- From: Ian Horrocks <horrocks@cs.man.ac.uk>
- Date: Tue, 26 Feb 2002 15:15:36 +0000
- To: www-webont-wg@w3.org, Jeff Heflin <heflin@cse.lehigh.edu>
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". 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]] 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. 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.]] 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.]] 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.]] 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"). 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. 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.]] 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. 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. 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. 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. 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. Regards, Ian
Received on Tuesday, 26 February 2002 10:16:42 UTC