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Re: comment horrocks-01

From: Jos De_Roo <jos.deroo@agfa.com>
Date: Thu, 13 Mar 2003 01:12:24 +0100
To: "Frank Manola <fmanola" <fmanola@mitre.org>
Cc: Ian Horrocks <horrocks@cs.man.ac.uk>, pat hayes <phayes@ai.uwf.edu>, w3c-rdfcore-wg@w3.org, w3c-rdfcore-wg-request@w3.org
Message-ID: <OF397CCDCE.5EC7607F-ONC1256CE8.000067C7-C1256CE8.000138C7@agfa.be>


Frank Manola wrote:
> pat hayes wrote:
>
> >
> > 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.
>
>
> I agree we need to pay some attention to this (I remember thinking that
> when Ian first made the comment), but I need some clarification of the
> pros and cons.
>
>
> >
> > 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.
>
>
> In Ian's message, he said "In particular, it would be inappropriate for
> *applications* to infer semantic differences in information represented
> in two ontologies based solely on differences in comments (in the same
> way that it would be inappropriate for code to behave differently when
> only the comments are changed)." [my emphasis]
>
> I'm having problems with who is doing the inferring/entailments in
> question, and what kinds they are.  It makes perfect sense for an
> *application* to decide that it wants to pay no attention to triples
> with rdf:comment properties, but it seems to me what we're being asked
> to do is to rule that out in some sense for *all* applications, by
> making this somehow basic in RDF.  If triples with rdf:comment
> properties are "pure comments", presumably they are still there in the
> graph [otherwise the XML comment option would have worked], but they are
> ignored in certain types of processing.  I would hope that I could still
> write an application that, say, used the number of comments (and maybe
> even the length of the values) to decide whether an ontology was
> "well-commented" in some sense?  So could you give me some test cases
> describing the sorts of entailments in RDF that you'd like to forbid
> (test cases describing the situations below, which I didn't entirely
> understand, would be fine).
>
>
> >
> > 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.)
>
>
> This seems similar to the class/instance distinction, which RDF also
> "muddles", because at this level we don't want to wire those
> distinctions in.
>
>
> >
> > 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.
>
>
> That sounds reasonable, assuming I understood what these extra
> entailments are that create the problem (see the "test cases" comment
> above).

If I understand it correctly, that could be easily done
and I've tried that (rdfs:comment triples always succeed).
Of course, that would then mean that the empty graph entails
<phayes@ai.uwf.edu> rdfs:comment "mail address of the cleverest guy on the
planet".

> >
> > 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?
> >
>
>
> Reification anyone?

de re ification?

> --Frank
>
>
>
>
> --
> Frank Manola                   The MITRE Corporation
> 202 Burlington Road, MS A345   Bedford, MA 01730-1420
> mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-875

-- ,
Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/
Received on Wednesday, 12 March 2003 19:13:27 EST

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