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comment horrocks-01

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 12 Mar 2003 14:53:55 -0600
Message-Id: <p05111b00ba952f82eb99@[64.134.139.17]>
To: w3c-rdfcore-wg@w3.org
Cc: Ian Horrocks <horrocks@cs.man.ac.uk>

I think we need to pay some attention to this. This request reflects 
an energetic exchange of views within Webont, and although it did not 
emerge as a consensual group comment, it clearly reflects a very deep 
issue for some potentially large user communities for RDF.

The issue is that the only available syntactic form for adding 
comments to RDF involves making RDF assertions, since rdf:comment is 
a genuine RDF property, so all such triples have genuine entailments. 
This means, in particular, that changing a comment in an ontology 
changes the formal entailments made by that ontology, so is a genuine 
logical change to that ontology. Whether or not this should be 
considered a bug or a feature is controversial, but there is no doubt 
that to those for whom it is a problem, it is a very serious and 
basic problem, something very close to a fatal can't-live-with 
objection to RDF.

It also means that one can set up inference chains which are probably 
not what any rational person would want to do, eg by defining an 
rdfs:subPropertyOf rdf:comment and then expecting to be able to use 
that to infer that something is an rdf:comment value. This 
distinction isn't particularly important (IMO) in RDF itself, but it 
becomes more trenchant in OWL, where quite subtle and indirect chains 
of reasoning could, in principle, allow one to draw unexpected  (and 
probably unintended) conclusions about an rdf:comment value, eg by 
virtue of there only being three comments in the graph, a cardinality 
constraint applying to a superproperty of rdf:comment and an 
assertion that rdf:comment was functional could produce an 
inconsistency, or maybe allow one to conclude that an invisible 
comment must exist even though it is not in the graph. (The ambiguity 
in what this would really mean illustrates one of the aspects which I 
think most bothers Ian and others, which is that this treatment of 
rdf:comment muddles the distinction between the logical content of an 
RDF graph and what might be called the syntactic decorations of it, 
and hence muddies the semantic clarity of the language by importing 
things - in the case, comment values - into the semantic domain which 
do not belong there. Personally I am happier in muddier semantic 
waters than Ian is, but I recognize that his views are widely shared.)

We could address this in various ways (dark triples, anyone?), but 
all but one of them are too ambitious at this stage, probably. One 
thing we could do relatively easily is for the MT to declare that all 
interpretations make all assertions of rdf:comment true. This in 
effect would cancel the entailments which bother Ian. What this 
amounts to in practice is that all comments are trivially entailed, 
so one cannot use entailment as a guide to associating a comment with 
a graph; one has to appeal to a more directly syntactic criterion, 
such as actually being in the graph.

On the other hand, this might bother some other users who would 
prefer to use entailment as a general RDF 'glue', even for such 
things as comments.

An alternative point of view is that problems will only arise if 
people fiddle with the machinery (which is forbidden in OWL-DL in any 
case), and that Ian's worries about development of large ontologies 
can probably be handled by providing some extra-RDF way of 
associating developer comments with RDF graphs, eg by adding non-RDF 
XML markup. This is rather a brush-off attitude, however, 
particularly if we do not actually provide any hints as to how this 
might be done.

Comments? Is there any other way to allow for 'genuine' comments in 
an RDF graph?

Pat
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Received on Wednesday, 12 March 2003 15:53:45 EST

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