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Re: Requirement: negation

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Fri, 2 Apr 2004 10:27:20 +0300
Message-Id: <2D9BD0A7-8477-11D8-B128-000A95EAFCEA@nokia.com>
Cc: "RDF Data Access Working Group" <public-rdf-dawg@w3.org>
To: "ext Rob Shearer" <Rob.Shearer@networkinference.com>


I agree with what Rob says below, and support this requirement.

I think we can avoid alot of the debate about the difficulties with
negation if we qualify the requirement with "insofar as the particular
knowledge source is aware of at the time of the query".

True, some may grumble about closed world assumptions, etc. but the
fact is that at the moment any software makes any decision, it is
operating in a closed world. The next time it is faced with that
decision, it may choose differently based on new knowledge, but an
agent can only work with what information it has on hand at any
given moment in time, and this requirement for negation reflects
that.

Patrick



On Apr 01, 2004, at 20:21, ext Rob Shearer wrote:

>
>
> Given RDF nodes for people and the property "hasAccountant" I think
> we've all agreed on the need to answer questions along the lines of 
> "get
> me everyone with an accountant" and "get me everyone whose accountant 
> is
> Fred".
>
> I think there's also a requirement to query for people who *don't* have
> accountants, or who don't have Fred as an accountant.
>
> Those on the telecon noted that negation is a bit complex in a system
> with incomplete information: what if somebody does have an accountant,
> but you just haven't heard about it yet?
>
> The thing is, such an assumption goes way beyond RDF semantics. RDF has
> no way of declaring, whether implicitly or explicitly, that a 
> particular
> relationship does not exist, thus there is *never* any distinction
> between "don't know" and "definitely not" within a particular RDF 
> model.
>
> In terms of a single RDF model, negation really is as simple as it
> sounds. If a triple doesn't exist, then it doesn't exist, and that's 
> the
> end of it.
>
> The proposed requirement is that queries be able to be phrased in terms
> of non-existence of some triple(s).
>
>
> It's a common misconception that OWL ontologies and such simply add new
> triples to existing RDF models, but this case demonstrates that it's
> abolutely not the case. OWL ontologies can declare the non-existence of
> a relationship, which can't be encoded in an RDF model.
>
> In practice, I see the true relationship between OWL and RDF as being
> that each of the many many different interpretations of an OWL ontology
> can be viewed as an RDF model. Some triples exist in every
> interpretation (e.g. the explicit triples, and a whole lot of inferred
> triples), some are present in only some interpretations (the realm of
> "incomplete information"), and some are not present in any
> interpretations (those which the ontology has inferred are
> impossible/contradictory).
>
> The query language developed at Network Inference makes this situation
> explicit, and in the process makes negation a very sensible operation.
> The non-existence of a triple is a perfectly fine thing to use as a
> boolean query predicate, and the distinction between the "maybe"s
> (incomplete information; triples in some models but not others) and the
> "no"s (triples which are present in no interpretation) is revealed by
> summarizing results over all possible models.
>
> (There's obviously a lot more to say about our particular query
> language, but I still think this working group shouldn't get too bogged
> down in actual languages yet. All I wanted to illustrate was how
> negation relates to RDF and OWL.)
>
>

--

Patrick Stickler
Nokia, Finland
patrick.stickler@nokia.com
Received on Friday, 2 April 2004 02:32:54 GMT

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