are over one billion pages on > > the Web, and the potential application of the Semantic Web to embedded > > devices and agents poses even larger amounts of information that must > > be handled. The web ontology language must support systems that can > > scale to these sizes." There is no differentiation here between > > ontologies and data/markup/annotations using terms from > > ontologies. Surely no one is suggesting that size of ontologies will > > be of the order of billions of pages? Moreover, the ontology language > > itself provides no mechanism for adding annotations - we are relying > > on RDF (or some similar mechanism) for that. What applications will do > > with semantic markup is as yet rather unclear (to me at least), and > > may vary widely between different applications. > > > > One thing that is clear is that it will not be possible to perform > > reasoning across the entire web w.r.t. the full semantics of any > > language that even remotely resembles DAML+OIL or comes anywhere close > > to satisfying the requirements set out in this document, and it > > doesn't seem to make much sense to state that as a requirement (or > > even a goal). > > > > How about changing the paragraph to say something like: > > > > [[There are over one billion pages on the Web, and the potential > > application of the Semantic Web to embedded devices and agents poses > > even larger amounts of information that must be handled. The > > ontologies needed to describe this information can be expected to be > > both large and complex. The web ontology language must therefore > > support very large ontologies, but it should also be as expressive as > > possible, so that users can state the kinds of knowledge that is > > important to their applications.]] > > The thing about the Semantic Web that I find most exciting is the > possibility of combining information from multiple, distributed, > seemingly unrelated resources in order to produce new knowledge or > answer difficult questions. This would require reasoning across large > portions of the Web. Although, we cannot have a complete DAML+OIL > reasoner that can accomplish this, we may be able to design various > incomplete reasoners that could perform the task satisfactorly. In any > case, I consider this to be an important goal (but not a requirement!). > However, perhaps I should change > the wording from "The web ontology language must support systems that > can scale to these sizes" to "The web ontology language SHOULD support > reasoning systems that scale well." A definite improvement. > > 4. Section 4, "Class and property equivalence". I would delete > > "definitions" from the text - it is meaningless w.r.t. the kind of > > language we are considering. The text should just say > > [[The language must include features for stating that two classes or > > properties are equivalent.]] > > Okay. Change accepted. > > > 5. Section 4, "Identifier equivalence". This is really just an > > extension of the previous point, which could be changed, e.g., to say: > > > > [[Class, property and individual equivalence: The language must > > include features for stating that two classes, properties or > > individuals (objects) are equivalent.]] > > Although I see your point, I think there is some value to keeping them > separate. In this case I think you should refer explicitly to individuals in order to avoid tautology and/or possible confusion. > > > 6. Section 4, "Ability to state closed worlds". I think that this is > > still much too vague to be included as a requirement and should be > > changed to being an objective (as Frank says, "there is clearly work > > to be done here"). > > In the current document, it already is an objective. Frank's new wording > is available on the mailing list, and assuming no one objects to it, > will be added after the telecon. > > > 7. Section 4, "Classes as instances". Ditto for this requirement. It > > is very vague as stated. Although there may be some cases where it is > > useful to have classes of classes, convincing examples are > > surprisingly few (many of the supposed examples I have seen have been > > ill conceived, and/or are of little benefit in terms of the knowledge > > captured in the ontology). The cost of such a requirement may also be > > very high in terms of the computational complexity of the resulting > > language. Again, I would suggest that this be made an objective rather > > than a requirement. > > The current version of the document has an example to help explain it. > Please see if this helps. It is a good example as far as it goes. My point about this kind of example is that it doesn't really demonstrate the utility of having meta-classes. The main purpose of the class-instance model of KR seems to be to have instances inherit properties from the class that they instantiate. What are the properties of Species are that are inherited by Orangutan? > > > 8. Section 5, "Chained properties". The text here needs > > revision. Composition is better than chaining (it is the standard > > term), and the sentence about "linking the range of one property to > > the domain of another property" is unclear/incorrect. Also, the > > statement about the use of variables may not be correct - it is > > certainly not clear given the vague specification of what variables > > means. I would suggest changing the text to: > > > > [[The language may support the composition of properties in statements > > about classes and properties. An example of the use of property > > composition would be the assertion that a property called uncleOf is > > the same (has the same extension) as the composition of the fatherOf > > and brotherOf properties.]] > > This change sounds okay to me. If no one has an objection, I'll add this > to our change list. > > > 9. Section 5, "Variables". This is so vague as to be meaningless in > > its current form. I'm not sure how to fix it. I would suggest simply > > removing it for now. > > Removing a requirement or an objective is a major change, and would > require group consensus before I would do it. Unless this is a > show-stopper, I suggest tabling the change. > > > 10. Section 5, "View mechanism". This is more or less incomprehensible > > to me, so what chance is there for readers outside the WG. It needs > > significant clarification or deletion. > > Same response as issue 9 above. > > > 11. Section 5, "Arithmetic primitives" and "String manipulation". I'm > > not sure that these are reasonable objectives w.r.t. the language > > itself (although they would clearly be useful in an integration > > tool). Even if they are to be retained, it might make more sense to > > state the requirement in terms on n-ary datatype predicates. > > I think to the greatest extent possible, ontologies should provide > axioms that allow terms to be related to terms in other ontologies. > That is, the axioms used by an integration tool should be shareable. > One way to do this is to publish "integration ontologies." Note that > this is an objective, and not a requirement, because I see this as > something that would be good for the Semantic Web, but I don't expect > us to get to with the first versino of the language. > > > 12. Section 5, "Aggregation and grouping". I would suggest that this > > should be part of the query language (as in SQL) and not of the > > ontology language itself. > > For similar reasons as the one above, I see this as useful for > establishing links between different ontologies. For example, a > corporate level ontology might have a numEmployees relations, while a > Human Resources ontology might have a employee relation. Integrating > these two ontologies would requre the ability to specify that > numEmployees is the count of the employee relation. > > > 13. Section 5, "Definitional constraints on conjunctive types". It > > seems strange to have this as an objective as it is just a special > > case of the requirement for being able to state class > > equivalence/subsumption relationships. > > This particular example was originally raised by Guus Schreiber in his > "art-image collection" use case. His point was although it could be > expressed in DAML+OIL, it required 24 lines and an unintuitve structure. > I think the point is it would be nice to have a simple way to express > this knowledge. I can do it in one (very long) line. Seriously, the verbosity is an RDF problem and not really a DAML+OIL problem - the DAML+OIL axiom stating this fact is similar to the "rule" as written, with any extra verbosity being accounted for by increased precision. (E.g., does style="LateGeorgian" mean that there must be at-least or exactly one style with a string datatype value "LateGeorgian", or no styles with a datatype value that is not "LateGeorgian", or something else? And if it means one of these things, then how do I express the others?). Ian > > > Regards, Ian > > Thanks for your comments. > > Jeff >
Received on Thursday, 28 February 2002 09:57:38 UTC