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Re: LANG: syntactic version for imports (and other things)

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 17 Sep 2002 18:10:17 -0500
Message-Id: <p05111b0ab9ad625e8cce@[65.217.30.172]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-webont-wg@w3.org

>From: Jeff Heflin <heflin@cse.lehigh.edu>
>Subject: Re: LANG: syntactic version for imports (and other things)
>Date: Mon, 16 Sep 2002 17:59:02 -0400
>
>>  "Peter F. Patel-Schneider" wrote:
>>  >
>>  > From: Jeff Heflin <heflin@cse.lehigh.edu>
>>  > Subject: Re: LANG: syntactic version for imports (and other things)
>>  > Date: Mon, 16 Sep 2002 10:06:27 -0400
>
>[...]
>
>>  > > Also, I expected to see some account of the meaning of this construct.
>>  > > You didn't like my entailment-based version, what would you suggest
>>  > > instead?
>>  >
>>  > The meaning is the obvious one.  The contents of the imported ontology
>>  > are considered to be part of the meaning of this ontology.
>>
>>  I have to admit I am a bit suprised at this answer, especially since it
>>  is coming from someone who normally holds semantic precision as
>>  something of uptmost importance.
>
>The meaning is quite clear, and quite precise.
>
>>  What are the "contents" of an ontology?
>
>An ontology is a document.  Its contents are the contents of the document.
>
>>  Is it the RDF syntax? The triples? The abstract syntax?  The conditions
>>  imposed on interpretations by the syntax?
>
>At the level of *syntax* it doesn't matter.  All you need to do is to take
>the document, in whatever form it is, and add it to the importing ontology,
>in whatever form *it* is.  Then the syntax-to-semantics mapping takes
>over.

I agree with Peter, but how about the following suggestion for 
re-phrasing this in terms of entailment. If an ontology A contains 
[import B] (in whatever notation turns out to be appropriate) and if 
B + A entails C then A entails C, where '+' means whatever is the 
appropriate 'conjunction' (merging) operation on ontologies.  (For 
bare RDF that would be a triples-graph merge, but OWL might have a 
more sophisticated notion involving the addition of some extra stuff 
to keep the merge tidy; whatever. The point here is that we will need 
this notion to be defined for ANY web ontology language, so whatever 
it is, 'imports' refers to that.)

Now, one can take this to be a constraint on what 'entails' means, or 
one can take it instead to be a specification constraint on behaviors 
of reasoners: that in order to be complete with respect to 
entailment, a reasoner needs to somehow 'merge' the content of the 
imported ontology (and hence, the transitive closure of other 
imported ontologies that they might require in order to be complete). 
But there are many ways one might do that: by copying and caching B 
into A, by retrieval from B during inference from A when needed, if 
that is possible, or even by querying B using a query language (under 
some circumstances). But that decision - or indeed, the decision to 
completely ignore B - can be left to the implementation. An engine 
that makes no pretensions to completeness - and I hereby predict that 
almost all deployed reasoners will not make any such claims, since 
they will be largely meaningless in a real web context - can choose 
to ignore imports statements, or to treat them very casually, for 
example, and still conform to this kind of spec.

I think any spec that *requires* imports to be implemented by a 
syntactic copy is simply not acceptable and will not be used; and 
that in any case it is not necessary.  And all such interpretations 
run foul of the syntactic vagaries of XML and RDF.

Pat


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Received on Tuesday, 17 September 2002 19:10:27 GMT

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