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Re: Equality and subclass axioms

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Tue, 28 Nov 2000 11:17:23 +0000 (GMT)
To: Jeff Heflin <heflin@cs.umd.edu>
cc: Jim Hendler <jhendler@darpa.mil>, www-rdf-logic@w3.org
Message-ID: <Pine.LNX.4.21.0011281115530.2083-100000@galahad.cs.man.ac.uk>
On Mon, 27 Nov 2000, Jeff Heflin wrote:

> Ian Horrocks wrote:
> > 
> > You didn't "negate" my axiom (you can never do that), you just added some
> > additional information (an additional constraint). Assuming it is true
> > that no model can allow triangles that are both three and four-sided, then
> > this is an example of the kind of "over-constraining" that I mentioned in
> > my email: our ontology now constrains allowable models to the extent that
> > none can ever contain an instance of triangle (i.e., we can infer that
> > triangle is equivalent to the class "Nothing"). If we use a reasoner to
> > check the ontology generated by our crawler, then it will detect this
> > fact, and can alert an intelligent (possibly human) agent to the fact that
> > there may be a problem with the axioms relating to triangle.
> > 
> 
> But how can a system know when a particular definition is
> "over-constrained" and when an equivalence to "Nothing" is actually
> intended? Is a human going have to step in every time "Nothing" is
> defined and say, "Yes, I really meant 'Nothing'?" I hope not, because I
> can see ontology integration as a frequent occurence. I think that
> semantic search engines will need to be able to integrate ontologies on
> the fly to meet the needs/context of each query issued by a user. I
> don't believe you can have a single integrated ontology that works for
> all queries.

I agree that there are many difficult questions here which I don't claim
to know the answer to. What I was trying to say was that, in my humble
opinion:

1. Trying to restrict the language so that no "conflicts" can ever arise
is not a viable solution: you would have to get rid of a lot more than
just equivalence axioms, and the resulting language would be so
impoverished as to be useless (c.f. the failure of early DL systems that
tried to make reasoning "easy" by severely limiting the expressive power
of the language).

2. Using reasoning to identify such conflicts can only help. What you do
about conflicts once they have been identified is your problem. You are of
course at liberty to ignore them, in which case you are no worse off than
you would have been without reasoning (unless you take the view that "what
I don't know can't hurt me").

Ian
Received on Tuesday, 28 November 2000 06:16:22 UTC

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