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Re: Representation of SWRL expressions in OWL-S

From: Drew McDermott <drew.mcdermott@yale.edu>
Date: Sun, 6 Feb 2005 10:15:59 -0500
Message-ID: <16902.13487.258584.947428@DVM-Powerbook.local>
To: public-sws-ig@w3.org


> [Manshan Lin]
> 
> Can we use SWRL to define universal quantifier?
> For example,
> all (x) ( A(x) and P(a,x) ) (B(x)) 
> can be defined as
> ( A(x) and P(a,x) ) implies (B(x)) .
> In this case, "x" is an universal variable.

The main point to keep in mind is that Owl-S must _quote_ SWRL
expressions, i.e., label them as XML literals, not to be taken as RDF
by any RDF inference engine looking at them.  The reason for this is
that much of the neat hierarchical structure in a typical
XML-ification of RDF is an illusion.  You must always keep in mind
that the hierarchy evaporates when the XML is turned into triples.
Hence any subset of the triples that can be reassembled into a
meaningful expression _will_ be reassembled.  So if SWRL expressions
occur unquoted, they will be asserted just as surely as if they had
occurred all by themselves.  Your intent may be to use, say, a
headless rule as a formula whose role in context is to be a
precondition, but in fact that rule will gain its freedom and be
interpreted as an actual assertion.

Think of an Owl-S plan as a chromosome, and of unquoted SWRL as
viruses.  Quotation devices then play the role of antibodies.

I belabor this because (a) it took me a while to figure it out, during
which time I was pummeled about the head repeatedly by Bijan until I
got it; (b) once you grant the point then your proposal heads toward
the "moot" category.  Yes, you can adopt a convention whereby unbound
variables are interpreted as universals.  You can also adopt a
convention whereby they are interpreted as existentials, or as
references to the Tao.  Personally, I would favor a convention whereby
quantifiers are explicit, on the grounds that, outside of logic
programming, people get confused about how to interpret a bare
variable.

                                             -- Drew
-- 

                                         -- Drew McDermott
                                            Yale University
                                            Computer Science Department

eczemella\footnote{A skin condition brought on by repeated exposure to
  XML.} 
Received on Sunday, 6 February 2005 15:16:40 GMT

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