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Re: Statings -- Much ado about nothing

From: Bob MacGregor <macgregor@ISI.EDU>
Date: Fri, 07 Feb 2003 15:35:13 -0800
Message-Id: <5.1.1.6.0.20030207151914.00bc57f0@tnt.isi.edu>
To: pat hayes <phayes@ai.uwf.edu>
Cc: bwm@hplb.hpl.hp.com, www-rdf-interest@w3.org

Pat,

>In the very message you cite, I say explicitly that since you know that a 
>statement has one subject, you can infer that <ex:a> = <ex:b>. I didn't 
>say that a statement could have two subjects. It can't.

We are using the open world assumption in RDF.  When I say two, or twenty 
subjects,
I'm referring to URIs, not denotations in the real world.  Since RDF cannot 
verify denotations,
that's the best you can expect.

>But there is no way to express *IN RDF* the uniqueness condition. How 
>could there be? Try it yourself: go on, write me a message with some 
>triples in it that would follow from knowing that there was only one 
>subject to a statement. Remember, you don't have any equality symbol.
>
>>  Pat Hayes states that the above example is 'peculiar', but
>>there is nothing in the RDF model theory that supports such an opinion.
>
>Not in the formal MT, no. Again, I challenge you: try writing some 
>truth-conditions which would support the intuitive account. I'll buy you a 
>dinner some time if you can do it.

We both know that there aren't any equality constraints in RDF.  But, the 
possible world
semantics says that if I have two triples
    S1, P, O  and
    S2, P, O
then for one of the possible interpretations, S1 and S2 denote the same 
thing.  Therefore,
if I write a stating ST that says
     subject ST S1
     subject ST S2
     predicate ST P
     object ST O
the verifier can't object to the validity of this stating (or any other).

>>Furthermore, if an RDF engine were to complain about it, I would
>>consider that engine to be operating as an *extension* of the RDF spec,
>>as it has currently been written.  Hence, it would not, strictly
>>speaking, be an RDF engine.
>
>Nonsense. Of course it would. The spec specifically gives rules for such 
>extensions.

First, you would have to provide more machinery (e.g., Seth's containedIn
property).  And, you would have to insist that if the triple you looked for 
wasn't
there, then the stating isn't valid (right now, the triple could reside in 
Timbuktu).
Then, you would need to get your new  property and semantics blessed by
W3C (or risk an explosion of disharmonious stating implementations).  If making
up new semantics for RDF was this easy, we wouldn't need an OWL committee
to scrutinize each new predicate.

>>So, how should we interpret Section 3.2.1 of the RDF Semantics document?
>It is intended to be informative.
>
> From a semantics point of view, it is completely null; we might as
>>well omit it.
>
>Its not completely null. It says quite a lot, as part of a spec. Semantics 
>doesn't have to be all formal MT.
>
>>  From a concept point of view, it gives the impression
>>of something being present (a relationship between statings and
>>statements) that is in fact not there at all.
>
>The relationship is there: that is just a fact. What you mean is, it 
>provides no formal constraints which enable you to lock it down precisely.
>
>>  In otherwords, the
>>effect is to mislead the average reader.
>>
>>So, my recommendation would be to delete the entire section.
>
>Fine with me, but I don't see how it would help anyone.

Right now, statings are, at best, half finished.  More semantics (more than 
zero)
is needed to turn them into a useful product.  At the very least, the
RDF documents that treat this subject should point out what's missing
from the spec that would be needed in any implementation that would
support automatic validation of statings.

Cheers, Bob
Received on Friday, 7 February 2003 19:03:56 GMT

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