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Re: rdf inclusion

From: Drew McDermott <drew.mcdermott@yale.edu>
Date: Fri, 26 Apr 2002 12:37:22 -0400 (EDT)
Message-Id: <200204261637.g3QGbMU21631@pantheon-po01.its.yale.edu>
To: www-rdf-logic@w3.org

   [Sandro Hawke]
   Here's a test case for rdf/daml inclusion:

      I have two RDF files of instance data using ontology
      O1.  O1 is available on the web at two different addresses,
	 http://www.example.org/O1-a and
	 http://www.example.org/O1-b 
      File a.rdf imports O1-a, file b.rdf imports O1-b, but are otherwise
      identical.  The contents of O1-a and O1-b are identical.

      Is the meaning of a.rdf and b.rdf supposed to be exactly the
      same for all systems, ...

This question would be better phrased as: "Are all systems supposed to
draw exactly the same inferences from a.rdf and b.rdf?"  I would
tentatively answer Yes.  (I'm taking "supposed to draw" to mean "would
be justified in drawing"; obviously, some systems will miss valid
inferences others will make.)

      ... or are applications allowed to hard-code
      "http://www.example.org/O1-a", along with perhaps some human
      understand gathered from a telephone conversation between the
      programmer and the ontology creator?

   My sense is that the string "http://www.example.org/O1-a" really is
   special and can be hard-coded, along with ontology information which
   is not machine-readable.   This feels more scruffy but more workable.

Can you clarify what you mean by "hard-coded"?  I'll assume you mean
that some symbols in 01-a have procedural attachments that are missing
in 01-b.  For instance, 01-a might verify (< 3 5) by calling a
built-in "<" function, whereas 01-b might have to resort to using
Peano arithmetic.  Of course, the fact that 01-a and 01-b are
indistinguishable means that 01-a also has Peano's axioms, so the call
to the "<" function just speeds the inference up.

The only way to get a difference between 01-a and 01-b is to give 01-a
a procedural attachment that goes beyond what 01-b knows, leaving 01-b
stuck when given any query of the form (< numeral numeral).  This is a
possible scenario, so if it became common practice for different
agents to make their own procedural attachments to symbols in various 
ontologies I would have to withdraw my tentative Yes.  But let's get
clear on what that would mean. Your phrase "along with ontology
information which is not machine-readable" seems to indicate that 01-a
does contain some specification that "<" is to be handled in a special
way, but that this spec can't be read by all machines.  In that case,
01-a and 01-b would no longer be identical.  01-a would have this
mysterious black box sitting in it that 01-b doesn't have.  In that
case machines that can't penetrate the black box should refuse to use
01-a, and my Yes is restored.

The only way to get the difference you want is to have the information
about 01-a's specialness reside *outside 01-a.*  Anyone anywhere can
decide that 01-a is special to it in some way just because of its
name.  But if you're going to allow that, why not have some agents
omit all the assertions in 01-a that contain the symbol 'daml'; while
others read 01-a backward and 01-b forward.  Meanwhile, 01-c, which
contains only the assertion <> zen:enlightenment <zen:satori>, is
interpreted by some agents as actually containing exactly the
assertions of 01-b.  So, to agents in on the trick, 01-a and 01-b,
which appear to be identical, are really completely different, and
01-b and 01-c, which appear to be different, are identical.  Can I
have my Yes back?

                                             -- Drew
Received on Friday, 26 April 2002 12:37:37 GMT

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