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Re: SEM: Light review of semantics document

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Thu, 7 Nov 2002 13:25:46 +0000
Message-ID: <15818.27098.314534.637923@merlin.horrocks.net>
To: pat hayes <phayes@ai.uwf.edu>
Cc: Jeff Heflin <heflin@cse.lehigh.edu>, www-webont-wg@w3.org

On November 6, pat hayes writes:
> 
> >Here's some initial comments on the Semantics document dated Nov. 3:
> >
> >1) Sect. 2.2. The syntax needs the ability to represent documents that
> >consist soley of facts (that is, something other than ontologies).
> 
> ? Can you explain what you mean by "other than ontologies" ?Do you 
> mean, not in OWL?
> 
> >Perhaps a top-level <content> or <data> production could be added. The
> >most import reason for this is that content documents (which are not
> >ontologies), will also need to be represented. Assuming imports is not
> >postponed, such documents will also need to contain imports directives.
> >
> >2) In the abstract syntax we have EquivalentClasses,
> >EquivalentProperties but SameIndividual. We should change them all to
> >the same basic form. Since sameXXXas is what is used in the exchange
> >syntax, I suggest: SameClass, SameProperty, and SameIndividual.
> >
> >3) Section 3.4: The discussion of imports does not take into account
> >documents that are not ontologies. I had a proposal (that I thought you
> >agreed to), that fixed this and other problems. Is this an oversight, or
> >are you waiting for the group to resolve the issue before making a
> >change.
> >
> >4) Sect. 4.1: The conditino for n-triple form seem overly restrictive.
> >Is this just meant for OWL/DL?
> >
> >5) Same section: The list of URI references that should not be mentioned
> >should include owl:imports (assuming it is not postponed)
> >
> >6) Sect. 5.2: I'm hesitant about all of the iff definitions. For
> >example, isn't iff for TransitiveProperty putting an undue burden on
> >reasoners? I understand that you can only infer something is an
> >owl:TransitiveProperty if it is transitive in all models, but it seems
> >that you might be able uses cardinalities to restrict a property to a
> >certain number of tuples and then list all of these tuples. In such a
> >case, wouldn't complete reasoners always have to run through at the
> >tuples to determine if the property was transitive? Seems like an
> >expensive operation to me and I don't really see the utility of it.
> >
> >Also, as I was reading the following thing occured to me, which is not
> >specific to the semantics document:
> >
> >The notion of complementOf in an open-world like the Semantic Web
> >worries me.
> 
> RIGHT!! I have been complaining about this ever since it was first 
> put into DAML. We should have a relative complement construction as 
> basic, rather than an absolute one, since we never know when one 
> ontology's universe is the same as another's.

I don't see the problem. A fact x is entailed just in case x is true
in all possible universes. An ontology constrains the set of possible
universes.  I don't see what you mean by an "ontology's universe" and
I don't see how the use of ontologies on the web is any different from
the use of ontologies in any other context.


> >We can never compute the complement because we can never
> >know the entire set of resources. I guess this will be used in rules in
> >such a way that we never actually need to know the extension of the
> >complement of a class, but are we sure there are no problems lurking
> >here? Is this something that should be a new issue, or have people given
> >this enough thought and decided its okay?
> 
> No, they havn't, and its not OK, and it needs some thought.

As above, how is this any different from standard logic? The MT makes
use of interpretations which include arbitrary sets, the contents of
which are as unknowable as the set of resources (actually, if it is
the case that resources must have some finite name (?), then they must
be finite in number, but I presume that we can ignore this detail).
In general there is no question of computing the complement of a
class, any more than there is of computing the "positive" of a class.

Ian


> 
> Pat
> 
> 
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Received on Thursday, 7 November 2002 08:26:24 GMT

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