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Re: REQDOC: New Draft

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Tue, 26 Feb 2002 15:15:36 +0000
Message-ID: <15483.42648.874038.556828@cs.man.ac.uk>
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 GMT

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